Probing General Relativity and the Extreme Environments around Black Holes during X-ray Flares

NuSTAR 2022: 10 years of the high-energy Universe in focus, Cagliari, Italy (21/06/2022)

Observing the reflection and reverberation of X-rays off the accretion disks around supermassive black holes gives us a unique insight into the extreme environment just outside the event horizon. We can measure structure of the accretion flow and the corona that produces the X-ray continuum emission, and observe the effects of General Relativity in the strong gravitational field around the black hole.

NuSTAR observations of bright X-ray flares and how they echo off the accretion disc have led to unprecedented discoveries. They show how the corona is evolving, giving rise to the extreme variability we observe. We are discovering how, during bright X-ray flares, the temperature of the corona drops as it is launched away from the disk, giving us important clues as to how the corona is powered by the accretion flow.

In addition to teaching us about the corona, the echoes of the flare from the accretion disk allow us to measure light bending around the black hole and test key predictions of General Relativity. Observations of these bright X-ray flares reveal the first observational evidence for the reverberation of the X-ray flare from material that should classically be hidden behind the shadow of the black hole, re-emerging as the reflected X-rays are bent around the black hole into our line of sight.

A statistical approach to measuring X-ray reverberation in gravitationally lensed quasars

Chandra Data Science Workshop (19/08/2021)

The measurement of X-ray reverberation around black holes, as variations in the continuum emission echo off the accretion disk, have enabled a breakthrough in mapping the extreme environments just outside the event horizon. Reverberation probes the structure of the inner accretion flow and reveals the nature of the corona that produces the X-ray continuum. Measurements of X-ray reverberation via conventional Fourier techniques, however, are limited to relatively modest supermassive black holes in nearby galaxies. Recently-developed statistical techniques, based on Gaussian processes, enable the measurement of X-ray reverberation around more massive black holes in radio galaxies by combining multiple observations to provide a longer time baseline. This framework provides a statistical basis to robustly characterize time lags in lower signal-to-noise data, enabling the measurement of X-ray reverberation in Chandra observations of gravitationally lensed quasars. X-ray reverberation measurements in lensed quasars extend our understanding to rapidly accreting black holes beyond just the local Universe and pave the way for breakthrough science with the next-generation Lynx observatory.

The Central Engine: Observations

43rd COSPAR Scientific Assembly, Sydney, Australia and online (31/01/2021) - invited

The accretion of matter onto supermassive black holes powers some of the most luminous objects we see in the Universe; active galactic nuclei (AGN). Intense emission is produced across the electromagnetic spectrum; optical and UV emission from the accretion disc and X-rays produced by a corona of accelerated particles close to the black hole, along with powerful winds and vast jets travelling at close to the speed of light that are seen from many black holes.

Observations of supermassive black holes across the electromagnetic spectrum reveal a wealth of information about the central engine. From the reflection and reverberation of X-rays o˙ the innermost regions the accretion disc, a three-dimensional picture is starting to emerge of the extreme environments around black holes. Reverberation reveals the structure of the disc and the corona, the e˙ects of strong light bending in the gravitational field around the black hole, and how the launching sites of jets may be connected to the corona and inner disc. During flares and other transient events, we see how the corona evolves, giving rise to extreme variations in luminosity and how the corona can be ejected in failed jet-launching events, while providing a unique opportunity to observe the e˙ects of General Relativity.

Observations of the central engines in AGN yield important insight into the small-scale processes close to the event horizon that enable black holes to power some of the most extreme objects in the Universe, launch vast jets at close to the speed of light and play their important feedback role in the formation of structure in the Universe.

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

SPIE Space Telescopes and Instrumentation 2020: Ultraviolet to Gamma Ray (13/12/2020)

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.

Venturing beyond the ISCO: Mapping the extreme environments around black holes

30th Texas Symposium on Relativistic Astrophysics (16/12/2019)
JSI Workshop ‘The New Faces of Black Holes,’ Annapolis, MD (11/11/2019)

The immediate vicinities of black holes represent some of the most extreme environments in the Universe, where accreting material in its final moments before plunging through the event horizon powers some of the most luminous sources in the Universe; bright X-ray emitting coronae and vast jets launched close to the speed of light. 

