Accretion and Ejection in Compact objects
Accretion onto compact objects like black holes (BH), neutron stars (NS) and white dwarves (WD), is one of the most efficient energy sources in the universe. When matter falls onto such compact objects, it spirals inwards to form an accretion disc. A large fraction of the accretion energy is often channeled into relativistic, collimated outflows known as jets. I investigate the multi-wavelength electromagnetic emission detected in a wide variety of astrophysical systems, from the supermassive BHs residing at the centers of distant Active Galactic Nuclei (AGN), to stellar-mass BHs and NSs in our own galaxy, with the aim to interpret the complex physics of astrophysical jets and discs. My research expands across the coupling between accretion and ejections in these systems, explaining the accretion processes in discs and the relativistic nature of jets, as well as scale invariance of BH physics across the entire mass scale. I analyze a wealth of broadband (radio-infraredoptical-ultraviolet-X-ray) state-of-the-art observations and use theoretical modeling, to probe the environment of extreme gravitational fields.
Listed below are projects I have either spearheaded or made contributions to :
Project |01
Optical Fundamental Plane of black hole activity
Saikia P., Koerding E. and Falcke H., 2015, MNRAS, 450, 2317
FIGURE 1. Projection of the Optical Fundamental Plane. LLAGN sample is shown in red, with the red solid line depicting the best-fitting line for the SMBH sample. The XRB sample is put on the graph as blue dots. Luminosities are given in erg/s while the masses are in the unit of solar mass.
Black holes, whether small or supermassive, are believed to behave similarly when pulling in matter (accretion) and launching jets — an idea called scale invariance. One way this is tested is through the Fundamental Plane of black hole activity, a 3D relation linking a black hole’s mass with its radio and X-ray brightness. In our study, we explored a new version of this plane by using the brightness of an optical emission line called [O III], which comes from gas near the black hole, instead of X-rays. We analyzed a sample of 39 supermassive black holes from the Palomar Spectroscopic Survey, each with radio, optical, and mass measurements. To test if these patterns hold at all scales, we added a group of stellar-mass black holes (X-ray binaries). Our results show that the supermassive black holes alone form a clear plane in 3D space defined by bolometric luminosity, radio brightness, and mass. Interestingly, the stellar-mass black holes fit naturally onto this same plane — supporting the idea that the same physical processes govern black holes across the mass scale. We also examined how radio-loud different low-luminosity active galaxies are, viewed through this Fundamental Plane.
Project |02
Lorentz factor distribution of relativistic jets from blazars
Saikia P., Koerding E. and Falcke H., 2016, MNRAS, 461, 2397
FIGURE 2. KS statistics for different power-law exponents depicting the blazar distributions for a fixed gamma range of 1–40 and a θ range of 0–30, showing a best-fitting value of −2.1 for the power-law index.
Blazars are a special type of active galaxy with a supermassive black hole at the center that launches a powerful, fast-moving jet of particles pointed almost directly toward Earth. Because of this, the light we see from blazars is boosted by relativistic effects, making it tricky to measure their true properties. One important property is the Lorentz factor, which tells us how fast the jet is moving. Usually, this is estimated using the apparent speed of the jet and how much its light is boosted, which is extremely difficult to do in most cases.
In this study, we introduce a new, independent method to estimate blazar Lorentz factors using something called the optical Fundamental Plane of black hole activity — a relationship connecting a black hole’s [O III] line brightness, radio brightness, and mass. By applying this to a well-studied sample of blazars, we measured their true radio brightness and used it to estimate their jet speeds. We found that blazar Lorentz factors follow a power-law distribution, with most jets moving at moderately high speeds and a few at extreme speeds, and we also explored how the angles at which we view these jets are distributed.
FIGURE 3. The distance of FIRST AGN sample (red), LINERs (yellow) and LLAGN (green) from the fundamental plane (index), along with the theoretical estimation for position of the radio lobes (blue). For easy visualization of the subsamples that have comparatively fewer sources, we plot the mean position ofevery subsample in dashed lines.
Active galaxies are powered by supermassive black holes that can launch jets of energy from their centers, and astronomers often use radio observations to study this activity. In this study, we investigated whether radio measurements from the 1.4 GHz FIRST survey reliably trace the core, or central, activity of active galaxies. Using a large sample of over 10,000 galaxies from the SDSS-FIRST catalogs, we tested how these radio measurements line up with the optical Fundamental Plane of black hole activity — a known relationship connecting a galaxy’s radio brightness, optical emission, and black hole mass.
We found that for most galaxies, the FIRST radio measurements are contaminated by extended emissions from lobes and surrounding material, not just the core jets. Only compact sources and LINERs (a type of weakly active galaxy with little extended emission) follow the expected Fundamental Plane when adjusted for beaming effects. This shows that 1.4 GHz FIRST data do not reliably reflect the central jet power in active galaxies, and should be used with caution in studies of black hole activity.
Project |04
AGN Spectra : Kinematical relationship between emission regions
Kovacevic et al., 2013, Proceedings of the XII Congress of Serbian Physics, 384-387
FIGURE 4. An example of line fitting (solid line) in the optical spectral range of SDSS J02003916-084555.01 (observed spectra shown in dotted lines).
Active galaxies often show rich and complex spectra, with one especially interesting feature being a set of emission lines from iron (Fe II ions). These iron lines raise many open questions, like how they’re produced, where exactly they come from in the galaxy’s structure, and how they relate to other properties of the galaxy. In this study, based on my Master’s thesis, I analyzed 100 active galactic nuclei (AGN) spectra from the Sloan Digital Sky Survey (SDSS) using a detailed multi-Gaussian fitting method, which separates each emission line into components coming from different regions with distinct physical conditions. We explored how different groups of iron emission lines relate to other optical and ultraviolet spectral features, and examined how these trends connect to the Baldwin Effect (the tendency for some emission lines to weaken at higher luminosities) and the famous anticorrelation between Fe II and [O III] emission, a key part of what’s known as Eigenvector 1 in AGN studies.
Project |05
LeMMINGs. I. The eMERLIN legacy survey of nearby galaxies. 1.5-GHz parsec-scale radio structures and cores
Baldi et al. 2018, MNRAS, 476, 3478
FIGURE 5. Few examples of eMERLIN 1.5-GHz radio maps of the galaxies with an identified radio core. For each galaxy, two panels are shown. The upper panel shows the full-resolution map, while the lower panel shows the low-resolution map obtained with a uv-tapered scale written in the panel (in k).
Black holes at the centers of galaxies can launch powerful jets of energy, and studying these jets in nearby galaxies helps us understand how black holes grow and interact with their surroundings. In this project, we present the first set of high-resolution radio images of 103 nearby galaxies from the Palomar sample, taken with the eMERLIN radio telescope array as part of the LeMMINGs survey. The sample includes both active galaxies (like Seyferts and LINERs) and quieter ones (like HII galaxies and absorption line galaxies). About half of them show clear jet-like structures, while the rest display compact cores or complex shapes. We found that LINERs behave like smaller versions of powerful radio galaxies, Seyferts produce less focused jets, and some HII galaxies might contain weakly active black holes or star-forming regions. By using the [O III] emission as a measure of how actively a black hole is accreting matter, we showed that active galaxies and some HII galaxies with jets follow a shared relationship known as the optical Fundamental Plane of black hole activity, hinting at a common connection between accretion discs and jets across different galaxy types.
Project |06
FIGURE 6. The radio luminosity function of the LLAGN in the Palomar sample. For each bin, the number of galaxies is shown at the top. Error-bars are assigned assuming Poisson statistics. The red line is the broken power-law to the RLF. It has a slope of -0.7 for the brighter, and -0.3 for the fainter sources.
Many galaxies host low-luminosity active galactic nuclei (LLAGN), powered by less active black holes at their centers, but their faint radio signals have often gone undetected. In this study, we carried out a high-resolution radio imaging survey of 76 such LLAGN that were previously not seen at 15 GHz with the Very Large Array.
We detected compact, parsec-scale radio signals in about 60% of them, including many LINERs, Seyferts, and transition objects. Using this new sample, we studied how their radio brightness relates to other properties like emission lines, X-rays, and optical luminosity, and refined the parameters of the Fundamental Plane of black hole activity. Most importantly, we found clear evidence that the distribution of LLAGN radio brightness — called the radio luminosity function (RLF) — has a break at lower luminosities, meaning there’s a limit to how faint these nuclei can get. This break happens at a specific, low mass accretion rate, and interestingly, nearby galaxies like those in our Local Group fall along the same trend as more powerful AGN in the universe.
