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 hole accretion and jet formation have long been thought to be scale invariant. One empirical relation suggesting scale invariance is the Fundamental Plane of black hole activity, which is a plane in the space given by black hole mass and the radio/X-ray luminosities. We search for an alternative version of this plane using the luminosity of [O III ] emission line instead of X-ray luminosity. We use a complete sample of 39 supermassive black holes selected from the Palomar Spectroscopic Survey with available radio and optical measurements and information on black hole mass. A sample of stellar mass X-ray binaries has also been included to examine if physical processes behind accretion is universal across the entire range of black hole mass. We present the results of multivariate regression analysis performed on the AGN sample and show that the sample stretches out as a plane in the 3D logarithmic space created by bolometric luminosity, radio luminosity and black hole mass. We reproduce the established Fundamental Plane of black hole activity in X-rays. We show that this plane can be obtained with the supermassive black hole sample alone and the X-ray binaries agrees to the found relation. We also discuss radio-loudness of various classes of low-luminosity AGN in view of our 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.
Blazar radiation is dominated by a relativistic jet which can be modelled at first approximation using just two intrinsic parameters – the Lorentz factor and the viewing angle. Blazar jet observations are often beamed due to relativistic effects, complicating the understanding of these intrinsic properties. The most common way to estimate blazar Lorentz factors needs the estimation of apparent jet speeds and Doppler beaming factors. We present a new and independent method of constructing the blazar Lorentz factor distribution, using the optical Fundamental Plane of black hole activity. The optical Fundamental Plane is a plane stretched out by both the supermassive black holes and the X-ray binaries, in the 3D space provided by their [OIII ] line luminosity, radio luminosity and black hole mass. We use the intrinsic radio luminosity obtained from the optical Fundamental Plane to constrain the boosting parameters of the VLBA Imaging and Polarimetry Survey blazar sample. We find a blazar bulk Lorentz factor distribution in the form of a power law with an index -2.1, for the Lorentz factor range of 1–40. We also discuss the viewing angle distribution of the blazars and the dependence of our results on the input parameters.
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.
For the first time, we use the 1.4 GHz FIRST radio luminosities on the optical fundamental plane, to investigate whether or not 1.4 GHz FIRST uxes can trace nuclear core activity. We use a SDSS- FIRST cross-correlated sample of 10149 active galaxies and analyse their positioning on the optical fundamental plane. We focus on various reasons that can cause the discrepancy between the observed FIRST radio uxes and the theoretically expected core radio uxes, and show what 1.4 GHz FIRST radio uxes are heavily contaminated by uxes from non-nuclear, extended components and other environmental factors. We show that the subsample of `compact sources', which should have negligible lobe contribution, statistically follow the fundamental plane when corrected for relativistic beaming, while all the other sources lie above the fundamental plane. The sample of LINERs, which should have negligible lobe and beaming contribution, also follows the fundamental plane. We conclude that 1.4 GHz FIRST uxes do not trace the core jet power and instantaneous nuclear activity in the AGN, and hence should not be used to represent core radio jet and study the fundamental plane of black hole activity.
Project |04
AGN Spectra : Kinematical relationship between emission regions
Kovacevic J., Popovic L.C., Saikia P. and Dimitrijevic M.S., 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).
The iron lines, which arise from a complex Fe II ion, are very interesting features in AGN spectra. There are many open questions connected with these lines: mechanisms of their excitation, location of their emission region in AGN structure, as well as correlations with other spectral properties, which physical cause is unknown. We present a study of optical Fe II emission in a sample of AGNs selected from the SDSS.
As a part of this project and for my Master's thesis, I have done spectral line fitting on 100 AGN spectra from Solar Digital Sky Survey (SDSS) DR-7, with a multi-gaussian procedure, considering each emission line to be composite of contributions from difference emission regions having different physical properties. We have examined the relationships between different groups of iron multiplets and some optical and UV spectral properties. We have also investigated the dependence between Baldwin effect and well-known anticorrelation between Fe II and [OIII] lines, which dominates in Boroson and Green's Eigenvector 1.
Project |05
LeMMINGs. I. The eMERLIN legacy survey of nearby galaxies. 1.5-GHz parsec-scale radio structures and cores
Baldi R. D., Williams D. R. A., McHardy I. M. ... Saikia P. ... 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).
