Augmenting astronomical X-ray detectors with AI for enhanced sensitivity and reduced background

D. R. Wilkins, A. Poliszczuk, B. Schneider, E. D. Miller, S. W. Allen, M. Bautz, T. Chattopadhyay, A. D. Falcone, R. Foster, C. E. Grant, S. Herrmann, R. Kraft, R. G. Morris, P. Nulsen, P. Orel and G. Schellenberger, 2024, Proc. SPIE “Space Telescopes and Instrumentation: Ultraviolet to Gamma Ray”, 13093-65

Bringing artificial intelligence (AI) alongside next-generation X-ray imaging detectors, including CCDs and DEPFET sensors, enhances their sensitivity to achieve many of the flagship science cases targeted by future X-ray observatories, based upon low surface brightness and high redshift sources. Machine learning algorithms operating on the raw frame-level data provide enhanced identification of background vs. astrophysical X-ray events, by considering all of the signals in the context within which they appear within each frame. We have developed prototype machine learning algorithms to identify valid X-ray and cosmic-ray induced background events, trained and tested upon a suite of realistic end-to-end simulations that trace the interaction of cosmic ray particles and their secondaries through the spacecraft and detector. These algorithms demonstrate that AI can reduce the unrejected instrumental background by up to 41.5 per cent compared with traditional filtering methods. Alongside AI algorithms to reduce the instrumental background, next-generation event reconstruction methods, based upon fitting physically-motivated Gaussian models of the charge clouds produced by events within the detector, promise increased accuracy and spectral resolution of the lowest energy photon events.

Towards efficient machine-learning-based reduction of the cosmic-ray induced background in X-ray imaging detectors: increasing context awareness

A. Poliszczuk, D. R. Wilkins, S. W. Allen, E. D. Miller, T. Chattopadhyay, B. Schneider, J. E. Darve, M. Bautz, A. D. Falcone, R. Foster, C. E. Grant, S. Herrmann, R. Kraft, R. G. Morris, P. Nulsen, P. Orel, G. Schellenberger and H. R. Stueber, 2024, Proc. SPIE “Space Telescopes and Instrumentation: Ultraviolet to Gamma Ray”, 13093-67

Traditional cosmic ray filtering algorithms used in X-ray imaging detectors aboard space telescopes perform event reconstruction based on the properties of activated pixels above a certain energy threshold, within 3x3 or 5x5 pixel sliding windows. This approach can reject up to 98% of the cosmic ray background. However, the remaining unrejected background constitutes a significant impediment to studies of low surface brightness objects, which are especially prevalent in the high-redshift universe. The main limitation of the traditional filtering algorithms is their ignorance of the long-range contextual information present in image frames. This becomes particularly problematic when analyzing signals created by secondary particles produced during interactions of cosmic rays with body of the detector. Such signals may look identical to the energy deposition left by X-ray photons, when one considers only the properties within the small sliding window. Additional information is present, however, in the spatial and energy correlations between signals in different parts of the frame, which can be accessed by modern machine learning (ML) techniques. In this work, we continue the development of an ML-based pipeline for cosmic ray background mitigation. Our latest method consist of two stages: first, a frame classification neural network is used to create class activation maps (CAM), localizing all events within the frame; second, after event reconstruction, a random forest classifier, using features obtained from CAMs, is used to separate X-ray and cosmic ray features. The method delivers >40% relative improvement over traditional filtering in background rejection in standard 0.3-10keV energy range, at the expense of only a small (<2%) level of lost X-ray signal. Our method also provides a convenient way to tune the cosmic ray rejection threshold to adapt to a user's specific scientific needs.

Wavelet spectral timing: X-ray reverberation from a dynamic black hole corona hidden beneath ultrafast outflows

