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.

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.

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.

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.

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.

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.

Low frequency X-ray timing with Gaussian processes and reverberation in the radio-loud AGN 3C 120

D.R. Wilkins, 2019, MNRAS 489, 1957

A framework is developed to perform Fourier-domain timing analysis on X-ray light curves with gaps, employing Gaussian processes to model the probability distribution underlying the observed time series from which continuous samples can be drawn. A technique is developed to measure X-ray reverberation from the inner regions of accretion discs around black holes in the low frequency components of the variability, on timescales longer than can be probed employing standard Fourier techniques. This enables X-ray reverberation experiments to be performed using data from satellites in low-Earth orbit such as NICER, NuSTAR and the proposed X-ray timing mission STROBE-X, and enables long timescale reverberation around higher mass AGN to be measured by combining multiple observations. Gaussian processes are applied to observations of the broad line radio galaxy 3C120 spanning two orbits with XMM-Newton to measure the relative time lags of successive X-ray energy bands. The lag-energy spectrum between 5E-6 and 3E-5Hz, estimated using Gaussian processes, reveals X-ray reverberation from the inner accretion disc for the first time in this radio-loud AGN. Time lags in the relativistically broadened iron K line are significantly detected. The core of the line lags behind the continuum by (3800 +/- 1500)s, suggesting a scale height of the corona of (13 +/- 8)rg above the disc. The ability to compare the structure of coronae in radio loud AGN to their radio quiet counterparts will yield important insight into the mechanisms by which black holes are able to launch jets.

On the illumination of neutron star accretion discs

D.R. Wilkins, 2018, MNRAS 475, 748

The illumination of the accretion disc in a neutron star X-ray binary by X-rays emitted from (or close to) the neutron star surface is explored through general relativistic ray tracing simulations. The applicability of the canonical suite of relativistically broadened emission line models (developed for black holes) to discs around neutron stars is evaluated. These models were found to describe well emission lines from neutron star accretion discs unless the neutron star radius is larger than the innermost stable orbit of the accretion disc at 6rg or the disc is viewed at high inclination, above 60deg where shadowing of the back side of the disc becomes important. Theoretical emissivity profiles were computed for accretion discs illuminated by hotspots on the neutron star surfaces, bands of emission and emission by the entirety of the hot, spherical star surface and in all cases, the emissivity profile of the accretion disc was found to be well represented by a single power law falling off slightly steeper than r^-3. Steepening of the emissivity index was found where the emission is close to the disc plane and the disc can appear truncated when illuminated by a hotspot at high latitude. The emissivity profile of the accretion disc in Serpens X-1 was measured and found to be consistent with a single unbroken power law with index q=3.5 (-0.4,+0.3), suggestive of illumination by the boundary layer between the disc and neutron star surface.

Revealing structure and evolution within the corona of the Seyfert galaxy I Zw 1

D.R. Wilkins, L.C. Gallo, C.V. Silva, E. Costantini, W.N. Brandt and G.A. Kriss, 2017, MNRAS 471, 4436

X-ray spectral timing analysis is presented of XMM-Newton observations of the narrow line Seyfert 1 galaxy I Zwicky 1 (I Zw 1) taken in 2015 January. After exploring the effect of background flaring on timing analyses, X-ray time lags between the reflection-dominated 0.3-1.0keV energy and continuum-dominated 1.0-4.0keV band are measured, indicative of reverberation off the inner accretion disc. The reverberation lag time is seen to vary as a step function in frequency; across lower frequency components of the variability, 3e-4 to 1.2e-3Hz a lag of 160s is measured, but the lag shortens to (59 +/- 4)s above 1.2e-3Hz. The lag-energy spectrum reveals differing profiles between these ranges with a change in the dip showing the earliest arriving photons. The low frequency signal indicates reverberation of X-rays emitted from a corona extended at low height over the disc while at high frequencies, variability is generated in a collimated core of the corona through which luminosity fluctuations propagate upwards. Principal component analysis of the variability supports this interpretation, showing uncorrelated variation in the spectral slope of two power law continuum components. The distinct evolution of the two components of the corona is seen as a flare passes inwards from the extended to the collimated portion. An increase in variability in the extended corona was found preceding the initial increase in X-ray flux. Variability from the extended corona was seen to die away as the flare passed into the collimated core leading to a second sharper increase in the X-ray count rate.