Project |07
A wildly flickering jet in the black hole X-ray binary MAXI J1535-571
Baglio et al. 2018, The Astrophysical journal, 867, 2
FIGURE 7. Hardness-intensity diagram (HID) for the 2017-8 outburst of J1535. The red dots show the hard state, the green dots hard-intermediate state, the violets dots represent soft-intermediate and light blue dots show soft states.
Black hole X-ray binaries are systems where a black hole pulls in matter from a companion star, often launching powerful jets of particles. In this study, we observed the black hole candidate MAXI J1535-571 during its 2017–2018 outburst, using optical, near-infrared (NIR), and mid-infrared (mid-IR) telescopes. Early in the outburst, the system showed a bright, steady jet producing optically thin synchrotron radiation — a type of light emitted by fast-moving particles in a magnetic field. Soon after, the source faded dramatically, especially at lower frequencies, while its X-ray spectrum softened. This pattern suggests the jet was suppressed as the system’s accretion state changed, a behavior known from other black hole binaries. Remarkably, before the fade, MAXI J1535-571 became one of the brightest mid-IR black hole binaries ever observed. We also carried out the first study of rapid mid-IR brightness changes in such a system, finding significant variability on minute timescales — much stronger than the tiny variations seen in optical light.
Project |08
Lorentz Factors of compact jets in Black hole X-ray binaries
Saikia et al. 2019, The Astrophysical journal, 887, 1, 21
Compact, continuously launched jets in black hole X-ray binaries (BHXBs) produce synchrotron emission spanning from radio to infrared (IR) and optical wavelengths. In most BHXBs, an IR excess — a noticeable increase in infrared light beyond what’s expected from the accretion disc alone — appears when the system is in the hard X-ray spectral state and the jet is active. In this study, we explored why some BHXBs show strong IR excesses while others do not. Our results show that the amplitude of this excess can be explained by the combined effects of inclination-dependent beaming of the jet’s synchrotron emission and the projected area of the accretion disc. We also find no correlation between the expected and observed IR excess when assuming no beaming (Lorentz factor = 1), providing strong evidence that the IR excess arises from relativistically beamed jet emission. Using how much the jet fades and recovers during state transitions, combined with known orbital parameters, we constrained — for the first time — the bulk Lorentz factor range of compact jets in several BHXBs, with well-measured values falling between 1.3 and 3.5.
FIGURE 8. Schematic diagram to qualitatively explain the expected IR excess observed for different BHXB sources, depending on their inclinations.
Project |09
Appearance of a compact jet in soft–intermediate state of 4U 1543−47
Russell et al. 2020, MNRAS, 495, 1, 182
FIGURE 9. X-ray light curve in the 3–6, 6–10, and 3–10 keV energy bands. The hard X-ray increase during the soft to hard transition is clearly visible after MJD 52473. If the IR excess has an optically thin synchrotron spectrum (with α = −0.7 from IR to X-ray energies, its light curve would be the blue filled diamonds)
Recent advancements in the understanding of jet–disc coupling in black hole candidate X-ray binaries have provided close links between radio jet emission and X-ray spectral and variability behaviour. In ‘soft’ X-ray states the jets are suppressed, but the current picture lacks an understanding of the X-ray features associated with the quenching or recovering of these jets. Here, we show that a brief, ∼4 d infrared brightening during a predominantly soft X-ray state of the BHXB 4U 1543-47 is contemporaneous with a strong X-ray type B quasi-periodic oscillation, a slight spectral hardening and an increase in the rms variability, indicating an excursion to the soft–intermediate state. This IR ‘flare’ has a spectral index consistent with optically thin synchrotron emission and most likely originates from the steady, compact jet. IR emission is produced in a small region of the jets close to where they are launched (∼0.1 light-seconds), and the time-scale of the IR flare in 4U 1543-47 is far too long to be caused by a single, discrete ejection. We also present a summary of the evolution of the jet and X-ray spectral/variability properties throughout the whole outburst, constraining the jet contribution to the X-ray flux during the decay.
Project |10
Probing Jet Launching in Neutron Star X-Ray Binaries: The Variable and Polarized Jet of SAX J1808.4–3658
Baglio et al. 2020, ApJ, 905, 87
FIGURE 10. Polarization level P (%) vs. position angle (°) in all epochs of the 2019 outburst for I band (black squares), R band (red circles), V band (green triangles), and B band (blue stars) data of SAX J1808.4-3658.
Accreting millisecond X-ray pulsars are fast-spinning neutron stars that pull in material from a companion star and can launch powerful jets. In this study, we observed SAX J1808.4–3658 during its 2019 outburst using optical photometry and polarimetry. As the outburst progressed, we noticed a red excess in its optical light, especially in the z, i, and R bands — a clear sign of optically thin synchrotron emission from a jet. Our polarimetric observations revealed low-level but variable linear polarization (0.2%–2%) across multiple optical bands, showing this polarization is real and comes from the source itself. The polarization was strongest at redder wavelengths during both the main outburst and later reflaring periods, supporting a jet origin rather than effects from the accretion disc or scattered material. Occasional brief episodes of higher polarization (1%–2%) were seen during the reflaring state, suggesting a jet base with strongly tangled magnetic fields. This study highlights how optical polarimetry is a powerful way to probe the magnetic structures in X-ray binary jets, much like it’s used to study jets in active galactic nuclei.
Project |11
LeMMINGs. II. The e-MERLIN legacy survey of nearby galaxies. The deepest radio view of the Palomar sample on parsec scale
Baldi et al. 2020, MNRAS, 500, 4749
FIGURE 11. e-MERLIN 1.5-GHz maps of a few of the detected and core-identified galaxies. The restoring beam is presented as a filled ellipse at one of the corners of each of the maps. The × marks indicate the optical galaxy centre taken from NED, while the + symbol marks the radio core position, if identified.
Studying radio jets from galaxies helps astronomers understand how black holes interact with their host galaxies. In this work, we present the second data release of high-resolution radio images from the LeMMINGs survey, observing 177 nearby galaxies with the e-MERLIN radio array at 1.5 GHz. Combined with the first release, this completes a deep radio survey of 280 local galaxies, including active types (LINERs and Seyferts) and quieter types (HII galaxies and absorption line galaxies, or ALGs). We detected radio emission in about 45% of galaxies, with over 100 showing compact cores at their centers. Around a third of these detections display jet-like structures. LINERs and Seyferts were the brightest sources, with LINERs showing FR I-like core-brightened jets and Seyferts often having more symmetric morphologies. Most HII galaxies showed compact cores or complex shapes, likely linked to star formation or weak black hole activity, and a few even revealed clear jets. Though less common, ALGs — typically found in old elliptical galaxies — surprisingly hosted some of the most powerful radio sources, resembling those seen in LINERs. This work provides one of the most complete views of how radio jets behave in the nearby Universe, helping us better understand how black holes affect galaxies of all types.
Project |12
LeMMINGs. III. The e-MERLIN legacy survey of the Palomar sample: exploring the origin of nuclear radio emission in active and inactive galaxies through the [O III]–radio connection
Baldi et al. 2021, MNRAS, 508, 2019
FIGURE 12. The fundamental plane of BH activity in the optical band for the LeMMINGs sample with e-MERLIN data, The filled symbols refer to the detected radio sources, while the empty symbols refer to non-detected radio sources.
What determines the nuclear radio emission in local galaxies? To address this question, we combine optical [O III] line emission, robust black hole (BH) mass estimates, and high-resolution e-MERLIN 1.5-GHz data, from the LeMMINGs survey, of a statistically complete sample of 280 nearby optically active (LINER and Seyfert) and inactive [H II and absorption line] galaxies. Using [O III] luminosity as a proxy for the accretion power, local galaxies follow distinct sequences in the optical–radio planes of BH activity, which suggest different origins of the nuclear radio emission for the optical classes. The 1.5-GHz radio luminosity of their parsec-scale cores is found to scale with BH mass and [O III] luminosity. Radio-quiet and radio-loud LINERs are powered by low accretion rate discs launching sub-relativistic and relativistic jets, respectively. Low-power slow jets and disc/corona winds from moderately high to high mass accretion discs account for the compact and edge-brightened jets of Seyferts, respectively. Jetted H II galaxies may host weakly active BHs. Fuel-starved BHs and recurrent activity account for ALG properties. In conclusion, specific accretion–ejection states of active BHs determine the radio production and the optical classification of local active galaxies.
Project |13
LeMMINGs. IV. The X-ray properties of a statistically complete sample of nuclei in active and inactive galaxies from Palomar sample
Williams et al. 2022, MNRAS, 510, 4909
FIGURE 13. Example X-ray spectrum of one of the X-ray detected sources, NGC224. The top panel shows the number of photons plotted against the energy in keV across the whole 0.3−10.0 keV band. The bottom panel shows the model subtracted from the data, divided by the error.