D. R. Wilkins, 2023, MNRAS 526, 3441

Spectral timing analyses based upon wavelet transforms provide a new means to study the variability of the X-ray emission from accreting systems, including AGN, stellar mass black holes and neutron stars, and can be used to trace the time variability of X-ray reverberation from the inner accretion disc. The previously-missing iron K reverberation time lags in the AGN IRAS 13224-3809 and MCG-6-30-15 are detected and found to be transitory in nature. Reverberation can be hidden during periods in which variability in the iron K band becomes dominated by ultrafast outflows (UFO). Following the time evolution of the reverberation lag between the corona and inner accretion disc, we may observe the short-timescale increase in scale height of the corona as it is accelerated away from the accretion disc during bright X-ray flares in the AGN I Zw 1. Measuring the variation of the reverberation lag that corresponds to the continuous, stochastic variations of the X-ray luminosity sheds new light on the disc-corona connection around accreting black holes. Hysteresis is observed between the X-ray count rate and the scale height of the corona, and a time lag of 10~40ks is observed between the rise in luminosity and the increase in reverberation lag. This correlation and lag are consistent with viscous propagation through the inner accretion disc, leading first to an increase in the flux of seed photons that are Comptonised by the corona, before mass accretion rate fluctuations reach the inner disc and are able to modulate the structure of the corona.

X-Ray/UVOIR Frequency-resolved Time Lag Analysis of Mrk 335 Reveals Accretion Disk Reprocessing

C. Lewin, E. Kara, E. Cackett, D. Wilkins, C. Panagiotou, J. García and J. Gelbord, 2023, ApJ 954, 33

UV and optical continuum reverberation mapping is a powerful tool for probing the accretion disk and inner broad-line region. However, recent reverberation mapping campaigns in the X-ray, UV, and optical have found lags consistently longer than those expected from the standard disk reprocessing picture. The largest discrepancy to date was recently reported in Mrk 335, where UV/optical lags are up to 12 times longer than expected. Here, we perform a frequency-resolved time lag analysis of Mrk 335, using Gaussian processes to account for irregular sampling. For the first time, we compare the Fourier frequency-resolved lags directly to those computed using the popular interpolated cross-correlation function method applied to both the original and detrended light curves. We show that the anticipated disk reverberation lags are recovered by the Fourier lags when zeroing in on the short-timescale variability. This suggests that a separate variability component is present on long timescales. If this separate component is modeled as reverberation from another region beyond the accretion disk, we constrain a size scale of roughly 15 lt-days from the central black hole. This is consistent with the size of the broad-line region inferred from Hβ reverberation lags. We also find tentative evidence for a soft X-ray lag, which we propose may be due to light travel time delays between the hard X-ray corona and distant photoionized gas that dominates the soft X-ray spectrum below 2 keV.

UV-Optical Disk Reverberation Lags despite a Faint X-Ray Corona in the Active Galactic Nucleus Mrk 335

E. Kara, A. Barth, E. Cackett, J. Gelbord, J. Montano, Y.-R. Li, L. Santana, K. Horne et al. (incl Wilkins), 2023, ApJ 947, 62

We present the first results from a 100-day Swift, NICER, and ground-based X-ray-UV-optical reverberation mapping campaign of the Narrow-line Seyfert 1 Mrk 335, when it was in an unprecedented low X-ray flux state. Despite dramatic suppression of the X-ray variability, we still observe UV-optical lags as expected from disk reverberation. Moreover, the UV-optical lags are consistent with archival observations when the X-ray luminosity was >10 times higher. Interestingly, both low- and high-flux states reveal UV-optical lags that are 6-11 times longer than expected from a thin disk. These long lags are often interpreted as due to contamination from the broad line region; however the u-band excess lag (containing the Balmer jump from the diffuse continuum) is less prevalent than in other active galactic nuclei. The Swift campaign showed a low X-ray-to-optical correlation (similar to previous campaigns), but NICER and ground-based monitoring continued for another 2 weeks, during which the optical rose to the highest level of the campaign, followed ~10 days later by a sharp rise in X-rays. While the low X-ray countrate and relatively large systematic uncertainties in the NICER background make this measurement challenging, if the optical does lead X-rays in this flare, this indicates a departure from the zeroth-order reprocessing picture. If the optical flare is due to an increase in mass accretion rate, this occurs on much shorter than the viscous timescale. Alternatively, the optical could be responding to an intrinsic rise in X-rays that is initially hidden from our line of sight.