We present the first data release of high-resolution (≤0.2 arcsec) 1.5-GHz radio images of 103 nearby galaxies from the Palomar sample, observed with the eMERLIN array, as part of the LeMMINGs survey. This sample includes galaxies which are active (LINER and Seyfert) and quiescent (HII galaxies and Absorption line galaxies, ALG), which are reclassified based upon revised emission-line diagrams. Half of the sample shows jetted morphologies. The remaining half shows single radio cores or complex morphologies. LINERs show radio structures more core-brightened than Seyferts.
We find that the LINER nuclei are the scaled-down version of FR I radio galaxies; Seyferts show less collimated jets; HII galaxies may host weak active BHs and/or nuclear star-forming cores; and recurrent BH activity may account for ALG properties. We also find that radio core luminosities correlate with black hole (BH) mass down to » 10^7 M⊙ , but a break emerges at lower masses. Using [O III] line luminosity as a proxy for the accretion luminosity, active nuclei and jetted HII galaxies follow an optical fundamental plane of BH activity, suggesting a common disc–jet relationship.
Project |06
15-GHz radio emission from nearby low-luminosity AGN
Saikia P., Körding E., Coppejans D., Falcke H., Williams D. et al. 2018, A&A, 616, A152
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.
We present a sub-arcsec resolution radio imaging survey of a sample of 76 low-luminosity active galactic nuclei (LLAGN) that were previously not detected with the Very Large Array at 15 GHz. Compact, parsec-scale radio emission has been detected above a flux density of 40 Jy in 60 % (45 of 76) of the LLAGN sample. We detect 20 of 31 (64 %) low-ionization nuclear emission-line region (LINER) nuclei, 10 of 14 (71 %) low-luminosity Seyfert galaxies, and 15 of 31 (48 %) transition objects. We use this sample to explore correlations between different emission lines and the radio luminosity. We also populate the X-ray and the optical fundamental plane of black hole activity and further refine its parameters. Finally, we find conclusive evidence that the nuclear 15 GHz radio luminosity function (RLF) of all the detected Palomar Sample LLAGN has a turnover at the low-luminosity end and is best-fitted with a broken power-law. The break in the power-law occurs at a critical mass accretion rate of 1.2 10^-3 M/yr. The local group stands closer to the extrapolation of the higher luminosity sources, and the classical Seyferts agree with the nuclear RLF of the LLAGN in the local universe.
Project |07
A wildly flickering jet in the black hole X-ray binary MAXI J1535-571
Baglio M. C., Russell D. M. ... Saikia P. ... 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.
We report on the results of optical, near-infrared (NIR) and mid-infrared observations of the black hole X-ray binary candidate MAXI J1535-571 during its 2017/2018 outburst. During the first part of the outburst (MJD 58004-58012), the source shows an optical-NIR spectrum that is consistent with an optically thin synchrotron power-law from a jet. After MJD 58015, however, the source faded considerably, the drop in flux is much more evident at lower frequencies. Before the fading, we measure a de-reddened flux density of & 100 mJy in the mid-infrared, making MAXI J1535-571 one of the brightest mid-infrared BHBs known so far. A significant softening of the X-ray spectrum is evident contemporaneous with the infrared fade. We interpret it as due to the suppression of the jet emission, similar to the accretion-ejection coupling seen in other BHBs. We also present the first mid-IR variability study of a BHB on minute timescales, with a fractional rms variability of the light curves of ~15-22 %, which is much higher than the optical fractional rms ( < 0.7 %).
Project |08
Lorentz Factors of compact jets in Black hole X-ray binaries
Saikia P., Russell D. M., Bramich D. M. et al. 2019, The Astrophysical journal, 887, 1, 21
FIGURE 8. Schematic diagram to qualitatively explain the expected IR excess observed for different BHXB sources, depending on their inclinations.