Understanding how black holes behave in nearby galaxies is key to figuring out how they grow and affect their surroundings. In this study, as part of the LeMMINGs e-MERLIN legacy survey, all 280 nearby galaxies from the well-known Palomar sample were observed in radio, and here we present Chandra X-ray observations for 213 of them, covering both active galaxies (like Seyferts and LINERs) and quieter ones (like HII galaxies and absorption line galaxies, or ALGs). We detected X-ray emission from the centers of about 70% of these galaxies, including nearly all Seyferts and most LINERs, as well as a surprising number of HII galaxies and ALGs. By measuring how many galaxies shine at different X-ray brightnesses, we built an X-ray luminosity function (XLF) for the local Universe and found it follows a simple power-law shape. Even more exciting, when comparing X-ray brightness, [O III] emission, and black hole masses, we found that some galaxies previously considered "inactive" actually behave like low-power active galaxies, suggesting that many more galaxies may be hiding weakly active black holes than previously thought.
Project |14
MeerKAT discovery of radio emission from the Vela X-1 bow shock
van den Eijnden et al. 2022, MNRAS, 510, 515
FIGURE 14. Radio continuum emission of the full MeerKAT field of view of Vela X-1, shown by the cross, and its surroundings, created by combining three observing runs. The combined exposure time is 90 minutes, yielding a 40 𝜇Jy RMS sensitivity. The observations were performed at L-band (1.3 GHz),
When massive stars or X-ray binaries race through space, their powerful stellar winds slam into the surrounding interstellar gas, creating arc-like structures known as bow shocks. These are typically seen in optical or infrared light, but detecting them in radio waves has been extremely rare. In this study, we report the exciting discovery of 1.3 GHz radio emission from the bow shock surrounding Vela X-1, a well-known X-ray binary system, using the MeerKAT radio telescope.
This study is especially significant because it marks the first radio detection of a stellar-wind-driven bow shock around an X-ray binary. The radio emission closely traces the structure previously seen in Hα images, confirming the shape and extent of the bow shock and revealing new details about its interaction with the surrounding gas. We explored different possible causes for the radio emission and found it could be explained by thermal (free-free) radiation from hot, ionized gas, especially if a dense patch of interstellar material is present near the system. While a non-thermal synchrotron scenario is possible, it would require an unusually high fraction of the star’s wind energy to be converted into fast-moving electrons, making it less likely.
This discovery opens up exciting new opportunities to use radio observations to detect and study bow shocks around other high-speed massive stars and X-ray binaries, giving us valuable clues about how these powerful systems shape their environments.
Project |15
A multi-wavelength study of GRS 1716−249 : constraints on its distance
Saikia et al. 2022, ApJ, 932, 38
FIGURE 15. The optical finding chart of GRS 1716−249 during outburst (MJD 57874.7) with the 2-m LCO telescope in the i′-band with 200s exposure time.
When black hole X-ray binaries go into outburst, they brighten dramatically across multiple wavelengths, offering a chance to study how matter behaves around black holes. In this study, we present a detailed multi-wavelength analysis of the black hole transient GRS 1716-249 during its 2016-2017 outburst, using optical, infrared, ultraviolet, X-ray, and radio data from a range of telescopes including Swift, NuSTAR, VLT, and the Very Large Array. While its X-ray behavior was typical for a black hole outburst, we found that GRS 1716-249 was unexpectedly faint in optical light if the widely assumed distance of 2.4 kiloparsecs is correct. By comparing its properties to other similar systems and using several lines of evidence, we argue that the source is actually much farther away than previously thought — likely somewhere between 4 and 17 kiloparsecs, with a most probable range of 4–8 kiloparsecs. This is important because a system’s distance affects estimates of its brightness, jet power, and how it fits into broader models of black hole behavior, meaning that many earlier conclusions about GRS 1716−249 may need to be revised.
Project |16
A misfired outburst in the neutron star X-ray binary Centaurus X-4
Baglio, Saikia et al. 2022, ApJ, 930, 20
FIGURE 16. The R and i' -band optical light curves of the residuals, obtained after the subtraction of the sinusoidal modulation from the original LCO light curves.
Neutron star X-ray binaries are systems where a neutron star pulls in material from a companion star, forming an accretion disc that can occasionally erupt in bright outbursts. In this study, we report the results of 13.5 years of optical monitoring of Centaurus X-4, a well-known neutron star system that has remained in quiescence since its last outburst in 1979. Over this long timespan, we tracked the gradual changes in the disc’s light and detected brief flares likely coming from the disc itself, along with a subtle modulation from the companion star’s changing shape as it orbits. Interestingly, we observed a slow, long-term fading in the disc’s brightness over about 6 years, followed by a 3-year rise in optical light, a pattern often seen before outbursts in other systems and predicted by the disc instability model. In late 2020, this buildup led to a brief increase in brightness across optical, UV, and X-rays, though it quickly faded without triggering a full outburst. We suggest that a heating wave started in the inner disc due to hydrogen ionization but stalled as it moved outward, likely because of the disc’s structure and weak irradiation from the central source. This long-term monitoring provides valuable insight into how accretion discs evolve between outbursts and helps test models of disc behavior in X-ray binaries.
Project |17
Radio detections of IR-selected runaway stellar bow shocks
van den Eijnden, Saikia et al. 2022, MNRAS, 512, 4, 5374
FIGURE 17. Radio (RACS; left column) and infrared (WISE; right column) images of G1 (top) and G3 (bottom). The physical scale is the same between the radio and IR images. In both images, same contours are drawn, based on the radio morphology.
When massive stars move at supersonic speeds through space, their strong stellar winds collide with the surrounding interstellar gas, creating bow shocks — curved, arc-like structures where the two flows meet. These are often detected in infrared light, but radio detections have remained extremely rare, despite expectations that fast-moving electrons in the shock should produce radio waves through synchrotron emission. In this study, we used newly released radio images from the Rapid ASKAP Continuum Survey (RACS) to search for radio counterparts to 50 infrared-detected bow shocks from the E-BOSS catalogues. We identified three confident and three likely radio detections, along with several additional candidates requiring follow-up observations for confirmation. This discovery significantly increases the number of known radio-detected stellar bow shocks, marking a big step forward in studying these fascinating structures. By analyzing the emission from these sources, we found a mix of thermal (free-free) and non-thermal (synchrotron)radiation, helping us better understand the physical processes happening at these shock fronts. For the undetected sources, we placed limits on their possible radio emission and considered how upcoming radio telescopes could reveal even fainter shocks. This work highlights how powerful next-generation radio surveys are for uncovering hidden features of the dynamic environments around massive stars, opening new windows into how they interact with and shape the galaxy around them.
Project |18
Radio observations of the Black Hole X-ray Binary EXO 1846−031 re-awakening from a 34-year slumber
Williams et al. 2022, MNRAS, 517 (2), 2801-2817
FIGURE 18. The MAXI/GSC X-ray Hardness–Intensity diagram (HID) of the 2019 outburst of EXO1846, showing a characteristic q-shaped hysteresis of X-ray binaries in outburst.
Black hole X-ray binaries are systems where a black hole pulls in matter from a companion star, often launching powerful jets during outbursts. In this study, we observed the 2019 outburst of the candidate BHXB EXO 1846−031 using a combination of radio telescopes (MeerKAT, VLA, AMI-LA) and X-ray instruments (Swift and MAXI). High-resolution radio images from the VLA captured moving components in the jet during the event. The radio and X-ray brightness followed a similar pattern, with a major peak followed by a short dip and a second flare. We estimated the minimum energy carried by these radio flares and, based on the X-ray flux and standard assumptions about outburst behavior, calculated the system’s distance to be around 2.4–7.5 kiloparsecs. By placing EXO 1846−031 on the radio/X-ray luminosity plane for black holes, we found it behaves like a ‘radio-quiet’ black hole — producing less radio emission for its X-ray brightness than expected — likely sitting around 4.5 kiloparsecs away. Using this distance and the jet’s inclination, we estimated the intrinsic jet speed to be about 0.29 times the speed of light, indicating relatively slow, sub-luminal jet motion.
Project |19
MeerKAT radio observations of the neutron star low-mass X-ray binary Cen X-4 at low accretion rates
van den Eijnden et al. 2022, MNRAS, 516, 2, 2641
FIGURE 19. The 1–10 keV X-ray – 5 GHz radio luminosity diagram for low-mass X-ray binaries, including Cen X-4 upper limits from this work.