X-ray Reverberation Mapping of Ark 564 using Gaussian Process Regression

C. D. Lewin, E. Kara, D. R. Wilkins, G. Mastroserio, J. A. García et al., 2022, ApJ 939, 109

Ark 564 is an extreme high-Eddington Narrow-line Seyfert 1 galaxy, known for being one of the brightest, most rapidly variable soft X-ray AGN, and for having one of the lowest temperature coronae. Here we present a 410-ks NuSTAR observation and two 115-ks XMM-Newton observations of this unique source, which reveal a very strong, relativistically broadened iron line. We compute the Fourier-resolved time lags by first using Gaussian processes to interpolate the NuSTAR gaps, implementing the first employment of multi-task learning for application in AGN timing. By fitting simultaneously the time lags and the flux spectra with the relativistic reverberation model RELTRANS, we constrain the mass at 2.3(+2.6,-1.3)×106M⊙, although additional components are required to describe the prominent soft excess in this source. These results motivate future combinations of machine learning, Fourier-resolved timing, and the development of reverberation models.

The multi-epoch X-ray tale of I Zwicky 1 outflows

D. Rogantini, E. Costantini, L. C. Gallo, D. R. Wilkins, W. N. Brandt and M. Mehdipour, 2022, MNRAS 516, 5171

The narrow-line Seyfert 1 galaxy I Zwicky 1 shows a unique and complex system of ionized gas in outflow, which consists of an ultra-fast wind and a two-component warm absorber. In the last two decades, XMM-Newton monitored the source multiple times enabling the study of the long-term variability of the various outflows. Plasma in photoionization equilibrium with the ionizing source responds and varies accordingly to any change of the ionizing luminosity. However, detailed modelling of the past Reflection Grating Spectrometer (RGS) data has shown no correlation between the plasma ionization state and the ionizing continuum, revealing a complex long-term variability of the multiphase warm absorber. Here, we present a new observation of I Zwicky1 by XMM-Newton taken in early 2020 characterized by a lower X-ray flux state. The soft X-ray spectrum from the RGS reveals the two components of the warm absorber with log ξ ~ -1.0 and log ξ ~ 1.7. Comparing our results with the previous observations, the ionization state of the two absorbing gas components is continuously changing, following the same unpredictable behaviour. The new results strengthen the scenario in which the ionization state of the warm absorber is driven by the density of the gas rather than the ionizing luminosity. In particular, the presence of a radiation driven, inhomogeneous clumpy outflow may explain both the variability in ionization throughout the years and the line-locked N V system observed in the ultraviolet band. Finally, the EPIC-pn spectrum reveals an ultra-fast wind with an outflow velocity of ~0.26c and ionization parameter of log ξ ~ 3.8.

A deep, multi-epoch Chandra HETG study of the ionized outflow from NGC 4051

A. Ogorzalek, A. L. King, S. W. Allen, J. C. Raymond and D. R. Wilkins, 2022, accepted for publication in MNRAS

Actively accreting supermassive black holes significantly impact the evolution of their host galaxies, truncating further star formation by expelling large fractions of gas with wide-angle outflows. The X-ray band is key to understanding how these black hole winds affect their environment, as the outflows have high temperatures (∼105−8K). We have developed a Bayesian framework for characterizing Active Galactic Nuclei (AGN) outflows with an improved ability to explore parameter space and perform robust model selection. We applied this framework to a new 700 ks and an archival 315 ks Chandra High Energy Transmission Gratings observation of the Seyfert galaxy NGC 4051. We have detected six absorbers intrinsic to NGC 4051. These wind components span velocities from 400 km s−1 to 30,000 km s−1. We have determined that the most statistically significant wind component is purely collisionally ionized, which is the first detection of such an absorber. This wind has T≈107 K and v≈880 km s−1 and remains remarkably stable between the two epochs. Other slow components also remain stable across time. Fast outflow components change their properties between 2008 and 2016, suggesting either physical changes or clouds moving in and out of the line of sight. For one of the fast components we obtain one of the tightest wind density measurements to date, log n/[cm−3]=13.0+0.01−0.02, and determine that it is located at ∼240 gravitational radii. The estimated total outflow power surpasses 5% of the bolometric luminosity (albeit with large uncertainties) making it important in the context of galaxy-black hole interactions.