Compact, continuously launched jets in black hole X-ray binaries (BHXBs) produce radio to optical/infrared synchrotron emission. In most BHXBs, an infrared (IR) excess (above the disc component) is observed when the jet is present in the hard spectral state. We investigate why some BHXBs have prominent IR excesses and some do not. We find that the amplitude of the IR excess can be explained by inclination dependent beaming of the jet synchrotron emission, and the projected area of the accretion disc. Furthermore, we see no correlation between the expected and the observed )IR excess for Lorentz factor 1, which is strongly supportive of relativistic beaming of the IR emission, confirming that the IR excess is produced by synchrotron emission in a relativistic outflow. Using the amplitude of the jet fade and recovery over state transitions and the known orbital parameters, we constrain for the first time the bulk Lorentz factor range of compact jets in several BHXBs (with all the well-constrained Lorentz factors lying in the range of = 1.3 - 3.5). Under the assumption that the Lorentz factor distribution of BHXB jets is a power-law, we also constrain the power-law index. We find that the very high amplitude IR fade/recovery seen repeatedly in the BHXB GX 339-4 favors a low inclination angle (< 15) of the jet.
Project |09
Appearance of a compact jet in soft–intermediate state of 4U 1543−47
Russell D. M., Casella P., .... Saikia P., ..... 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 M. C., Russell D. M., .... Saikia P., ..... 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.
We report on an optical photometric and polarimetric campaign on the accreting millisecond X-ray pulsar SAX J1808.4–3658 during its 2019 outburst. The emergence of a low-frequency excess in the spectral energy distribution in the form of a red excess above the disk spectrum (seen most prominently in the z, i, and R bands) is observed as the outburst evolves. This is indicative of optically thin synchrotron emission due to a jet. Our optical (BVRI) polarimetric campaign shows variable linear polarization (LP) throughout the outburst. We show that this is intrinsic to the source, with low-level but significant detections (0.2%–2%) in all bands. The LP spectrum is red during both the main outburst and the reflaring state, favoring a jet origin for this variable polarization over other interpretations, such as Thomson scattering with free electrons from the disk or the propelled matter. During the reflaring state, a few episodes with stronger LP levels (1%–2%) are observed. The low-level, variable LP is suggestive of strongly tangled magnetic fields near the base of the jet. These results clearly demonstrate how polarimetry is a powerful tool for probing the magnetic field structure in X-ray binary jets, as for active galactic nuclei jets.
Project |11
LeMMINGs. II. The e-MERLIN legacy survey of nearby galaxies. The deepest radio view of the Palomar sample on parsec scale
Baldi R. D., Williams D. R. A., McHardy I. M. ... Saikia P. ... 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.
We present the second data release of high-resolution (≤0.2 arcsec) 1.5-GHz radio images of 177 nearby galaxies from the Palomar sample, observed with the e-MERLIN array, as part of the LeMMINGs survey. Together with the 103 targets of the first LeMMINGs data release, this represents a complete sample of 280 local active (LINER and Seyfert) and inactive galaxies HII galaxies and Absorption Line Galaxies, ALG). This large program is the deepest radio survey of the local Universe, regardless of the host and nuclear type: we detect radio emission for 125/280 galaxies (44.6 per cent), of whiich, 106 targets show a core which coincides within 1.2 arcsec with the optical nucleus. Although we observed mostly cores, around one third of the detected galaxies features jetted morphologies. LINERs and Seyferts are the most luminous sources, whereas HII galaxies are the least. LINERs show FRI-like core-brightened radio structures, while Seyferts reveal the highest fraction of symmetric morphologies. The majority of HII galaxies have single radio core or complex extended structures, which probably conceal a nuclear starburst and/or a weak active nucleus (seven of them show clear jets). ALGs, which are typically found in evolved ellipticals, although the least numerous, exhibit on average the most luminous radio structures, similar to LINERs.
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 R. D., Williams D. R. A., Beswick R.J., ... Saikia P. ... 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 D. R. A., Pahari M., Baldi R. D., ... Saikia P. ... 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.
All 280 of the statistically-complete Palomar sample of nearby (<120 Mpc) galaxies with declination more than 20 degrees have been observed at 1.5 GHz as part of the LeMMINGs e-MERLIN legacy survey. Here, we present Chandra X-ray observations of the nuclei of 213 of these galaxies, including a statistically-complete sub-set of 113 galaxies in the declination range 40 degrees to 65 degrees. We observed galaxies of all optical spectral types, including 'active' galaxies like LINERs and Seyferts, and 'inactive' galaxies like HII galaxies and absorption line galaxies. We detect X-ray emission coincident within 2-arcsec of the nucleus in 150/213 galaxies, including 13/14 Seyferts, 68/77 LINERs, 13/22 ALGs and 56/100 HII galaxies, but cannot completely rule out contamination from non-AGN processes in fainter sources. We construct an X-ray Luminosity function (XLF) and find that the local galaxy XLF, when including all AGN types, can be represented as a single power-law of slope −0.54±0.06. Using [O III] line measurements and BH masses from the literature, we show that LINERs, HII galaxies and ALGs follow similar correlations to low luminosities, suggesting that some 'inactive' galaxies may harbour AGN.