Neutron star X-ray binaries are systems where a neutron star pulls in material from a companion star, sometimes launching jets of particles as it accretes. Centaurus X–4 (Cen X–4) is one of the best-known examples, having shown outbursts in 1969 and 1979 but remaining quiet ever since. Because it’s relatively close to us and has stayed in a quiescent state for decades, it’s a valuable target for studying how accretion and jet activity behave when a system isn’t actively flaring. In this study, we observed Cen X–4 using the MeerKAT radio telescope alongside X-ray monitoring from NICER and Swift. While the system remained in a fully quiescent X-ray state during the first observation, the other three tracked a brief, faint episode of X-ray activity in early 2021 — a so-called ‘mis-fired’ outburst that never fully developed. Cen X–4 was not detected in radio in any of these observations, and we placed new, tighter limits on how faint its radio emission must be at these X-ray levels. By adding these results to the radio–X-ray luminosity diagram for neutron star systems, we confirmed that Cen X–4 is radio fainter than transitional millisecond pulsars like PSR J1023+0038 at the same X-ray brightness. These findings help refine our understanding of how weakly accreting neutron stars behave at low luminosities, and highlight the importance of long-term monitoring to catch rare moments of activity.
Project |20
Seven reflares, a mini-outburst and an outburst: High amplitude optical variations in the black hole X-ray binary Swift J1910.2−0546
Saikia et al. 2023, ApJ, 949, 2, 104
FIGURE 20. Color-magnitude diagrams (CMD) of J1910.2 in V vs V -i′, The black solid lines show points from single-temperature blackbody models heating up and cooling. The grey lines show a different normalisation to better fit only the 2012 outburst.
In this study, we present 10 years of optical monitoring (2012–2022) of the candidate black hole system Swift J1910.2-0546 using the Faulkes Telescopes and the Las Cumbres Observatory (LCO) network. After its bright 2012 outburst, the system displayed a highly unusual pattern of at least seven large optical flares in 2013, each brightening by about 3 magnitudes and spaced 42–49 days apart. In 2014, it experienced a smaller double-peaked outburst. We also tracked the system’s 2022 outburst and compared it to these earlier events. During the flares in both 2013 and 2014, the system became bluer when brighter, with optical colors consistent with a disc heating and cooling between 4500 and 9500 K — right around the temperature where hydrogen begins to ionize. Comparing this behavior to other X-ray binaries, we found that Swift J1910.2-0546’s pattern of repeated, large-amplitude optical flares is highly unusual. We suggest these flares are caused by a series of heating and cooling fronts repeatedly bouncing through the accretion disc, a behavior rarely seen in other systems. This long-term monitoring offers new insights into how black hole accretion discs behave between major outbursts
Project |21
Lemmings. V. Nuclear activity and bulge properties: a detailed multi-component decomposition of e-MERLIN Palomar galaxies with HST
Dullo et al. 2023, A&A, 675, A105
FIGURE 21. The left hand panel shows the composite (HST ACS+SDSS) surface brightness, P.A. and B4 profiles of the doubled-barred lenticular LeMMINGs galaxy NGC 2859. The right hand side panel shows the SDSS image of NGC 2859. The top and bottom insets show the surface brightness contours of the galaxy’s SDSS and HST ACS images, respectively. North is up, and east is to the left.
Galaxies come in a variety of shapes and structures, and understanding how their central regions relate to the activity of their central black holes is an important question in astronomy. In this study, we combined high-resolution images from the Hubble Space Telescope (HST) with 1.5 GHz radio observations from the e-MERLIN LeMMINGs survey to investigate how the optical structures of galaxy cores relate to their nuclear radio emission. Using new surface brightness profiles for 163 nearby galaxies, we performed detailed multi-component modeling, fitting up to six structural components like bulges, discs, bars, rings, spiral arms, active galactic nuclei (AGN), and nuclear star clusters at once. We found that if features like bars or rings aren’t included, a galaxy’s bulge mass can be significantly overestimated. Our results also showed that the fraction of galaxies detected in radio increases with bulge mass, suggesting a connection between central mass concentration and radio activity. Additionally, galaxies with core-Sérsic bulges — a type of central structure with a flattened inner light profile — tend to be rounder, show higher radio luminosities, and often have boxy or elliptical isophote shapes. Interestingly, while earlier studies suggested a sharp divide between different galaxy core structures and their radio properties, our findings do not support a strong dichotomy.
Project |22
Lemmings. VI. Connecting nuclear activity to bulge properties of active and inactive galaxies: radio scaling relations and galaxy environment
Dullo et al. 2023, MNRAS, 522 (3), 3412-3438
FIGURE 22. Correlations between the 𝑒-MERLIN 1.5 GHz radio core luminosity and bulge stellar mass and absolute V-band bulge magnitude for our sample of 173 galaxies, separated by spectral classes.
Astronomers have long known that a galaxy’s central black hole activity seems to be connected to the structure of its bulge — the dense, central region of stars — but the exact relationship is still not fully understood. In this study, we explored this connection by combining Hubble Space Telescope (HST) measurements of bulge structure and brightness with 1.5 GHz radio data from the e-MERLIN LeMMINGs survey for a large sample of 173 nearby galaxies, including both ‘active’ types (like Seyferts and LINERs) and ‘inactive’ types (like HII galaxies and absorption line galaxies, or ALGs). Interestingly, we found that at a given bulge mass, the strength of nuclear radio activity and the likelihood of hosting an AGN show no dependence on the galaxy’s environment. However, radio-loud galaxies are more likely to have early-type (elliptical or lenticular) shapes compared to radio-quiet galaxies, although both types appear similar in terms of bulge concentration (Sérsic index) and shape (ellipticity). Results on the fine details of the bulge light profile’s inner slope were inconclusive.
Project |23
Clockwise evolution in the hardness–intensity diagram of the black hole X-ray binary Swift J1910.2−0546
Saikia et al. 2023, MNRAS, 524, 4543
FIGURE 23. X-ray hardness–intensity diagrams of Swift J1910.2−0546 using (a) Swift/XRT count rates at 2–10, 1.5–10, and 0.6–1.5 keV energy ranges (with arrows overplotted to show the evolution of the HID during the outburst), (b) Swift/XRT count rates at 3–10, 6–10, and 3–6 keV energy ranges, Different colours and symbols are used to represent the various states and stages of the outburst (see text). The orange pentagon in the first two panels represents the minimum of the X-ray dip on 2012 September 5th (MJD 56175.5, see Section 3.1).
Black hole X-ray binaries occasionally undergo outbursts, dramatically brightening across different wavelengths as matter flows onto the black hole. In this study, we examined optical data from the 2012 outburst of the candidate black hole system Swift J1910.2−0546, using the Faulkes Telescope and the Las Cumbres Observatory (LCO). We tracked how the system’s brightness and color changed as it moved through different spectral states — stages of accretion activity characterized by shifts in X-ray and optical behavior. By comparing optical and X-ray light curves and constructing spectral energy distributions, we identified the main sources of optical emission. Notably, when the system transitioned to a pure hard state, we observed a sharp optical brightening and a dramatic color change, signaling the appearance of a jet. At other times, the optical and UV emission was mainly produced by the X-ray-irradiated accretion disc. High-cadence optical monitoring revealed a possible periodic signal, which, if caused by a superhump (a small variation linked to the disc’s shape), suggests an extremely short orbital period of 2.25–2.47 hours — potentially the shortest known for a black hole X-ray binary. Finally, using the system’s brightness during state changes, we estimated its distance to be around 4.5–20.8 kiloparsecs, consistent with its position in the global optical/X-ray correlation for X-ray binaries. This study sheds light on how black hole systems evolve during outbursts and how jets and discs contribute to their multi-wavelength emission.
Project |24
Matter ejections behind the highs and lows of the transitional millisecond pulsar PSR J1023+0038
Baglio et al. 2023, A&A 677, A30
FIGURE 24. Schematic visual representation of the evolution of the inner flow into an outflow at the high X-ray emission mode. When the system is in the high mode, a small-size inner flow is present, together with a faint steady jet that is launched along the pulsar rotational axis and gives rise to the observed low-level radio and millimetre emission. As the pulsar rotates, the pulsar wind (marked with solid green lines) wobbles around the equatorial plane (see e.g. Bogovalov 1999) and shocks off the electrons in the inner flow at two opposite sides (red spots) at a distance that is slightly larger than the light cylinder radius (≃80 km). At each pulsar rotation, synchrotron emission at the shock at X-ray, UV, and optical frequencies is modulated at the spin period at one side (bright red spot), while it is absorbed by material in the inner flow at the other side (light red spot; Papitto et al. 2019).