Reducing the background in X-ray imaging detectors via machine learning

D. R. Wilkins, S. W. Allen, E. D. Miller, M. Bautz, T. Chattopadhyay, R. Foster, C. E. Grant, S. Hermann, R. Kraft, R. G. Morris, P. Nulsen, G. Schellenberger, 2022, Proc. SPIE "Space Telescopes and Instrumentation: Ultraviolet to Gamma Ray", 12181, 155

The sensitivity of astronomical X-ray detectors is limited by the instrumental background. The background is especially important when observing low surface brightness sources that are critical for many of the science cases targeted by future X-ray observatories, including Athena and future US-led flagship or probe-class X-ray missions. Above 2keV, the background is dominated by signals induced by cosmic rays interacting with the spacecraft and detector. We develop novel machine learning algorithms to identify events in next-generation X-ray imaging detectors and to predict the probability that an event is induced by a cosmic ray vs. an astrophysical X-ray photon, enabling enhanced filtering of the cosmic ray-induced background. We find that by learning the typical correlations between the secondary events that arise from a single primary, machine learning algorithms are able to successfully identify cosmic ray-induced background events that are missed by traditional filtering methods employed on current-generation X-ray missions, reducing the unrejected background by as much as 30 per cent.

High-density disc reflection spectroscopy of low-mass active galactic nuclei

L. Mallick, A. C. Fabian, J. A. Garcia, J. A. Tomsick, M. L. Parker, T. Dauser, D. R. Wilkins et al., 2022, MNRAS 513, 4361

The standard alpha-disc model predicts an anticorrelation between the density of the inner accretion disc and the black hole mass times square of the accretion rate, as seen in higher mass (MBH > 106 M⊙) active galactic nuclei (AGNs). In this work, we test the predictions of the alpha-disc model and study the properties of the inner accretion flow for the low-mass end (MBH ≍ 105-6 M⊙) of AGNs. We utilize a new high-density disc reflection model where the density parameter varies from ne = 1015 to 1020 cm-3 and apply it to the broad-band X-ray (0.3-10 keV) spectra of the low-mass AGN sample. The sources span a wide range of Eddington fractions and are consistent with being sub-Eddington or near-Eddington. The X-ray spectra reveal a soft X-ray excess below ~1.5 keV which is well modelled by high-density reflection from an ionized accretion disc of density ne ~ 1018 cm-3 on average. The results suggest a radiation pressure-dominated disc with an average of 70 per cent fraction of the disc power transferred to the corona, consistent with that observed in higher mass AGNs. We show that the disc density higher than 1015 cm-3 can result from the radiation pressure compression when the disc surface does not hold a strong magnetic pressure gradient. We find tentative evidence for a drop in black hole spin at low-mass regimes.

Acceleration and cooling of the corona during X-ray flares from the Seyfert galaxy I Zw 1

D. R. Wilkins, L. C. Gallo, E. Costantini, W. N. Brandt, R. D. Blandford, 2022, MNRAS 512, 761

We report on X-ray flares that were observed from the active galactic nucleus I Zwicky 1 (I Zw 1) in 2020 January by the NuSTAR and XMM-Newton observatories. The X-ray spectrum is well-described by a model comprised of the continuum emission from the corona and its reflection from the accretion disc around a rapidly spinning (a > 0.94) black hole. In order to model the broadband spectrum, it is necessary to account for the variation in ionisation across the disc. Analysis of the X-ray spectrum in time periods before, during and after the flares reveal the underlying changes to the corona associated with the flaring. During the flares, the reflection fraction drops significantly, consistent with the acceleration of the corona away from the accretion disc. We find the first evidence that during the X-ray flares, the temperature drops from 140(-20,+100)keV before to 45(-9,+40)keV during the flares. The profile of the iron K line reveals the emissivity profile of the accretion disc, showing it to be illuminated by a compact corona extending no more than 7(-2,+4)rg over the disc before the flares, but with tentative evidence that the corona expands as it is accelerated during the flares. Once the flares subsided, the corona had collapsed to a radius of 6(-2,+2)rg. The rapid timescale of the flares suggests that they arise within the black-hole magnetosphere rather than in the accretion disc, and the variation of the corona is consistent with the continuum arising from the Comptonisation of seed photons from the disc.