Towards modelling X-ray reverberation in AGN: Piecing together the extended corona

D.R. Wilkins, E.M. Cackett, A.C. Fabian and C.S. Reynolds, 2016, MNRAS 458, 200

Models of X-ray reverberation from extended coronae are developed from general relativistic ray tracing simulations. Reverberation lags between correlated variability in the directly observed continuum emission and that reflected from the accretion disc arise due to the additional light travel time between the corona and reflecting disc. X-ray reverberation is detected from an increasing sample of Seyfert galaxies and a number of common properties are observed, including a transition from the characteristic reverberation signature at high frequencies to a hard lag within the continuum component at low frequencies, as well a pronounced dip in the reverberation lag at 3keV. These features are not trivially explained by the reverberation of X-rays originating from simple point sources. We therefore model reverberation from coronae extended both over the surface of the disc and vertically. Causal propagation through its extent for both the simple case of constant velocity propagation and propagation linked to the viscous timescale in the underlying accretion disc is included as well as stochastic variability arising due to turbulence locally on the disc. We find that the observed features of X-ray reverberation in Seyfert galaxies can be explained if the long timescale variability is dominated by the viscous propagation of fluctuations through the corona. The corona extends radially at low height over the surface of the disc but with a bright central region in which fluctuations propagate up the black hole rotation axis driven by more rapid variability arising from the innermost regions of the accretion flow.

Flaring from the supermassive black hole in Mrk 335 studied with Swift and NuSTAR

D.R. Wilkins, L.C. Gallo, D. Grupe, K. Bonson, S. Komossa and A.C. Fabian, 2015, MNRAS 454, 4440

Monitoring of the narrow line Seyfert 1 galaxy Markarian 335 (Mrk 335) with the Swift satellite discovered an X-ray flare beginning 2014 August 29. At the peak, the 0.5-5keV count rate had increased from that in the low flux state by a factor of 10. A target of opportunity observation was triggered with NuSTAR, catching the decline of the flare on 2014 September 20. We present a joint analysis of Swift and NuSTAR observations to understand the cause of this flare. The X-ray spectrum shows an increase in directly observed continuum flux and the softening of the continuum spectrum to a photon index of 2.49 (-0.07,+0.08) compared to the previous low flux observations. The X-ray spectrum remains well-described by the relativistically blurred reflection of the continuum from the accretion disc whose emissivity profile suggests that it is illuminated by a compact X-ray source, extending at most 5.2rg over the disc. A very low reflection fraction of 0.41 (-0.15,+0.15) is measured, unexpected for such a compact corona. The X-ray flare is, hence, interpreted as arising from the vertical collimation and ejection of the X-ray emitting corona at a mildly relativistic velocity, causing the continuum emission to be beamed away from the disc. As the flare subsides, the base of this jet-like structure collapses into a compact X-ray source that provides the majority of the radiation that illuminates the disc while continuum emission is still detected from energetic particles further out, maintaining the low reflection fraction.

Driving Extreme Variability — Measuring the evolving coronae and evidence for jet launching in AGN

D.R. Wilkins, 2015. Accepted for publication in Astron. Nach. – Proceedings of the XMM-Newton 2015 Science Workshop, "The Extremes of Black Hole Accretion"

Relativistically blurred reflection from the accretion disc provides a powerful probe of the extreme environments close to supermassive black holes; the inner regions of the accretion flow and the corona that produces the intense X-ray continuum. Techniques by which the geometry and extent of the corona can be measured through the observed X-ray spectrum are reviewed along with the evolution in the structure of the corona that is seen to accompany variations in the X-ray luminosity both on long and short timescales. Detailed analyses of the narrow line Seyfert 1 galaxies Markarian 335 and 1H0707-495, over observations with XMM-Newton as well as Suzaku and NuSTAR spanning nearly a decade reveal that increases in the X-ray luminosity coincide with an expansion of the corona to cover a larger area of the inner accretion disc. Underlying this long timescale variability lie more complex patterns of behaviour on short timescales. Flares in the X-ray emission during a low flux state of Mrk 335 observed in 2013 and 2014 are found to mark a reconfiguration of the corona while there is evidence that the flares were caused by a vertical collimation and ejection of coronal material, reminiscent of an aborted jet-launching event. Measurements of the corona and reflecting accretion disc are combined to infer the conditions on the inner disc that lead to the flaring event.