Transitional millisecond pulsars are a rare and fascinating class of systems that switch between behaving like millisecond radio pulsars and low-mass X-ray binaries. In these systems, a neutron star alternates between emitting regular radio pulses and entering a faint X-ray-bright state where it accretes material from a companion star through a disc. During this active state, the system rapidly flips between two distinct emission modes — a bright high mode and a dim low mode — in a pattern that’s abrupt, unpredictable, and still not fully understood. In this study, we report the results of the most comprehensive multi-wavelength observing campaign ever conducted on the prototype transitional pulsar PSR J1023+0038, using 12 different telescopes over two nights in June 2021. By analyzing the system’s full broadband spectral energy distribution (SED)during both modes, we discovered that these mode switches are driven by changes in the innermost regions of the accretion disc. These changes lead to discrete ejections of material, launched alongside a steady compact jet — a conclusion confirmed by detecting a brief millimetre flare with ALMA precisely as the system switched from high to low mode.
Project |25
Bursts from Space: MeerKAT – the first citizen science project dedicated to commensal radio transients
Andersson A., Lintott C., et al. 2023, MNRAS 523, 2219
FIGURE 25. Variability plane for the 168 sources found by citizen scientists to be variable, along with those they find to not be so in grey. The colour bar denotes the fraction of classifications as a transient/variable source. Known variable sources are circled. Most known transients are found by citizen scientists, while many new sources are identified and show a wide spread of values in this parameter space
Modern radio telescopes like MeerKAT can now scan huge areas of the sky with incredible sensitivity and speed, generating enormous amounts of data and opening up new opportunities to discover rare and unpredictable cosmic events, known as radio transients. In this study, we describe the results from Bursts from Space: MeerKAT, the first-ever citizen science project dedicated to finding radio transients, which invited volunteers to help classify data from MeerKAT’s weekly surveys. Launched in late 2021, the project attracted over 1,000 volunteers who made nearly 89,000 classifications in just three months, uncovering 142 new variable radio sources. Combining these with known transients, we estimated that about 2.1% of radio sources vary at 1.28 GHz on weekly-to-yearly timescales, consistent with earlier studies. Among the discoveries were a pulsar, an OH maser star, flaring stars, and jets from X-ray binaries. Cross-matching with optical data from MeerLICHT and considering effects like interstellar scintillation, most of the new variables are likely active galactic nuclei (AGN) — galaxies powered by supermassive black holes. This work not only shows how citizen scientists can meaningfully contribute to real astronomical research but also provides one of the largest catalogs of candidate radio variables to date. The project’s success is now helping to train machine learning algorithms to automatically identify future transients, proving the power of combining human and AI efforts to explore the dynamic radio sky.
Project |26
The omnipresent flux-dependent optical dips of the black hole transient Swift J1357.2-0933
Panizo-Espinar et al. 2024, A&A, 682, A19
FIGURE 26. DRP evolution with X-ray fluxes for the four outbursts of J1357.
Swift J1357.2-0933 is a black hole X-ray binary known for its unusual behavior — showing repeated, fast optical dips during outbursts without any obvious equivalent pattern in X-rays. In this study, we present high-speed optical observations from its 2019 and 2021 outbursts, adding to earlier data from 2011 and 2017. We found that these optical dips appeared during every observed outburst, though in the two most recent and fainter events, the dips were shallower and occurred less frequently. By analyzing the properties of these dips across all four outbursts, we discovered that they don’t follow a consistent pattern over time, but instead correlate with the system’s overall brightness — being deeper and more frequent when the black hole is brighter in both X-rays and optical light. This trend may even continue into the system’s faint, quiescent state. These findings are exciting because they suggest a possible link between the optical dips and outflows or winds from the accretion disc, a connection hinted at in earlier studies. Understanding these dips could reveal new clues about how material moves around black holes and how their surrounding discs behave during outbursts.
Project |27
Evidence for low power radio jet−ISM interaction at 10 parsec in the dwarf AGN host NGC 4395
Nandi et al. 2024, ApJ, 959, 2, 116
FIGURE 27. Schematic diagram of our proposed scenario in the inner region of NGC 4395. The jet on its travels outwards from the central radio core, interacts with the medium and ionizes the gas via shock excitation. The radio core coincides with the optical Gaia position, the peak of the [OIII] emission and the peak of the 237 GHz emission. Ionised [OIII] has a cone-like structure, with the radio jet along the axis and causing the outflows.
Active galactic nuclei (AGN) — powered by supermassive black holes — can launch jets and outflows that interact with the gas in their host galaxies, sometimes affecting star formation. While this kind of feedback process is well-studied in massive galaxies on large scales, it’s still unclear how small a region these effects can influence, especially in low-mass or dwarf galaxies. In this study, we present evidence for a radio jet interacting with the interstellar medium (ISM) on a remarkably small scale — about 10 parsecs — in the nearby dwarf galaxy NGC 4395. High-resolution 15 GHz radio images revealed a triple radio structure with two asymmetric jet-like features on either side of a central core, matching the optical position of the AGN. These jets line up with regions of ionized gas traced by [O III] emission, suggesting the jets are interacting with and shocking the surrounding gas. Spectral modeling confirmed the presence of shock-ionized gas, and while faint, 237 GHz continuum emission was detected at the AGN’s position. Interestingly, the CO(2–1) molecular gas emission — a tracer of star-forming material — was found displaced by about 20 parsecs, likely pushed aside by the jet’s activity. The alignment of warm molecular hydrogen emission with the jet direction and disrupted gas morphology suggests that star formation is being suppressed in the central 10 parsec region. This discovery provides one of the clearest examples of AGN feedback operating on such small scales, offering new insight into how even weak radio jets can shape their environments in dwarf galaxies.
Project |28
Chasing the break: Tracing the full evolution of a black hole X-ray binary jet with multi-wavelength spectral modeling
Echiburu ́-Trujillo C. et al. 2024, ApJ, 962, 2, 116
FIGURE 28. Broad-band spectral evolution of J1820 over the course of its 2018/2019 outburst. The points represent the data and the solid lines represent the best-fit model. Colors indicate different epochs/accretion states and arrows mark the position of the spectral break for each individual epoch of the same color. The best-fit models and data points are scaled for better visualization, as identified in the legends (increasing with time). The optical, UV and X-ray data are corrected for reddening and absorption. Panel a displays the broad-band spectral models corresponding to the rising hard state. We clearly observe different broad-band spectral shapes of J1820 throughout the outburst.
Understanding how black holes launch powerful jets of energy into space is one of the most fascinating challenges in astrophysics, and our study of the black hole system MAXI J1820+070 offers a uniquely detailed look into this process. We analyzed one of the most comprehensive sets of multi-wavelength observations ever assembled for a black hole X-ray binary (BH XRB) outburst—spanning from radio to X-ray light and collected using 17 instruments across 19 different epochs during the 2018–2019 outburst. By modeling the broad-band spectra with a combination of jet, accretion flow, and companion star emission, we were able to track how a key feature in the jet spectrum—called the spectral break—evolved dramatically, shifting by over three orders of magnitude in frequency. This spectral break is crucial because it gives us a window into the region where the jet is launched. Our results show how the jet turns on, fades, and re-ignites in step with changes in the black hole’s accretion state, providing strong evidence that this behavior is not unique to MAXI J1820+070, but likely a common feature in BH XRBs. Importantly, we also find a tight link between the structure of the inner accretion flow and the jet base, helping to clarify how the two are physically connected.
Project |29
A multi-wavelength study of the hard and soft states of MAXI J1820+070 during its 2018 outburst
Banerjee et al. 2024, ApJ, 964, 189
FIGURE 29. Broad-band (Optical to hard X-ray) unabsorbed SED (upper panel) and residuals (lower panel), in the form of ratio (data/model), corresponding to Model 1C (hard state observation). The total model is represented by a solid black line in the upper panel. Data are rebinned for plotting purpose.
How do black holes light up the universe in different wavelengths during outbursts? To answer this, we used multi-band observations from AstroSat and LCO to study the black hole X-ray binary MAXI J1820+070 during its 2018 outburst, capturing both its hard and soft accretion states. In the soft state, we found unexpected excess emission in the soft X-ray, ultraviolet (UV), and optical bands—beyond what’s typically expected from the inner accretion disk and standard X-ray emission. These excesses include a hot soft X-ray component, distinct UV emission lines, and black-body components in both UV and optical light, likely caused by the inner regions of the disk and corona irradiating the cooler, outer parts of the disk. Using these data, we also estimated the black hole’s mass to be about 8 times that of the Sun and its spin to be relatively high (0.85). In the hard state, these UV and optical excesses become even more prominent, again modeled with thermal components and emission lines.
Our detailed broadband modeling shows that the X-ray emission in this state comes from two separate coronal regions—one producing broad reflection features close to the black hole, and another, harder one creating narrower features farther out. These findings reveal a dynamic, multi-layered structure in the accretion flow and corona, offering new insights into how energy is processed and re-emitted across the electromagnetic spectrum in black hole systems.