Light bending and X-ray echoes from behind a supermassive black hole

D.R. Wilkins, L.C. Gallo, E. Costantini, W.N. Brandt and R.D. Blandford, 2021, Nature 595, 657-660

The innermost regions of accretion disks around black holes are strongly irradiated by X-rays that are emitted from a highly variable, compact corona, in the immediate vicinity of the black hole. The X-rays that are seen reflected from the disk and the time delays, as variations in the X-ray emission echo or reverberate off the disk provide a view of the environment just outside the event horizon. I Zwicky 1 (I Zw 1), is a nearby narrow line Seyfert 1 galaxy. Previous studies of the reverberation of X-rays from its accretion disk revealed that the corona is composed of two components; an extended, slowly varying component over the surface of the inner accretion disk, and a collimated core, with luminosity fluctuations propagating upwards from its base, which dominates the more rapid variability. Here we report observations of X-ray flares emitted from around the supermassive black hole in I Zw 1. X-ray reflection from the accretion disk is detected through a relativistically broadened iron K line and Compton hump in the X-ray emission spectrum. Analysis of the X-ray flares reveals short flashes of photons consistent with the re-emergence of emission from behind the black hole. The energy shifts of these photons identify their origins from different parts of the disk. These are photons that reverberate off the far side of the disk and bent around the black hole and magnified by the strong gravitational field. Observing photons bent around the black hole confirms a key prediction of General Relativity.

Discovery of soft and hard X-ray time lags in low-mass AGNs

L. Mallick, D. R. Wilkins, W. N. Alston, A. Markowitz, B. De Marco, M. L. Parker, A. M. Lohfink and C. S. Stalin, 2021, MNRAS 503, 3775

The scaling relations between the black hole (BH) mass and soft lag properties for both active galactic nuclei (AGNs) and BH X-ray binaries (BHXRBs) suggest the same underlying physical mechanism at work in accreting BH systems spanning a broad range of mass. However, the low-mass end of AGNs has never been explored in detail. In this work, we extend the existing scaling relations to lower mass AGNs, which serve as anchors between the normal-mass AGNs and BHXRBs. For this purpose, we construct a sample of low-mass AGNs (MBH<3×106M⊙) from the XMM-Newton archive and measure frequency-resolved time-delays between the soft (0.3-1 keV) and hard (1-4 keV) X-ray emissions. We report that the soft band lags behind the hard band emission at high frequencies ∼[1.3-2.6] × 10-3 Hz, which is interpreted as a sign of reverberation from the inner accretion disc in response to the direct coronal emission. At low frequencies (∼[3-8] × 10-4 Hz), the hard-band lags behind the soft-band variations, which we explain in the context of the inward propagation of luminosity fluctuations through the corona. Assuming a lamppost geometry for the corona, we find that the X-ray source of the sample extends at an average height and radius of ∼10rg and ∼6rg, respectively. Our results confirm that the scaling relations between the BH mass and soft lag amplitude/frequency derived for higher mass AGNs can safely extrapolate to lower mass AGNs, and the accretion process is indeed independent of the BH mass.

Identifying charged particle background events in X-ray imaging detectors with novel machine learning algorithms

D. R. Wilkins, S. W. Allen, E. D. Miller, M. Bautz, T. Chattopadhyay, S. Fort, C. E. Grant, S. Herrmann, R. Kraft, R. G. Morris and P. Nulsen, 2020, Proc. SPIE "Space Telescopes and Instrumentation 202020: Ultraviolet to Gamma Ray", 11444, 308

Space-based X-ray detectors are subject to significant fluxes of charged particles in orbit, notably energetic cosmic ray protons, contributing a significant background. We develop novel machine learning algorithms to detect charged particle events in next-generation X-ray CCDs and DEPFET detectors, with initial studies focusing on the Athena Wide Field Imager (WFI) DEPFET detector. We train and test a prototype convolutional neural network algorithm and find that charged particle and X-ray events are identified with a high degree of accuracy, exploiting correlations between pixels to improve performance over existing event detection algorithms. 99 per cent of frames containing a cosmic ray are identified and the neural network is able to correctly identify up to 40 per cent of the cosmic rays that are missed by current event classification criteria, showing potential to significantly reduce the instrumental background, and unlock the full scientific potential of future X-ray missions such as Athena, Lynx and AXIS.