Driving extreme variability: The evolving corona and evidence for jet launching in Markarian 335

D.R. Wilkins and L.C. Gallo, 2015. MNRAS 449, 129-146

Variations in the X-ray emission from the narrow line Seyfert 1 galaxy, Markarian 335 (Mrk 335), are studied on both long and short timescales through observations made between 2006 and 2013 with XMM-Newton, Suzaku and NuSTAR. Changes in the geometry and energetics of the corona that give rise to this variability are inferred through measurements of the relativistically blurred reflection seen from the accretion disc. On long timescales, we find that during the high flux epochs the corona has expanded, covering the inner regions of the accretion disc out to a radius of 26(-7,+10)rg. The corona contracts to within 12rg and 5rg in the intermediate and low flux epochs, respectively. While the earlier high flux observation made in 2006 is consistent with a corona extending over the inner part of the accretion disc, a later high flux observation that year revealed that the X-ray source had become collimated into a vertically-extended jet-like corona and suggested relativistic motion of material upward. On short timescales, we find that an X-ray flare during a low flux epoch in 2013 corresponded to a reconfiguration from a slightly extended corona to one much more compact, within just 2~3rg of the black hole. There is evidence that during the flare itself, the spectrum softened and the corona became collimated and slightly extended vertically as if a jet-launching event was aborted. Understanding the evolution of the X-ray emitting corona may reveal the underlying mechanism by which the luminous X-ray sources in AGN are powered.

The Comptonisation of accretion disc X-ray emission: Consequences for X-ray reflection and the geometry of AGN coronae

D.R. Wilkins and L.C. Gallo, 2015, MNRAS 448, 703-712

We consider the Comptonisation of the photons that make up the relativistically blurred reflection that is commonly detected from the accretion discs of AGN by the coronae of energetic particles believed to give rise to the powerful X-ray continua by the inverse-Compton scattering of thermal seed photons from the disc. Recent measurements of the emissivity profiles of accretion discs as well as reverberation time lags between the primary X-ray continuum and the reflection suggest that this corona is situated at a low height above the disc and extends radially, tens of gravitational radii over the disc surface, hence should also Compton scatter the reflected X-rays. We find that the detection of blurred reflection from as close in as the innermost stable circular orbits (ISCOs) of maximally rotating black holes is consistent with such coronae, but requires that the corona be patchy, consisting perhaps of a number of isolated flares throughout the region. Considering only the requirement that it be possible to detect reflection from the ISCO, we find that at any given moment, the covering fraction of the inner part of the accretion disc by the corona needs to be less than 85 per cent, though allowing for the detection of 'reflection-dominated' spectra in which the total reflected flux exceeds that seen in the continuum requires covering fractions as low as 50 or 25 per cent.

Caught in the act: Measuring the changes in the corona that cause the extreme variability of 1H0707-495

D.R. Wilkins, E. Kara, A.C. Fabian and L.C. Gallo, 2014, MNRAS 443, 2746-2756

The X-ray spectra of the narrow line Seyfert 1 galaxy, 1H0707-495, obtained with XMM-Newton, from time periods of varying X-ray luminosity are analysed in the context of understanding the changes to the X-ray emitting corona that lead to the extreme variability seen in the X-ray emission from active galactic nuclei (AGN). The emissivity profile of the accretion disc, illuminated by the X-ray emitting corona, along with previous measurements of reverberation time lags are used to infer the spatial extent of the X-ray source. By fitting a twice-broken power law emissivity profile to the relativistically-broadened iron K fluorescence line, it is inferred that the X-ray emitting corona expands radially, over the plane of the accretion disc, by 25 to 30 per cent as the luminosity increases, contracting again as the luminosity decreases, while increases in the measured reverberation lag as the luminosity increases would require also variation in the vertical extent of the source above the disc. The spectrum of the X-ray continuum is found to soften as the total X-ray luminosity increases and we explore the variation in reflected flux as a function of directly-observed continuum flux. These three observations combined with simple, first-principles models constructed from ray tracing simulations of extended coron self-consistently portray an expanding corona whose average energy density decreases, but with a greater number of scattering particles as the luminosity of this extreme object increases.