Project |30
Particle acceleration at the bow shock of runaway star LS 2355: non-thermal radio emission but no γ-ray counterpart
van den Eijnden et al. 2024, MNRAS, 532, 2920–2933
FIGURE 30. The EMU field showing the 24 × 24 arcmin2 field around LS 2355. The arrow indicates the proper motion of LS 2355, corrected for local
Galactic rotation, with the two dashed lines showing the uncertainty on the direction.
When massive stars move at supersonic speeds through space, their powerful stellar winds can create dramatic, curved structures called bow shocks, as they slam into the surrounding interstellar medium (ISM). These bow shocks are important because they’re one of the main ways individual stars inject energy and momentum into their environment, but they’ve mostly been studied in the infrared (IR), and detecting their high-energy or radio emissions has proven difficult. In this study, we present a detailed multiwavelength investigation of the bow shock driven by the runaway star LS 2355, focusing especially on its rare non-thermal emission—radiation not produced by simple heating. Using updated data from the Fermi gamma-ray source catalog, we rule out an earlier suggestion that LS 2355 was linked to an unidentified gamma-ray source. However, we do make an exciting discovery: deep radio observations from the ASKAP telescope reveal a non-thermal radio counterpart to LS 2355’s bow shock—only the third such detection ever confirmed. We also combine data from WISE (infrared) and Gaia (stellar motion) to analyze the surrounding ISM and revise LS 2355’s speed, finding it to be moving more slowly than previously thought. Our results suggest the bow shock's unusual emission properties can be explained by its interaction with a nearby dense H II region and a strong local magnetic field. Interestingly, we also find that the expected thermal radio emission is likely being suppressed, matching the observations and offering new clues about how stellar winds and magnetic fields shape the interstellar environment.
Project |31
LeMMINGs. Multi-wavelength constraints on the co-existence of nuclear star clusters and AGN in nucleated galaxies
Dullo et al. 2024, MNRAS, 532, 4729–4751
FIGURE 31 . 1D multicomponent decompositions of the major-axis surface brightness profiles for a dozen nucleated LeMMINGs galaxies, selected as representative examples from the 100 nucleated galaxies in our sample. We fit up to six model components which are summed up to a full model with up to 16 free parameters.
One of the biggest open questions in galaxy evolution is how nuclear star clusters (NSCs) and supermassive black holes (SMBHs) grow together at the centers of galaxies, and how this co-evolution is tied to the overall properties of their host galaxies. Recent discoveries have shown that about 10% of galaxies with NSCs also host hybrid nuclei—central regions containing both a star cluster and an actively feeding black hole, which powers an active galactic nucleus (AGN). Taking advantage of powerful new multiwavelength data from the LeMMINGs survey, we carry out the most detailed study so far of these hybrid nuclei across a diverse sample of 100 nearby galaxies, ranging from ellipticals and lenticulars to spirals and irregulars, and spanning over three orders of magnitude in stellar mass. We identify the central nuclei using advanced modeling of Hubble Space Telescope images and classify AGNs based on consistent measurements from radio (1.5 GHz e-MERLIN), X-ray (Chandra), and optical emission-line data. Our analysis shows that about 67% of galaxies in this sample host nuclear components. Even more striking, we identify 30 galaxies that show clear signs of being hybrid nuclei—bright in optical, radio, and X-rays—suggesting that at least 30% of NSC-hosting galaxies also harbor actively accreting black holes, a significantly higher fraction than previously recognized. These hybrid systems are especially common in galaxies with intermediate-to-high stellar masses and occur three times more frequently than past studies have indicated. This work offers compelling new evidence that NSCs and SMBHs often coexist and interact, shedding light on the complex central engines that shape galaxy evolution.
Project |32
Radio observations of the 2022 outburst of the transitional Z-Atoll source XTE J1701− 462
Gasealahwe et al. 2024, MNRAS, 533, 1800–1807
FIGURE 32. The radio (MeerKAT), X-ray (MAXI), hardness intensity (10–20)/(2–4) keV and radio (MeerKAT) spectral index light curves of the 2022/23 outburst of XTE J1701 in the first, second, third and bottom panel, respectively. The IXPE observations taken from Cocchi et al. (2023) are indicated with the grey dashed lines and are shown to coincide with our radio detections. The red dashed lines run through the peaks of the radio flares and the radio spectral indices are shown for all the radio detections except the final one from MJD 59939.31 since the source was not detected in several of the lower band frequencies.
The neutron star system XTE J1701−462 has played a key role in our understanding of accretion in low-mass X-ray binaries (LMXBs). Discovered in 2006, it was the first source to clearly demonstrate that the two main classes of accreting neutron stars—called "Atoll" and "Z" sources—are distinguished by how much material they accrete. During its 2006–2007 outburst, the source also showed signs of producing a relativistic jet, confirming that neutron stars, like black holes, can launch powerful outflows at high accretion rates. In 2022, the source entered a new outburst, and we report results from 29 radio observations with the MeerKAT telescope, along with simultaneous X-ray monitoring from MAXI. We first detected radio emission on September 16, 2022, and continued to do so until mid-December, followed by non-detections (upper limits) until March 2023. The radio light curve shows at least three flare-like events, with the brightest requiring a minimum energy of about 10³⁸ ergs—a significant amount of energy output. Comparing this event with the 2006/7 outburst, we find the recent one produced detectable radio emission for a longer period, though this may be due to better observational coverage. We also measured the radio polarization during a time when X-ray polarization was detected by IXPE, and found it to be below 9%, providing constraints on jet geometry and magnetic fields.
Project |33
Do Neutron Star Ultraluminous X-Ray Sources Masquerade as Intermediate-mass Black Holes in Radio and X-Ray?
Panurach et al. 2024, APJ, 977, 211
FIGURE 33. Our sample of extragalactic neutron star ULXs (shown by cross symbols) projected onto the fundamental plane, showing the result if one were to naively estimate“black hole” masses via the radio and X-ray luminosities/limits (here, we have used Equation (15) of A. Merloni et al. 2003). The gray symbols show XRBs and AGNs from A. Merloni et al. (2003), and the dashed line shows their best- t function. Since it is common for studies to employ joint radio and X-ray observations to search for AGNs in low-mass galaxies, this gure illustrates the parameter space where neutron star ULXs could potentially masquerade as IMBH/low-mass AGNs.
Ultraluminous X-ray sources (ULXs) are unusually bright objects found outside the centers of galaxies, and for a long time, astronomers thought they were powered by stellar-mass black holes accreting matter at extreme (super-Eddington) rates. In rare cases, they were even considered possible candidates for the elusive intermediate-mass black holes (IMBHs). However, the discovery of pulsations in eight ULXs has confirmed that some are actually powered by neutron stars, not black holes—raising the question of whether these neutron star ULXs could confuse searches for IMBHs, especially in radio/X-ray surveys. In this study, we present the first comprehensive radio survey of seven known neutron star ULXs, using both new and archival data from the VLA and ATCA, as well as previously published results. Out of the seven sources, only one—the Galactic ULX Swift J0243.6+6124—was confidently detected in the radio. Of the six extragalactic sources, just one showed nearby radio emission, but it appears to come from a background H II region rather than the ULX itself. These results suggest that neutron star ULXs do not typically emit strong radio waves, and are unlikely to be mistaken for IMBH candidates in radio/X-ray searches—as long as background sources like star-forming regions are accounted for. This provides reassurance that compact radio counterparts to ULXs are more likely to point to black holes, not neutron stars.
Project |34
Evolution of the Accretion Disk and Corona during the Outburst of the Neutron Star Transient MAXI J1807+132
Rout et al. 2024, ApJ, 978, 12
FIGURE 34. Time evolution of the uxes of the individual model components.
Neutron star low-mass X-ray binaries are known for their rich and complex behavior during outbursts, offering valuable insights into how matter behaves in extreme gravity. In this study, we present a detailed spectral and timing analysis of the neutron star transient MAXI J1807+132 during its 2023 outburst, using observations from the NICER X-ray observatory. The outburst began with a dramatic increase in X-ray brightness, rising 20-fold in just one day, and showed clear state transitions, including the well-known hysteresis effect where the source follows different tracks in its rise and decay. By applying a three-component spectral model, the analysis reveals that the accretion disk is truncated in the hard state but reaches all the way to the innermost stable circular orbit during the intermediate and soft states. Moreover, the study finds that the frequencies of characteristic timing features are tightly correlated with the disk’s inner edge, helping to trace the evolution of the corona throughout the outburst. Interestingly, after the main outburst, the source experienced a bright reflare, during which it displayed high variability (~10%) despite having a soft X-ray spectrum.