Project |14
MeerKAT discovery of radio emission from the Vela X-1 bow shock
van den Eijnden J., Heywood I., ... Saikia P. ... 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),
Vela X-1 is a runaway X-ray binary system hosting a massive donor star, whose strong stellar wind creates a bow shock as it interacts with the interstellar medium. This bow shock has previously been detected in Hα and IR, but, similar to all but one bow shock from a massive runaway star (BD+43o3654), has escaped detection in other wavebands. We report on the discovery of 1.3 GHz radio emission from the Vela X-1 bow shock with the MeerKAT telescope. The MeerKAT observations reveal how the radio emission closely traces the Hα line emission, both in the bow shock and in the larger-scale diffuse structures known from existing Hα surveys. The Vela X-1 bow shock is the first stellar-wind-driven radio bow shock detected around an X-ray binary. In the absence of a radio spectral index measurement, we explore other avenues to constrain the radio emission mechanism. We find that thermal/free-free emission can account for the radio and Hα properties, for a combination of electron temperature and density consistent with earlier estimates of ISM density and the shock enhancement. In this explanation, the presence of a local ISM over-density is essential for the detection of radio emission. Alternatively, we consider a non-thermal/synchrotron scenario, evaluating the magnetic field and broad-band spectrum of the shock. However, we find that exceptionally high fractions (≳13%) of the kinetic wind power would need to be injected into the relativistic electron population to explain the radio emission. Finally, we speculate about the detectability of radio bow shocks and whether it requires exceptional ISM or stellar wind properties.
Project |15
A multi-wavelength study of GRS 1716−249 : constraints on its distance
Saikia P., Russell D. M., 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.
We present a detailed study of the Galactic black hole transient GRS 1716−249 during its 2016–2017 outburst at optical (Las Cumbres Observatory), mid-infrared (Very Large Telescope), near-infrared (Rapid Eye Mount telescope), and ultraviolet (Swift’s Ultraviolet/Optical Tele- scope) wavelengths, along with archival radio (Very Large Array, Australian Telescope Compact Array, Long Baseline Array) and X-ray (Swift, NuSTAR) data, and constrain its physical parameters. We discuss how previous estimates of the system parameters of the source are based on various incorrect assumptions, and so are likely to be inaccurate.By comparing our GRS 1716−249 data-set to those of other outbursting black hole X-ray binaries, we find that while GRS 1716−249 shows similar X-ray behaviour, it is noticeably optically fainter, if the literature distance of 2.4 kpc is adopted. Using several lines of reasoning, we argue that the source distance is further than previously assumed in the literature, likely within 4–17 kpc, with a most likely range of ∼ 4–8 kpc.
Project |16
A misfired outburst in the neutron star X-ray binary Centaurus X-4
Baglio M. C., Saikia P., Russell D. M., 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. Only the points from before the beginning of the 2020 Sun constraint are shown. Superimposed with dashed lines, the linear fits of the long-term trends are shown, where possible.
We report on a long-term optical monitoring of the neutron star X-ray binary Centaurus X-4 performed during the last 13.5 years. This source has been in quiescence since its outburst in 1979. Our monitoring reveals the overall evolution of the accretion disc; we detect short-duration flares, likely originating also in the disc, superimposed with a small-amplitude (<0.1 mag) ellipsoidal modulation from the companion star due to geometrical effects. A long-term (~2300 days) downward trend, followed by a shorter (~1000 days) upward one, is observed in the disc light curve. Such a rise in the optical has been observed for other X-ray binaries preceding outbursts, as predicted by the disc instability model. For Centaurus X-4, the rise of the optical flux proceeded for ~3 years, and culminated in a flux increase at all wavelengths (optical-UV-X-rays) at the end of 2020. This increase faded after ~2 weeks, without giving rise to a full outburst. We suggest that the propagation of an inside-out heating front was ignited due to a partial ionization of hydrogen in the inner disc. The propagation might have stalled soon after the ignition due to the increasing surface density in the disc that the front encountered while propagating outwards. The stall was likely eased by the low level irradiation of the outer regions of the large accretion disc, as shown by the slope of the optical/X-ray correlation, suggesting that irradiation does not play a strong role in the optical, compared to other sources of emission.