The origin of the lag spectra observed in AGN: Reverberation and the propagation of X-ray source fluctuations

D.R. Wilkins and A.C. Fabian, 2013, MNRAS 430, 247-258

The X-ray emission from active galactic nuclei (AGN) is highly variable. Measurements of time lags (characterised by lag spectra) between variability in the light curves in energy bands corresponding to directly observed continuum emission from the corona around the black hole and to X-rays reflected from the accretion disc adds a further dimension to studies of the structure and energetics of these systems. We seek to understand these measurements in terms of the physical parameters of the X-ray source (its location, extent, etc.) through the calculation of theoretical lag spectra for a range of source parameters in general relativistic ray tracing simulations, combined with knowledge of the observed variability of the X-ray emission from AGN. Due to the proximity of the emission to the central black hole, Shapiro delays are important and the effects of general relativity should be considered when interpreting the lags as the light travel time between the source and reflector. We show that it is important to consider dilution of the lag by the contribution of both the primary and reflected spectral components to the observed energy bands rather than observing pure continuum and reflected emission, reducing the measured lag by up to 75 per cent compared to the 'intrinsic' time lag due to light travel times. We find that the observed lag spectrum of the narrow line Seyfert 1 galaxy 1H 0707-495 implies an X-ray source extending radially outwards to around 35rg and at a height of around 2rg above the plane of the accretion disc, consistent with the constraints obtained independently by considering the emissivity profile of the accretion disc. By investigating the influence of the propagation of X-ray luminosity fluctuations through the source region we find it is possible to reproduce the shape of the low frequency part of the lag spectrum (where the hard 'primary' band lags behind the soft 'reflected' band) as the effect of luminosity fluctuations originating in the centre of the X-ray source, close to the black hole, and propagating outwards.

Understanding X-ray reflection emissivity profiles in AGN: Locating the X-ray source

D.R. Wilkins and A.C. Fabian, 2012, MNRAS 424, 1284-1296

The illumination pattern (or emissivity profile) of the accretion disc due to the reflection of X-rays in AGN can be understood in terms of relativistic effects on the rays propagating from a source in a corona surrounding the central black hole, both on their trajectories and on the accretion disc itself. Theoretical emissivity profiles due to isotropic point sources as well as simple extended geometries are computed in general relativistic ray tracing simulations performed on graphics processing units (GPUs). Such simulations assuming only general relativity naturally explain the accretion disc emissivity profiles determined from relativistically broadened emission lines which fall off steeply (with power law indices of between 6 and 8) over the inner regions of the disc, then flattening off to almost a constant before tending to a constant power law of index 3 over the outer disc. Simulations for a variety of source locations, extents and geometries show how the emissivity profiles depend on these properties, and when combined with reverberation time lags allow the location and extent of the primary X-ray source to be constrained. Comparing the emissivity profile determined from the broadened iron K emission line in spectra of 1H 0707-495 obtained in January 2008 to theoretical emissivity profiles and applying constraints from reverberation lags suggest that there exists an extended region of primary X-ray emission located as low as 2rg above the accretion disc, extending outwards to a radius of around 30rg.

Testing the accuracy of radiative cooling approximations in SPH simulations

Daniel R. Wilkins and Cathie J. Clarke, 2011, MNRAS 419, 3368-3377

Hydrodynamical simulations of star formation have stimulated a need to develop fast and robust algorithms for evaluating radiative cooling. Here we undertake a critical evaluation of what is currently a popular method for prescribing cooling in SPH simulations, i.e. the polytropic cooling due originally to Stamatellos et al. This method uses the local density and potential to estimate the column density and optical depth to each particle and then uses these quantities to evaluate an approximate expression for the net radiative cooling. We evaluate the algorithm by considering both spherical and disc-like systems with analytic density and temperature structures. In spherical systems, the total cooling rate computed by the method is within around 20 for the astrophysically relevant case of opacity dominated by ice grains and is correct to within a factor of order unity for a range of opacity laws. In disc geometry, however, the method systematically under-estimates the cooling by a large factor at all heights in the disc. For the self-gravitating disc studied, we find that the method under-estimates the total cooling rate by a factor of 200. This discrepancy may be readily traced to the method's systematic over-estimate of the disc column density and optical depth, since (being based only on the local density and potential) it does not take into account the low column density route for photon escape normal to the disc plane. We note that the discrepancy quoted above applies in the case that the star's potential is not included in the column density estimate and that even worse agreement is obtained if the full (star plus disc) potential is employed. These results raise an obvious caution about the method's use in disc geometry whenever an accurate cooling rate is required, although we note that there are situations where the discrepancies highlighted above may not significantly affect the global outcome of simulations. Finally, we draw attention to our introduction of an analytic self-gravitating disc structure that may be of use in the calibration of future cooling algorithms.