Project |35
Finding radio transients with anomaly detection and active learning based on volunteer classifications
Andersson et al. 2025, MNRAS, 538, 1397–1414
FIGURE 35. The (η, V ) feature space, colour coded by observational field. Stars indicate volunteer-verified anomalies (transients). There are clear field-dependent systematics affecting, for example, the MAXI J1820+070 field.
As next-generation telescopes like the Square Kilometre Array (SKA) begin to produce enormous amounts of data, astronomers face a major challenge: how to efficiently detect rare, short-lived signals called radio transients buried within the noise. In this study, we test whether unsupervised machine learning, specifically anomaly detection algorithms, can help automate this search. Using 1.3 GHz light curves from the SKA precursor telescope MeerKAT, we applied two anomaly detection techniques from the ASTRONOMALY package to three different sets of descriptive features and evaluated their performance against known transients labeled by citizen science volunteers. Our results are promising: even without prior labeling, the algorithms were able to recover over half of all known transients within just the top 10% of data ranked by anomaly score. While the specific anomaly detection model mattered less, the choice of features used to describe the data was key, especially for balancing detection rates with the practical need to minimize human follow-up. We also explored active learning, where human input is used to label just 2% of the data, and found it could improve recall by up to 20 percentage points. This is the first study to apply such methods to radio transient discovery, paving the way for real-time detection systems in the era of large-scale radio surveys.
Project |36
X-ray & optical polarization aligned with the radio jet ejecta in GX 339–4
Mastroserio et al. 2025, ApJL, 978, L19
FIGURE 36. ATCA 9 GHz image of GX 339–4 showing the two radio sources, with the central source being associated with the core position of GX 339–4 and the NE component being the discrete knot that was ejected from the system along the jet axis.
Black hole X-ray binaries like GX 339–4 are powerful laboratories for studying how matter behaves near black holes—and for the first time, this study presents X-ray polarization measurements of GX 339–4 during its 2023–2024 outburst, offering new insights into the structure of its innermost regions. Using NASA’s IXPE (Imaging X-ray Polarimetry Explorer), the team detected a significant polarization signal (about 1.3% in the 3–8 keV range) during the soft-intermediate state, indicating that X-rays are being emitted in a preferred direction—likely shaped by magnetic fields or disk geometry.
What makes this result especially exciting is that simultaneous optical polarizationmeasurements from the Very Large Telescope show a similar orientation, suggesting a consistent geometry across wavelengths. Additionally, radio observations from ATCA detected a moving jet knot, launched just after a major X-ray state transition. The direction of this jet aligns with both the X-ray and optical polarization angles, providing rare direct evidence that the polarized light traces the jet’s orientation on the sky. This multiwavelength campaign not only links jet ejection to changes in the inner accretion flow, but also demonstrates how polarization can reveal the hidden geometry of black hole systems.
Project |37
Determining the Nature of IC 10 X-2: A Comprehensive Study of the Optical/IR Emission from an Extragalactic BeHMXB
Alnaqbi, Gelfand, Saikia et al. 2025, ApJ, 978, 170
FIGURE 37. An illustration of the geometry of the IC 10 X-2 system
High-mass X-ray binaries are crucial for understanding how massive stars evolve in extreme environments, and this study offers the most detailed optical and infrared analysis yet of one particularly intriguing system: IC 10 X-2, located in a small, star-forming galaxy similar to those in the early Universe. Using five years of regular observations from the Zwicky Transient Facility and Las Cumbres Observatory, we discovered periodic outbursts and color changes every ~26.5 days, most likely tied to the orbital motion of the system. These repeating flares point to the presence of a Be star companion, with the flares triggered as a neutron star passes through the dense equatorial diskof the star and accretes material. In addition to this regular behavior, the system shows short-lived episodes of extreme color changes, likely caused by clumpy structures in the stellar wind or disk that either obscure the star or enhance accretion.
Project |38
Detecting the Black Hole Candidate Population in M51’s Young Massive Star Clusters: Constraints on Accreting Intermediate-mass Black Holes
Dage et al. 2025, ApJ, 979, 82
FIGURE 38. Color–magnitude diagram of all LEGUS cluster candidates, along with the verified star clusters and contaminants. The yellow pentagons represent the X-ray sources matched to high-probability (>50%) matches to cluster candidates, lower-probability (<50%) matches to cluster candidates, or contaminants.
Intermediate-mass black holes (IMBHs), with masses between 100 and 100,000 times that of the Sun, remain one of the biggest mysteries in black hole astrophysics, particularly when it comes to how they form and grow. We investigated whether these elusive black holes might reside in young massive star clusters within the nearby galaxy M51 by combining archival X-ray data from Chandra, optical catalogs from Hubble, and new radio observations from the VLA.
Out of 44 bright X-ray sources in M51, we found 17 that could be associated with star clusters, after filtering out contaminants like background galaxies. However, we detected no accompanying radio signals that would suggest the presence of accreting, "hard-state" IMBHs with masses of ~10⁴ M☉ or more. This absence implies that such IMBHs, if present, are either inactive or below our detection threshold—but future, more sensitive radio telescopes like the SKA or ngVLA could uncover lower-mass or more weakly accreting candidates. This work is exciting because it tightens the constraints on where IMBHs may be hiding, bringing us closer to understanding their role in cosmic evolution.
Project |39
Long-term optical variations in Swift J1858. 6–0814: evidence for ablation and comparisons to radio properties
Rhodes, Russell, Saikia et al. 2025, MNRAS, 536, 3421–3430
FIGURE 39. At phase 0.7, the projected surface area of the star and ablated material is larger and therefore the reprocessed radiation along the observer’s line of sight is larger resulting in a higher flux density. Whereas at phase 0.0, the companion star completely eclipses the NS and so all reprocessed radiation from the star and ablated material travels away from the observer.
We present optical monitoring of the neutron star low-mass X-ray binary Swift J1858.6–0814 during its 2018–2020 outburst and subsequent quiescence. Throughout the entire outburst phase covered by our observations, we find strong optical variability, although the average flux remains relatively steady. The optical spectral energy distribution is generally blue, consistent with emission from an accretion disc, punctuated by occasional red flares, which we interpret as optically thin synchrotron emission. The fractional rms variability amplitude is comparable in both the radio and optical bands, suggesting that long-term variability is driven by changes in the accretion flow. These accretion-driven changes manifest optically and subsequently propagate into the jet, producing the observed radio variability.
We also detect orbital modulation in the optical light curves, with the flux peaking at phase ∼0.7. The modulation amplitude is consistent across all optical bands, which points to reprocessing by the accretion disc, companion star, and ablated material as the cause of this phase dependence. Evidence for ablation in this system adds to the growing understanding of how neutron star X-ray binaries may evolve into isolated millisecond pulsars, with mass loss and irradiation-driven outflows playing a central role.
Project |40
Peering into the heart of darkness with VLBA : Radio Quiet AGN in the JWST North Ecliptic Pole Time-Domain Field
Saikia et al. 2025, submitted to ApJ
FIGURE 40. WISE colour–colour diagram showing W1−W2 versus W2−W3 colors for the VLBA detections with WISE measurements (red diamonds), VLBA detections with WISE upper limits (green circles), and non-detectons (blue open squares).
This study provides a compelling new window into how supermassive black holes shape their host galaxies, using some of the sharpest radio observations ever conducted in a deep extragalactic field. We present initial results from a high-resolution Very Long Baseline Array (VLBA) survey of the JWST North Ecliptic Pole Time-Domain Field, where we detect compact radio emission in 12 out of 106 sources—an ~11% detection rate—at sub-milliarcsecond scales and microjansky sensitivities. These detections reveal pc-scale, non-thermal radio emission with brightness temperatures exceeding 10⁵ K, clear signatures of active galactic nuclei (AGN) powered by black holes rather than by star formation alone. Interestingly, we find that in most of these galaxies, star formation contributes less than half of the compact radio signal, and in some cases, the radio emission is almost entirely AGN-driven. This challenges the common assumption that "radio-quiet" AGN are dominated by star formation and demonstrates that black holes can still leave a detectable imprint. By combining VLBA data with JWST/NIRCam imaging and multiwavelength catalogs, we identify host galaxy types and estimate redshifts and star formation rates, highlighting how JWST’s unparalleled infrared sensitivity allows us to more effectively disentangle AGN activity from stellar processes. These results not only showcase the power of VLBA and JWST in tandem but also lay crucial groundwork for future studies with the Square Kilometre Array and next-generation surveys aiming to trace black hole growth across cosmic time.
Project |41
A Multi-wavelength Characterization of the 2023 Outburst of MAXI J1807+ 132: Manifestations of Disk Instability and Jet Emission
Rout et al. 2025, accepted at ApJ
FIGURE 41. Optical polarization degree (P) versus polarization angle (θ) for the three epochs. A clear rotation of θ by approximately∼ 100 degree is observed in the R and I bands between the first two epochs and the third. Upper limits on P are reported at the 99.97% credible interval, while all other uncertainties are quoted at the ± 1σ credible level.