The advent of X-ray timing studies has revealed unprecedented amounts of information about the extreme environments around black holes. The reverberation of the coronal X-ray emission off the inner regions of the accretion disc allows the region outside the event horizon to be mapped. X-ray reverberation reveals the geometry and the dynamic nature of the corona. The structure of the corona is revealed, with the discovery of a collimated core, reminiscent of a jet embedded within an extended corona, along with how these components evolve to give rise to the extreme X-ray variability that is observed.

The next step is to detect and understand what happens to material after it crosses innermost stable circular orbit (ISCO), predicted by General Relativity where gravity is sufficiently strong that stable circular orbits cannot exist. General relativistic ray tracing simulations show how signatures of material inside the ISCO, plunging into the black hole, are manifested in observations of X-ray reverberation. Simulations reveal how emission specifically reverberating off of material in the plunging region may be detected with the next generation X-ray observatories.

The ability to directly detect the presence of an innermost stable orbit and plunging region would provide a unique test of general relativity in the strong field limit, only accessible around black holes. Probing the dynamics of material in the plunging region will reveal how the accretion flow behaves in its final moments and how it may launch jets, accelerate coronae and power some of the most extreme systems in the Universe.

Venturing Beyond the ISCO: Probing the black hole plunging region

17th American Astronomical Society HEAD Meeting, Monterey, CA (20/03/2019)

The immediate vicinities of black holes represent some of the most extreme environments in the Universe, where accreting material in its final moments before plunging through the event horizon powers some of the most luminous sources in the Universe; bright X-ray emitting coronae and vast jets launched close to the speed of light. General relativity predicts that in close proximity to the black hole, within the innermost stable circular orbit (ISCO), gravity is sufficiently strong that stable circular orbits cannot exist and upon reaching this radius, material within the accretion disc must plunge into the black hole. In recent years, the advent of X-ray timing studies has revealed unprecedented amounts of information about the extreme environments around black holes. In particular the detection of the coronal X-ray emission reverberating off the inner regions of the accretion disc has revealed the geometry and the dynamic nature of the corona. The next step is to detect the material within the ISCO and understand its dynamics as it plunges into the black hole. Analysis typically assumes that no X-rays are detected from material within the ISCO. General relativistic ray tracing simulations, however, show how signatures of material inside the ISCO, plunging into the black hole are manifested in observations of X-ray reverberation. Simulations reveal how emission specifically reverberating off of material in the plunging region may be detected with the next generation of X-ray observatories such as Athena, and specialized X-ray timing missions such as STROBE-X. The ability to directly detect the presence of an innermost stable orbit and plunging region would provide a unique test of general relativity in the strong field limit, only accessible around black holes and would be an important component in validating black hole spin measurements. Probing the dynamics of material in the plunging region will reveal how the accretion flow behaves in its final moments and how it may launch jets, accelerate coronae and power some of the most extreme systems in the Universe.

Mapping supermassive black holes with X-ray reverberation

17th American Astronomical Society HEAD Meeting, Monterey, CA (19/03/2019) — invited

From the reverberation of X-rays off the innermost regions the accretion disc, a three-dimensional picture is starting to emerge of the extreme environments around black holes; the structure of the disc and the corona that produces intense X-ray emission, as well of how the launching sites of jets may be connected to the corona and inner disc. Spectral timing analysis of accreting black holes, compared to the predictions of general relativistic ray tracing simulations, enables the structure of the corona and accretion disc to be mapped. We discover how the corona evolves on long and short timescales, giving rise to orders of magnitude variation in luminosity. X-ray reverberation studies are revealing, for the first time, structure within the corona including a persistent collimated core akin to the base of a jet, even in radio-quiet sources, alongside a second component associated with the accretion disc itself.This gives us important insight into the small-scale processes close to the event horizon that black holes to power some of the most luminous objects in the Universe, launch vast jets at close to the speed of light and play their important feedback role in the formation of structure in the Universe.

Mapping the extreme environments around black holes

42nd COSPAR Assembly, Pasadena, CA (20/07/2018)

From the reflection and reverberation of X-rays from the innermost regions of AGN accretion discs, a three-dimensional picture is starting to emerge of the structure of the disc and the corona producing the intense X-ray emission, as well of how the launching sites of jets may be connected to the corona and inner disc. 

Spectral timing analysis of AGN observed by the great X-ray observatories, compared to the predictions of general relativistic ray tracing simulations, reveals not only the mechanisms of variability in the X-ray emission, but enables the structure of the corona and accretion disc to be mapped. We discover how the corona evolves on long and short timescales, giving rise to orders of magnitude variation in luminosity as well as the processes the corona can undergo during transient events, most notably the collimation and ejection of portions of the corona during X-ray flares, reminiscent of the aborted launching of a jet.