Project |17
Radio detections of IR-selected runaway stellar bow shocks
van den Eijnden J., Saikia P., Mohamed S. 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.
Massive stars moving at supersonic peculiar velocities through the interstellar medium (ISM) can create bow shocks, arc-like structures at the interface between the stellar wind and the ISM. Many such bow shocks have been detected and catalogued at IR wavelengths, but detections in other wavebands remain rare. Strikingly, while electrons are expected to be accelerated in the bow shock and their non-thermal emission may include synchrotron emission at low frequencies, only two massive runaway stellar bow shocks have to date been detected in the radio band. Here, we examine a sample of fifty IR-detected bow shocks from the E-BOSS catalogues in recently released radio images from the Rapid ASKAP Continuum Survey (RACS). We identify three confident and three likely counterparts, as well as three inconclusive candidates requiring confirmation via follow-up observations. These detections significantly increase the number of known radio massive stellar bow shocks and highlight the advantage of dedicated searches with current and next-generation radio telescopes. We investigate the underlying radio emission mechanism for these radio sources, finding a mix of free-free-dominated and synchrotron-dominated systems. We also discuss the non-detected targets by putting constraints on their emission properties and investigating their detectability with future observations. Finally, we propose several future avenues of research to advance the study and understanding of bow shocks at radio frequencies.
Project |18
Radio observations of the Black Hole X-ray Binary EXO 1846−031 re-awakening from a 34-year slumber
Williams D.R.A., Motta S.E.,.. Saikia P., et al. 2022, MNRAS, Accepted
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.
We present radio [MeerKAT, VLA and AMI-LA] and X-ray [Swift and MAXI] data from the 2019 outburst of the candidate Black Hole X-ray binary EXO 1846−031. We compute a Hardness–Intensity diagram, which shows the characteristic q-shaped hysteresis of BHXBs in outburst. The VLA observations provide sub-arcsecond-resolution images at key points in the outburst, showing moving radio components. The radio and X-ray light curves broadly follow each other, showing a peak on ∼MJD 58702, followed by a short decline before a second peak between ∼MJD 58731–58739. We estimate the minimum energy of these radio flares from equipartition. From the Swift X-ray flux on MJD 58905 and assuming the soft-to-hard transition happened at 0.3–3 per cent Eddington, we calculate a distance range of 2.4–7.5 kpc. We computed the radio/X-ray plane for EXO 1846−031 in the ‘hard’ state, showing that it is most likely a ‘radio-quiet’ BH, preferentially at 4.5 kpc. Using this distance and a jet inclination angle of 73 degrees, the VLA data place limits on the intrinsic jet speed of 0.29𝑐, indicating 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 J., Fender R., .... Saikia P., .... 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.
Centaurus X–4 (Cen X–4) is a relatively nearby neutron star low-mass X-ray binary that showed outbursts in 1969 and 1979, but has not shown a full outburst since. Due to its proximity and sustained period of quiescence, it is a prime target to study the coupling between accretion and jet ejection in quiescent neutron star low-mass X-ray binaries. Here, we present four MeerKAT radio observations at 1.3 GHz of Cen X–4, combined with NICER and Swift X-ray monitoring. During the first and most sensitive observation, Cen X–4 was in a fully quiescent X-ray state. The three later and shorter observations targeted a brief period of faint X-ray activity in January 2021, which has been referred to as a ‘mis-fired’ outburst. Cen X–4 is not detected in any of the four MeerKAT observations. We place these radio non-detections on the X-ray – radio luminosity diagram, improving the constraints on the correlation between the two luminosities from earlier quiescent radio studies. We confirm that Cen X–4 is radio fainter than the transitional milli-second pulsar PSR J1023+0038 at the same X-ray luminosity. We discuss the radio behaviour of accreting neutron stars at low X-ray luminosity more generally and finally comment on future observing campaigns.
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 P., Russell D. M., Pirbhoy S., 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.