Returning radiation in strong gravity around black holes: Reverberation from the accretion disc

D.R. Wilkins, J.A. García, T. Dauser and A.C. Fabian, 2020, MNRAS 498, 3302

We study reflected X-ray emission that returns to the accretion disc in the strong gravitational fields around black holes using General Relativistic ray tracing and radiative transfer calculations. Reflected X-rays that are produced when the inner regions of the disc are illuminated by the corona are subject to strong gravitational light bending, causing up to 47 per cent of the reflected emission to be returned to the disc around a rapidly spinning black hole, depending upon the scale height of the corona. The iron K line is enhanced relative to the continuum by 25 per cent, and the Compton hump by up to a factor of three. Additional light travel time between primary and secondary reflections increases the reverberation time lag measured in the iron K band by 49 per cent, while the soft X-ray lag is increased by 25 per cent and the Compton hump response time is increased by 60 per cent. Measured samples of X-ray reverberation lags are shown to be consistent with X-rays returning to the accretion disc in strong gravity. Understanding the effects of returning radiation is important in interpreting reverberation observations to probe black holes. Reflected X-rays returning to the disc can be uniquely identified by blueshifted returning iron K line photons that are Compton scattered from the inner disc, producing excess, delayed emission in the 3.5-4.5keV energy range that will be detectable with forthcoming X-ray observatories, representing a unique test of General Relativity in the strong field limit.

Characterizing continuum variability in the radio-loud narrow-line Seyfert 1 galaxy IRAS 17020+4544

A.G. Gonzalez, L.C. Gallo, P. Kosec, A.C. Fabian, W.N. Alston, M. Berton and D.R. Wilkins, 2020, MNRAS 496, 3708

We present results of temporal and spectral analyses on four XMM─Newton EPIC pn observations of IRAS 17020+4544, a narrow-line Seyfert 1 galaxy with evidence of a radio jet. Analysis of the light curves reveals that this radio-loud source does not behave like the bulk population of its radio-quiet counterparts. A trend of spectral hardening with increased flux is found. Variability is found to increase with energy, though it decreases as the spectrum hardens. The first 40 ks of the most recent observation behave uniquely among the epochs, exhibiting a softer spectral state than at any other time. Possible non-stationarity at low energies is found, with no such effect present at higher energies, suggesting at least two distinct spectral components. A reverberation signature is confirmed, with the lag-frequency, lag-energy, and covariance spectra changing significantly during the soft-state epoch. The temporal analysis suggests a variable power law in the presence of a reflection component, thus motivating such a fit for the 0.3−10 keV EPIC pn spectra from all epochs. We find an acceptable spectral fit using the timing-motivated parameters and report the detection of a broad Fe K emission line, requiring an additional model component beyond the reflection spectrum. We discuss links between this source and other narrow-line Seyfert 1 sources that show evidence of jet activity, finding similarities among this currently very limited sample of interesting objects.

Venturing beyond the ISCO: Detecting X-ray emission from the plunging regions around black holes

D.R. Wilkins, C.S. Reynolds and A.C. Fabian, 2020, MNRAS 493, 5532

We explore how X-ray reverberation around black holes may reveal the presence of the innermost stable circular orbit (ISCO), predicted by General Relativity, and probe the dynamics of the plunging region between the ISCO and the event horizon. Being able to directly detect the presence of the ISCO and probe the dynamics of material plunging through the event horizon represents a unique test of general relativity in the strong field regime. X-ray reverberation off of the accretion disc and material in the plunging region is modelled using general relativistic ray tracing simulations. X-ray reverberation from the plunging region has a minimal effect on the time-averaged X-ray spectrum and the overall lag-energy spectrum, but is manifested in the lag in the highest frequency Fourier components, above 0.01 c^3 (GM)^-1 (scaled for the mass of the black hole) in the 2-4keV energy band for a non-spinning black hole or the 1-2keV energy band for a maximally spinning black hole. The plunging region is distinguished from disc emission not just by the energy shifts characteristic of plunging orbits, but by the rapid increase in ionisation of material through the plunging region. Detection requires measurement of time lags to an accuracy of 20 per cent at these frequencies. Improving accuracy to 12 per cent will enable constraints to be placed on the dynamics of material in the plunging region and distinguish plunging orbits from material remaining on stable circular orbits, confirming the existence of the ISCO, a prime discovery space for future X-ray missions.

The soft state of the black hole transient source MAXI J1820+070: emission from the edge of the plunge region?