The latest X-ray reverberation studies are revealing, for the first time, structure within the corona including a persistent collimated core akin to the base of a jet, even in radio-quiet sources, alongside a second component associated with the accretion disc itself.

This gives us important insight into the small-scale processes close to the event horizon that enable supermassive black holes to power some of the most luminous objects in the Universe, launch vast jets at close to the speed of light and play their important feedback role in the formation of structure in the Universe.

Revealing structure and the jet connection within AGN coronae

29th Texas Symposium on Relativistic Astrophysics, Cape Town, South Africa (06/12/2017)

Detailed analysis of the reflection and reverberation of X-rays from the innermost regions of AGN accretion discs are revealing, for the first time, the structure and processes within the corona that produces the intense X-ray continuum emission and the extreme variability that is seen.

Observations of AGN by the great X-ray observatories, compared to the predictions of general relativistic ray tracing simulations, have enabled the measurement of the geometry of the corona. They show how it evolves on long and short timescales, giving rise to orders of magnitude variation in luminosity and reveal processes the corona can undergo during transient events, most notably the collimation and ejection of portions of the corona during X-ray flares, reminiscent of the aborted launching of a jet.

The latest X-ray reverberation studies are revealing, for the first time, structure within the corona including a persistent collimated core akin to the base of a jet, even in radio-quiet sources, alongside a second component associated with the accretion disc itself.

This gives us important insight into the processes by which energy is liberated during accretion and a picture is starting to emerge of how these extreme objects are powered and even how jets may be launched.

Flaring and jet launching from supermassive black holes

2016 NuSTAR Science Meeting, Pasadena, CA (16/11/2016)

Beyond the lamppost: Probing the structure of the extended corona

16th American Astronomical Society HEAD Meeting, Sun Valley, ID (23/08/2017) — invited

Revealing structure within the coronae of Seyfert galaxies

The X-ray Universe 2017, Rome, Italy (08/06/2017)

Detailed analysis of the reflection and reverberation of X-rays from the innermost regions of AGN accretion discs reveals the structure and processes that produce the intense continuum emission and the extreme variability we see, right down to the innermost stable orbit and event horizon of the black hole. Observations of Seyfert galaxies spanning more than a decade have enabled measurement of the geometry of the corona and how it evolves, leading to orders of magnitude of variability. They reveal processes the corona undergoes during transient events, notably the collimation and ejection of the corona during X-ray flares, reminiscent of the aborted launching of a jet. Recent reverberation studies, including those of the Seyfert galaxy I Zwicky 1 with XMM-Newton, are revealing structures within the corona for the first time. A persistent collimated core is found, akin to the base of a jet embedded in the innermost regions. The evolution of both the collimated and extended portions point to the mechanisms powering the X-ray emission and variability. This gives us important constraints on the processes by which energy is liberated from black hole accretion flows and by which jets are launched, allowing us to understand how these extreme objects are powered.

Beyond the Lamppost: Lag spectra arising from extended coronae

Lorentz Center Workshop ‘The X-ray Spectral Timing Revolution,’ Leiden, The Netherlands (02/02/2016) — invited

Driving extreme variability: Evolving coronae and evidence for jet launching in AGN

2015 XMM-Newton Science Workshop – The Extremes of Black Hole Accretion, Madrid, Spain (10/06/2015)

The evolving corona and evidence for jet launching from the supermassive black hole in Markarian 335

225th AAS Meeting, Seattle, WA (07/01/2015)

Driving Extreme Variability: Measuring Evolving AGN Coronae

14th AAS HEAD Meeting, Chicago, IL (20/08/2014)

X-ray Reflection as a Probe of Accreting Black Holes

223rd AAS Meeting, Washington DC (09/01/2014) — dissertation talk

Probing Accreting Black Holes with X-ray Reflection – New observational tools?

RAS National Astronomy Meeting, St. Andrews, UK (02/07/2013)
First UK LOFT Science Meeting, RAS, London (24/06/2013)

Understanding X-ray reflection in AGN

Athena Science Workshop, MPE, Garching (14/06/2011)

The emissivity profiles of AGN accretion discs

High Energy View of Accreting Objects (AGN Session), Crete (05/10/2010)