We present long-term (2012–2022) optical monitoring of the candidate black hole X-ray binary Swift J1910.2−0546 with the Faulkes Telescopes and Las Cumbres Observatory (LCO) network. Following its initial bright 2012 outburst, we find that the source displayed a series of at least 7 quasi- periodic, high amplitude (∼ 3 mags) optical reflares in 2013, with a recurrence time increasing from ∼ 42 days to ∼ 49 days. In 2014, the source experienced a mini-outburst with two peaks in the optical. We also study the recent 2022 outburst of the source at optical wavelengths, and perform a comparative analysis with the earlier rebrightenings. During the reflaring in both 2013 and 2014, the source showed bluer-when-brighter behavior, having optical colors consistent with a blackbody heating and cooling between 4500 and 9500 K, i.e. the temperature range in which hydrogen starts to ionize. Finally, we compare the flaring behavior of the source to re-brightening events in other X-ray binaries. We show that the repeated reflarings of Swift J1910.2−0546 are highly unusual, and propose that they arise from a sequence of repetitive heating and cooling front reflections travelling through the accretion disk.
Lemmings. V. Nuclear activity and bulge properties: a detailed multi-component decomposition of e-MERLIN Palomar galaxies with HST
Dullo B.T., Knapen J.H., ..., Saikia P., et al. 2023, accepted for publication at A&A
Project |21
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.
We use high-resolution HST imaging and e-MERLIN 1.5-GHz observations of galaxy cores from the LeMMINGs survey to investigate the relation between optical structural properties and nuclear radio emission for a large sample of galaxies. We perform accurate, multi-component decompositions of new surface brightness profiles extracted from HST images for 163 LeMMINGs galaxies and fit up to six galaxy components (e.g., bulges, discs, AGN, bars, rings, spiral arms, and nuclear star clusters) simultaneously with Sérsic and/or core-Sérsic models. We show that the bulge mass can be significantly overestimated in many galaxies when components such as bars, rings and spirals are not included in the fits. We also find that the radio detection fraction increases with bulge mass. Furthermore, we observe core-Sérsic bulges tend to be systematically round and to possess high radio core luminosities and boxy-distorted or pure elliptical isophotes. However, there is no evidence for the previously alleged strong tendency of galaxies’ central structures (i.e., a sharp Sérsic/core-Sérsic dichotomy) with their radio loudness, isophote shape and flattening.
Lemmings. VI. Connecting nuclear activity to bulge properties of active and inactive galaxies: radio scaling relations and galaxy environment
Dullo B.T., Knapen J.H., et al. 2023, accepted for publication at MNRAS
Project |22
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.
Multiwavelength studies indicate that nuclear activity and bulge properties are closely related, but the details remain unclear. To study this further, we combine Hubble Space Telescope bulge structural and photometric properties with 1.5 GHz, 𝑒-MERLIN
nuclear radio continuum data from the LeMMINGs survey for a large sample of 173 ‘active’ galaxies (LINERs and Seyferts) and ‘inactive’ galaxies (H iis and absorption line galaxies, ALGs). We find that at fixed bulge mass, the radio loudness, nuclear radio activity and the (optical and radio) AGN fraction exhibit no dependence on environment. Radio-loud hosts preferentially possess an early-type morphology than radio-quiet hosts, the two types are however indistinguishable in terms of bulge Sérsic index and ellipticity, while results on the bulge inner logarithmic profile slope are inconclusive. We finally discuss the importance of bulge mass in determining the AGN triggering processes, including potential implications for the nuclear radio emission in nearby galaxies.
Project |23
Clockwise evolution in the hardness–intensity diagram of the black hole X-ray binary Swift J1910.2−0546
Saikia P., Russell D. M., Pirbhoy S., 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).
We present a detailed study of optical data from the 2012 outburst of the candidate black hole X-ray binary Swift J1910.2−0546 using the Faulkes Telescope and Las Cumbres Observatory (LCO). We analyse the peculiar spectral state changes of Swift J1910.2−0546 in different energy bands, and characterize how the optical and UV emission correlates with the unusual spectral state evolution. Using various diagnostic tools like the optical/X-ray correlation and spectral energy distributions, we disentangle the different emission processes contributing towards the optical flux of the system. When Swift J1910.2−0546 transitions to the pure hard state, we find significant optical brightening of the source along with a dramatic change in the optical colour due to the onset of a jet during the spectral state transition. For the rest of the spectral states, the optical/UV emission is mostly dominated by an X-ray irradiated disc. From our high cadence optical study, we have discovered a putative modulation. Assuming that this modulation arises from a superhump, we suggest Swift J1910.2−0546 to have an orbital period of 2.25–2.47 h, which would make it the shortest orbital period black hole X-ray binary known to date. Finally, from the state transition luminosity of the source, we find that the distance to the source is likely to be ∼4.5–20.8 kpc, which is also supported by the comparative position of the source in the global optical/X-ray correlation of a large sample of X-ray binaries.
