Luminous accreting stellar mass and supermassive black holes produce power-law continuum X-ray emission from a compact central corona. Reverberation time lags occur due to light travel time-delays between changes in the direct coronal emission and corresponding variations in its reflection from the accretion flow. Reverberation is detectable using light curves made in different X-ray energy bands, since the direct and reflected components have different spectral shapes. Larger, lower frequency, lags are also seen and are identified with propagation of fluctuations through the accretion flow and associated corona. We review the evidence for X-ray reverberation in active galactic nuclei and black hole X-ray binaries, showing how it can be best measured and how it may be modelled. The timescales and energy-dependence of the high frequency reverberation lags show that much of the signal is originating from very close to the black hole in some objects, within a few gravitational radii of the event horizon. We consider how these signals can be studied in the future to carry out X-ray reverberation mapping of the regions closest to black holes.

The focussing optics of NuSTAR have enabled high signal-to-noise spectra to be obtained from many X-ray bright Active Galactic Nuclei (AGN) and Galactic Black Hole Binaries (BHB). Spectral modelling then allows robust characterization of the spectral index and upper energy cutoff of the coronal power-law continuum, after accounting for reflection and absorption effects. Spectral-timing studies, such as reverberation and broad iron line fitting, of these sources yield coronal sizes, often showing them to be small and in the range of 3 to 10 gravitational radii in size. Our results indicate that coronae are hot and radiatively compact, lying close to the boundary of the region in the compactness -temperature (Θ − ℓ) diagram which is forbidden due to runaway pair production. The coincidence suggests that pair production and annihilation are essential ingredients in the coronae of AGN and BHB and that they control the shape of the observed spectra.

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The geometry of the accretion flow around stellar-mass black holes can change on timescales of days to months 1-3 . When a black hole emerges from quiescence it has a very hard X-ray spectrum produced by a hot corona 4, 5 , and then transitions to a soft spectrum dominated by emission from a geometrically thin accretion disc extending to the innermost stable circular orbit 6, 7 .Much debate, however, persists over how this transition occurs, whether it is driven largely by a reduction in the truncation radius of the disc 8, 9 or in the spatial extent of the corona 10, 11 . Observations of X-ray reverberation lags in supermassive black hole systems 12, 13 suggest that the corona is compact and that the disc extends in close to the central black hole 14, 15 . Observations of stellar mass black holes, however, reveal equivalent (mass-scaled) reverberation lags that are much larger 16 , leading to the suggestion that the accretion disc in the hard state of stellar mass black holes is truncated out to hundreds of gravitational radii 17, 18 . Here we report X-ray observations of the new black hole transient MAXI J1820+070 19, 20 . We find that the reverberation time lags between the continuum-emitting corona and the irradiated accretion disc are 6-20 times shorter than previously seen. The timescale of the reverberation lags shortens by an order of magnitude over a period of weeks, while the shape of the broadened iron K emission line remains remarkably constant. This suggests a reduction in the spatial extent of the corona, rather than a change in the inner edge of the accretion disc.MAXI J1820+070 19 (ASASSN-18ey 21 ) was discovered on 2018 March 11 with the Monitor of All-sky X-ray Image (MAXI) on board the International Space Station. The next day, the Neutron star Interior Composition Explorer (NICER) 22 started obtaining detailed observations and has continued observing since, at a cadence of 1-3 days 20 . The NICER X-ray Timing Instrument consists of an aligned collection of 52 active paired X-ray "concentrator" optics and silicon drift detectors, which record the arrival times and energies of individual X-ray photons. It provides a timing resolution of < 100 ns (25x faster than NASA's previous best X-ray timing instrument, the Rossi X-ray Timing Explorer) and the highest ever soft band peak effective area of 1900 cm 2 (nearly twice that of timing-capable EPIC-pn camera on XMM-Newton), all while providing good spectral resolution (145 eV at 6 keV), minimal pile-up on bright sources and very little deadtime. MAXI J1820+070 regularly reached 25000 counts/s in NICER's 0.2-12 2/23 keV band, while still providing high-fidelity spectral and timing products (for comparison, the XMM-Newton detectors become piled up for count rates of 600-800 counts/s 23 ). This high count rate allows us to probe timescales that are nearly an order of magnitude shorter than possible with XMM-Newton.Due to the enormity of the dataset, in this letter, we only describe the spectral-timing results of a subset of the total NICER observations ...

X-ray reverberation, where light-travel time delays map out the compact geometry around the inner accretion flow in supermassive black holes, has been discovered in several of the brightest, most variable and well-known Seyfert galaxies. In this work, we expand the study of X-ray reverberation to all Seyfert galaxies in the XMMNewton archive above a nominal rms variability and exposure level (a total of 43 sources). ∼50 per cent of source exhibit iron K reverberation, in that the broad iron K emission line responds to rapid variability in the continuum. We also find that on long timescales, the hard band emission lags behind the soft band emission in 85 per cent of sources. This 'low-frequency hard lag' is likely associated with the coronal emission, and so this result suggests that most sources with X-ray variability show intrinsic variability from the nuclear region. We update the known iron K lag amplitude vs. black hole mass relation, and find evidence that the height or extent of the coronal source (as inferred by the reverberation time delay) increases with mass accretion rate.

Rapid detection of compact binary coalescence (CBC) with a network of advanced gravitational-wave detectors will offer a unique opportunity for multi-messenger astronomy. Prompt detection alerts for the astronomical community might make it possible to observe the onset of electromagnetic emission from CBC. We demonstrate a computationally practical filtering strategy that could produce early-warning triggers before gravitational radiation from the final merger has arrived at the detectors.

The recent detection of X-ray reverberation lags, especially in the Fe Kα line region, around Active Galactic Nuclei (AGN) has opened up the possibility of studying the time-resolved response (reflection) of hard X-rays from the accretion disk around supermassive black holes. Here, we use general relativistic transfer functions for reflection of X-rays from a point source located at some height above the black hole to study the time lags expected as a function of frequency and energy in the Fe Kα line region. We explore the models and the dependence of the lags on key parameters such as the height of the X-ray source, accretion disk inclination, black hole spin and black hole mass. We then compare these models with the observed frequency and energy dependence of the Fe Kα line lag in NGC 4151. Assuming the optical reverberation mapping mass of 4.6 × 10 7 M ⊙ we get a best fit to the lag profile across the Fe Kα line in the frequency range (1 − 2) × 10 −5 Hz for an X-ray source located at a height h = 7 +2.9 −2.6 R G with a maximally spinning black hole and an inclination i < 30 • .

MAXI J1820+070 (optical counterpart ASASSN-18ey) is a black hole candidate discovered through its recent very bright outburst. The low extinction column and long duration at high flux allow detailed measurements of the accretion process to be made. In this work, we compare the evolution of X-ray spectral and timing properties through the initial hard state of the outburst. We show that the inner accretion disc, as measured by relativistic reflection, remains steady throughout this period of the outburst. Nevertheless, subtle spectral variability is observed, which is well explained by a change in coronal geometry. However, characteristic features of the temporal variability -lowfrequency roll-over and QPO frequency -increase drastically in frequency, as the outburst proceeds. This suggests that the variability timescales are governed by coronal conditions rather than solely by the inner disc radius. We also find a strong correlation between X-ray luminosity and coronal temperature. This can be explained by electron pair production with a changing effective radius and a non-thermal electron fraction of ∼ 20%.