A.C. Fabian, D.J. Buisson, P. Kosec, C.S. Reynolds, D.R. Wilkins et al., 2020, MNRAS 493, 5389

The Galactic black hole X-ray binary MAXI J1820+070 had a bright outburst in 2018 when it became the second brightest X-ray source in the sky. It was too bright for X-ray CCD instruments such as XMM-Newton and Chandra, but was well observed by photon-counting instruments such as Neutron star Inner Composition Explorer (NICER) and Nuclear Spectroscopic Telescope Array (NuSTAR). We report here on the discovery of an excess-emission component during the soft state. It is best modelled with a blackbody spectrum in addition to the regular disc emission, modelled as either diskbb or kerrbb. Its temperature varies from about 0.9 to 1.1 keV, which is about 30-80 per cent higher than the inner disc temperature of diskbb. Its flux varies between 4 and 12 per cent of the disc flux. Simulations of magnetized accretion discs have predicted the possibility of excess emission associated with a non-zero torque at the innermost stable circular orbit (ISCO) about the black hole, which, from other NuSTAR studies, lies at about 5 gravitational radii or about 60 km (for a black hole, mass is 8 M☉). In this case, the emitting region at the ISCO has a width varying between 1.3 and 4.6 km and would encompass the start of the plunge region where matter begins to fall freely into the black hole.

Blueshifted absorption lines from X-ray reflection in IRAS 13224-3809

A.C. Fabian, C.S. Reynolds, J. Jiang, C. Pinto, L.C. Gallo, M.L. Parker, A.N. Lasenby, W.N. Alston, D.J.K. Buisson, E.M. Cackett, B. DeMarco, J. García, E. Kara, P. Kosec, M.J. Middleton, J.M. Miller, G. Miniutti, D.J. Walton, D.R. Wilkins and A.J. Young, 2020, MNRAS 492, 2518

We explore a disc origin for the highly blueshifted, variable absorption lines seen in the X-ray spectrum of the narrow-line Seyfert 1 galaxy IRAS 13224-3809. The blueshift corresponds to a velocity of ∼0.25c. Such features in other active galactic nuclei are often interpreted as ultrafast outflows. The velocity is of course present in the orbital motions of the inner disc. The absorption lines in IRAS 13224-3809 are best seen when the flux is low and the reflection component of the disc is strong relative to the power-law continuum. The spectra are consistent with a model in which the reflection component passes through a thin, highly ionized absorbing layer at the surface of the inner disc, the blueshifted side of which dominates the flux due to relativistic aberration (the disc inclination is about 70°). No fast outflow need occurs beyond the disc.

A dynamic black hole corona in an active galaxy through X-ray reverberation mapping

William N. Alston, Andrew C. Fabian, Erin Kara, Michael L. Parker, Michal Dovciak, Ciro Pinto, Jiachen Jiang, Matthew J. Middleton, Giovanni Miniutti, Dominic J. Walton, Dan R. Wilkins, Douglas J. K. Buisson, Maria D. Caballero-Garcia, Edward M. Cackett, Barbara De Marco, Luigi C. Gallo, Anne M. Lohfink, Chris S. Reynolds, Phil Uttley, Andrew J. Young, Abderahmen Zogbhi, 2020, Nature Astronomy 4, 597

X-ray reverberation echoes are assumed to be produced in the strongly distorted spacetime around accreting supermassive black holes. This signal allows us to spatially map the geometry of the inner accretion flow - a region which cannot yet be spatially resolved by any telescope - and provides a direct measure of the black hole mass and spin. The reverberation timescale is set by the light travel path between the direct emission from a hot X-ray corona and the reprocessed emission from the inner edge of the accretion disc. However, there is an inherent degeneracy in the reverberation signal between black hole mass, inner disc radius and height of the illuminating corona above the disc. Here, we use a long X-ray observation of the highly-variable active galaxy, IRAS 13224-3809, to track the reverberation signal as the system evolves on timescales of a day. With the inclusion of all the relativistic effects, modelling reveals that the height of the X-ray corona increases with increasing luminosity, providing a dynamic view of the inner accretion region. This simultaneous modelling allows us to break the inherent degeneracies and obtain an independent timing-based estimate for the mass and spin of the black hole. The uncertainty on black hole mass is comparable to the leading optical reverberation method, making X-ray reverberation a powerful technique, particularly for sources with low optical variability.