Project |24
Matter ejections behind the highs and lows of the transitional millisecond pulsar PSR J1023+0038
Baglio M. C., Coti Zelati F., 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 an emerging class of sources that link low-mass X-ray binaries to millisecond radio pulsars in binary systems. These pulsars alternate between a radio pulsar state and an active low-luminosity X-ray disc state. During the active state, these sources exhibit two distinct emission modes (high and low) that alternate unpredictably, abruptly, and incessantly. X-ray to optical pulsations are observed only during the high mode. The root cause of this puzzling behaviour remains elusive. This paper presents the results of the most extensive multi-wavelength campaign ever conducted on the transitional pulsar prototype, PSR J1023+0038, covering from the radio to X-rays. The campaign was carried out over two nights in June 2021 and involved 12 different telescopes and instruments, including XMM-Newton, HST, VLT/FORS2 (in polarimetric mode), ALMA, VLA, and FAST. By modelling the broadband spectral energy distributions in both emission modes, we show that the mode switches are caused by changes in the innermost region of the accretion disc. These changes trigger the emission of discrete mass ejections, which occur on top of a compact jet, as testified by the detection of at least one short-duration millimetre flare with ALMA at the high-to-low mode switch.
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 (e.g. XRBs) in our fields 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
The newest generation of radio telescopes is able to survey large areas with high sensitivity and cadence, producing data volumes that require new methods to better understand the transient sky. Here, we describe the results from the first citizen science project dedicated to commensal radio transients, using data from the MeerKAT telescope with weekly cadence. Bursts from Space: MeerKAT was launched late in 2021 and received ∼89 000 classifications from over 1000 volunteers in 3 months. Our volunteers discovered 142 new variable sources which, along with the known transients in our fields, allowed us to estimate that at least 2.1 per cent of radio sources are varying at 1.28 GHz at the sampled cadence and sensitivity, in line with previous work. We provide the full catalogue of these sources, the largest of candidate radio variables to date. Transient sources found with archival counterparts include a pulsar (B1845-01) and an OH maser star (OH 30.1–0.7), in addition to the recovery of known stellar flares and X-ray binary jets in our observations. Data from the MeerLICHT optical telescope, along with estimates of long time-scale variability induced by scintillation, imply that the majority of the new variables are active galactic nuclei. This tells us that citizen scientists can discover phenomena varying on time-scales from weeks to several years. The success both in terms of volunteer engagement and scientific merit warrants the continued development of the project, while we use the classifications from volunteers to develop machine learning techniques for finding transients
Project |26
The omnipresent flux-dependent optical dips of the black hole transient Swift J1357.2-0933
Panizo-Espinar G., Muñoz-Darias T.et al. 2023, A&A, 682, A19
Swift J1357.2-0933 is a black hole transient of particular interest due to the optical, recurrent dips found during its first two outbursts (in 2011 and 2017), with no obvious X-ray equivalent. We present fast optical photometry during its two most recent outbursts, in 2019 and 2021. Our observations reveal that the optical dips were present in every observed outburst of the source, although they were shallower and showed longer recurrence periods in the two most recent and fainter events. We perform a global study of the dips properties in the four outbursts, and find that they do not follow a common temporal evolution. In addition, we discover a correlation with the X-ray and optical fluxes, with dips being more profound and showing shorter recurrence periods for brighter stages. This trend seems to extend even to the faintest, quiescent states of the source. Finally, we discuss these results in the context of the possible connection between optical dips and outflows found in previous works.
FIGURE 26. DRP evolution with X-ray fluxes for the four outbursts of J1357.
Project |27
Evidence for low power radio jet−ISM interaction at 10 parsec in the dwarf AGN host NGC 4395
Nandi P., Stalin C. S., et al. 2023, 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.