This multiwavelength study of the neutron star transient MAXI J1807+132 during its 2023 outburst sheds new light on the complex behavior of low-luminosity systems like atolls, which remain less understood than their brighter counterparts. By tracking the system from before the outburst through its return to quiescence, we detect a clear delay between the optical and X-ray rises—strongly supporting the disk instability model involving a truncated accretion disk. As the outburst decays, coordinated optical, infrared, and radio observations point to synchrotron emission from a compact jet, reinforcing the presence of jet activity even in relatively faint neutron star systems. A particularly novel and exciting result is the first-ever detection in an X-ray binary of an abrupt, near-90-degree rotation in optical polarization immediately before a minor flare, which may signal internal changes in the jet's magnetic field structure just before quenching. The post-outburst phase reveals rapid, high-amplitude reflares across multiple wavelengths, posing an intriguing puzzle for future theoretical models. These findings highlight how coordinated, broad-band observations can uncover subtle but critical features of jet formation and disk evolution in neutron star binaries.
Project |42
On the distance to the black hole X-ray binary Swift J1727.8-1613
Burridge et al. 2025, submitted
FIGURE 42. A consolidated view of the H i spectra for both the target, J1727 (red), and our reference source, J1733 (blue). We observed no significant (>3σ) absorption outside the velocities chosen as the x-axis range. The y-axis represents the H i absorption percentage. The dashed lines indicate the x- and y-axes origins. The vertical dotted lines indicate the maximum velocity at which significant absorption is observed.
Understanding how far away a black hole is may seem simple, but it’s actually one of the biggest challenges in astrophysics—and it matters a lot for figuring out how these extreme systems evolve. In this study, we take a fresh look at the distance to the black hole X-ray binary Swift J1727.8−1613, a recently discovered system that’s become a key target for multiwavelength campaigns. Using new hydrogen (H I) absorption data from the MeerKAT radio telescope, we find that the traditional method of estimating distances from H I absorption has limitations in this direction of the sky. Still, it gives us a useful lower bound of about 3.6 kpc.
To go further, we combine ultraviolet reddening measurements and known properties of the companion star to estimate a more realistic distance of around 5.5 kpc. This revised distance suggests that the black hole was likely born with a powerful “natal kick”—a velocity boost of about 190 km/s—possibly from an asymmetric supernova. This result is exciting because it sheds light on how black holes are formed and launched into motion in our Galaxy, and it demonstrates how combining different techniques gives us a more complete and accurate picture of these complex systems.
Project |43
Comprehensive Radio Monitoring of the Black Hole X-ray Binary Swift J1727.8−1613 during its 2023–2024 Outburst
Hughes et al. 2025, accepted at APJ
FIGURE 43. X-ray hardness intensity diagram of Swift J1727 when it was detectable by MAXI/GSC. The marker colours correspond to the scaled 10 GHz radio flux densities.
Understanding how black holes launch and evolve relativistic jets remains one of the most compelling challenges in high-energy astrophysics, particularly during transient outbursts of low-mass X-ray binaries. In this study, we present an unprecedented, community-driven effort to compile nearly 10 months of multi-frequency radio observations of the black hole low-mass X-ray binary Swift J1727.8−1613, which underwent its first recorded outburst in August 2023. Bringing together data from 197 observing epochs across 7 radio facilities—spanning a wide frequency range of ~0.3 to 230 GHz—we provide the most comprehensive radio light curves of this source to date.
These light curves are made publicly available to support interpretation across all wavelengths, especially in coordination with X-ray, optical, and infrared campaigns. Phenomenologically, we observe multiple bright radio flares linked to discrete jet ejections, with peak fluxes reaching ∼1 Jy, and find strikingly steep spectral indices (α < −1, and occasionally α < −1.5), suggesting strong radiative losses. These results underscore the dynamic nature of jet activity during the outburst and highlight the importance of dense temporal sampling across wide frequency coverage. This work not only delivers valuable observational constraints on jet physics in black hole binaries but also sets a precedent for collaborative, open-access radio campaigns during future transient events.
Project |44
JWSTs PEARLS: NIRCam imaging and NIRISS spectroscopy of a 𝑧=3.6 star-forming galaxy lensed into a near-Einstein Ring by a 𝑧=1.258 massive elliptical galaxy
Adams et al. 2025, submitted
FIGURE 44. The results of the lens modelling process with lenstronomy.
A spectacular example of nature’s cosmic magnifying glass has been uncovered in JWST and HST data: we have discovered a new galaxy-galaxy gravitational lensing system, where a massive foreground galaxy bends and magnifies the light from a much more distant galaxy behind it. This rare system, named NEPJ172238.9+655143.1, features a background star-forming galaxy at redshift z ≈ 3.6 that is stretched into a near-perfect Einstein ring around a foreground elliptical galaxy at z ≈ 1.26. Thanks to the incredible resolution and sensitivity of JWST, we are able to study this system in great detail—including measuring the masses of the galaxies, estimating how much light is being magnified, and even investigating how much of the foreground galaxy’s mass comes from stars versus dark matter. What's especially exciting is that this lensing system shows how powerful JWST is for finding and studying distant galaxies that would otherwise be too faint to observe. It also gives us a chance to better understand the balance of visible and invisible matter in massive galaxies in the early universe—an essential piece of the cosmic puzzle.
Project |45
Jets from a stellar-mass black hole are as relativistic as those from supermassive black holes
Zhang et al. 2025, submitted
FIGURE 45. Four sample images of the source and the discrete blobs as seen with MeerKAT. The source position (yellow) and the discrete blobs (red for E1 and blue for E2) are marked with crosses. The image of the source and the discrete blob obtained in the observation on MJD 59,470 is shown in a). The position of the discrete blob can not be measured accurately due to confusion of the two components, causing obvious deviation of the estimated position from the trend for E1. b) to d) show clearly the two components, i.e., the core emission from the source at the center and the discrete blobs at the bottom left of the images.
Black hole jets are among the most extreme and energetic phenomena in the universe, capable of transporting energy and matter across vast distances. In AGN, jets from supermassive black holes are known to play a key role in regulating galaxy evolution by injecting energy into their environments and suppressing star formation. Interestingly, smaller-scale versions of these jets are also launched by stellar-mass black holes in our own galaxy—so-called X-ray binaries. These systems offer a unique opportunity to study jet physics on human-accessible time scales. However, for decades it has been assumed that the jets from stellar-mass black holes are less relativistic—that is, slower—than those from their supermassive counterparts, limiting their perceived value as analogues.
In this study, we challenge that assumption with the discovery of two distinct, relativistic jet ejections from the Galactic black hole X-ray binary 4U 1543−47 during a single outburst. Using high-resolution radio interferometry, we measure the jets’ apparent speeds and find they correspond to a Lorentz factor of about 8, with a 95% confidence minimum of 4.6. This means the jets were launched at over 99.5% the speed of light—on par with the fastest AGN jets. This discovery is exciting not only because it overturns long-standing beliefs about the limitations of stellar-mass black hole jets, but also because it confirms that the physical processes driving jet acceleration are scale-invariant across black hole masses.
Project |46
A relativistic jet from a neutron star breaking out of its natal supernova remnant
Gasealahwe et al. 2025, submitted
FIGURE 46. Cir X-1 and the newly revealed jet-punched bubbles (labelled NW and SE Bubble). We identify and label the core, jets, shock fronts, rings, pockets. We further separate the nebula into the bulb region and name the extension the earlobe.
Neutron star jets are thought to play an important role in feedback processes, yet direct observations of their interaction with the surrounding medium—especially in the earliest stages—are rare. In this study, we present deep radio imaging of the young neutron star X-ray binary Cir X-1, still embedded in its natal supernova remnant and undergoing regular outbursts every 16.5 days, likely due to periastron passages in an eccentric orbit. These observations reveal large-scale, jet-inflated radio bubbles aligned with the smaller-scale jets near the binary core, providing the first clear evidence of neutron star jets breaking out of their birth environment. We estimate a minimum energy of ∼10⁴⁵ erg for the bubbles. Spectral index mapping indicates predominantly optically thin synchrotron emission across the nebula, with pockets of flatter-spectrum emission. To understand the system’s morphology, we performed relativistic hydrodynamic simulations with the PLUTO code, reproducing the observed structures by modeling a supernova explosion followed within ∼100 years by a powerful, short-lived jet phase (<1000 years). Our results support the idea that Cir X-1 is a young analogue of SS 433 and offer new insight into how neutron star jets evolve and interact with their environments.