Black hole driven outflows in galaxies hosting active galactic nuclei (AGN) may interact with their interstellar medium (ISM) affecting star formation. Such feedback processes, reminiscent of those seen in massive galaxies, have been reported recently in some dwarf galaxies. However, such studies have usually been on kiloparsec and larger scales and our knowledge on the smallest spatial scales to which these feedback processes can operate is unclear. Here we demonstrate radio jet−ISM interaction on the scale of an asymmetric triple radio structure of ∼ 10 parsec size in NGC 4395. This triple radio structure is seen in the 15 GHz continuum image and the two asymmetric jet-like structures are situated on either side of the radio core that coincides with the optical Gaia position. The high resolution radio image and the extended [OIII]λ5007 emission, indicative of an outflow, are spatially coincident and are consistent with the interpretation of a low power radio jet interacting with the ISM. Modelling of the spectral lines using MAPPINGS, and estimation of temperature using optical integral field spectroscopic data suggest shock ionization of the gas. The continuum emission at 237 GHz, though weak, was found to spatially coincide with the AGN. However, the CO(2−1) line emission was found to be displaced by around 20 parsec northward of the AGN core. The spatial coincidence of molecular H2λ2.4085 along the jet direction, the morphology of ionised [OIII]λ5007 and displacement of the CO(2−1) emission argues for conditions less favourable for star formation in the central ∼ 10 parsec region.
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., Tetarenko A. J., et al. 2023, ApJ, 962, 2, 116
FIGURE 28. Broad-band spectral evolution of J1820 over the course of its 2018/2019 outburst. In each spectrum, 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.
We present quasi-simultaneous, multi-wavelength observations of the Galactic black hole system MAXI J1820+070, throughout its 2018–2019 outburst. Our data set includes cover- age from the radio through X-ray bands from 17 different instruments/telescopes, and encompasses 19 epochs over a 7 month time period, resulting in one of the most well-sampled multi-wavelength data sets of a BH XRB outburst to date. With our data, we compile and model the broad-band spectra of this source using a phenomenological model that includes emission from the jet, companion star, and accretion flow. This modeling allows us to track the evolution of the spectral break in the jet spectrum, a key observable that samples the jet launching region. We find that the spectral break location changes over at least ≈ 3 orders of magnitude in electromagnetic frequency over this period. Using these spectral break measurements, we link the full cycle of jet behavior, including the rising, quenching, and re-ignition, to the changing accretion flow properties as the source evolves through its different accretion states. Our analyses show a consistent jet behavior with other sources in similar phases of their outbursts, reinforcing that the jet quenching and recovery may be a global feature of BH XRB systems in outburst. Our results also provide valuable evidence supporting a close connection between the geometry of the inner accretion flow and the base of the jet.
Project |29
A multi-wavelength study of the hard and soft states of MAXI J1820+070 during its 2018 outburst
Banerjee S., Dewangan G. C., et al. 2023, accepted for publication at ApJ
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.
We present AstroSat far-UV, soft/hard X-ray and LCO optical spectral study of the black hole X-ray binary MAXI J1820+070 in the hard and soft states during its 2018 outburst. In the soft state, we detect soft X-ray, UV, and optical excess components over and above the intrinsic accretion disk emission (∼0.58 keV) and a steep X-ray power-law (Γ∼2.2) component. The soft X-ray excess (also found by NuSTAR) is consistent with a high-temperature black-body (∼0.8 keV). The UV excess consists of five emission lines and a low-temperature black-body (kT∼3.9 eV). An additional black-body component with kT∼0.77 eV is required to represent the optical excess. These UV and optical excess components, most likely, arise due to the irradiation of the outer accretion disk by the inner disk and coronal emission. Besides, using the continuum spectral fitting method for the soft state spectrum, we find the mass and spin of the BH to be 8.0 M⊙ and 0.85, respectively. In the hard state, we find greatly enhanced UV and optical excess components that are described similarly by two single-temperature black-body components (∼3.27eV and ∼0.8eV, respectively) and three emission lines in the UV band. A satisfactory fit to the broad-band data in the hard state suggests that the Xray emission is described by a two-component corona, their associated reflection components, and the emission from a disk (∼ 0.19 keV). The softer coronal component (Γ ∼ 1.6) dominates the bolometric X-ray luminosity and produces broader relativistic reflection features, whereas the harder component (Γ ∼ 1.2) gets reflected far from the inner disk giving rise to narrow reflection features.