The geometry of the accretion flow around stellar-mass black holes can change on timescales of days to months1–3. When a black hole emerges from quiescence (that is, it ‘turns on’ after accreting ...material from its companion) it has a very hard (high-energy) X-ray spectrum produced by a hot corona4,5 positioned above its accretion disk, and then transitions to a soft (lower-energy) spectrum dominated by emission from the geometrically thin accretion disk, which extends to the innermost stable circular orbit6,7. Much debate persists over how this transition occurs and whether it is driven largely by a reduction in the truncation radius of the disk8,9 or by a reduction in the spatial extent of the corona10,11. Observations of X-ray reverberation lags in supermassive black-hole systems12,13 suggest that the corona is compact and that the disk extends nearly to the central black hole14,15. Observations of stellar-mass black holes, however, reveal equivalent (mass-scaled) reverberation lags that are much larger16, leading to the suggestion that the accretion disk in the hard-X-ray state of stellar-mass black holes is truncated at a few hundreds of gravitational radii from the black hole17,18. Here we report X-ray observations of the black-hole transient MAXI J1820+07019,20. We find that the reverberation time lags between the continuum-emitting corona and the irradiated accretion disk are 6 to 20 times shorter than previously seen. The timescale of the reverberation lags shortens by an order of magnitude over a period of weeks, whereas 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 disk.
We carried out a systematic analysis of time lags between X-ray energy bands in a large sample (32 sources) of unabsorbed, radio quiet active galactic nuclei (AGN), observed by XMM-Newton. The ...analysis of X-ray lags (up to the highest/shortest frequencies/time-scales), is performed in the Fourier-frequency domain, between energy bands where the soft excess (soft band) and the primary power law (hard band) dominate the emission. We report a total of 15 out of 32 sources displaying a high-frequency soft lag in their light curves. All 15 are at a significance level exceeding 97 per cent and 11 are at a level exceeding 99 per cent. Of these soft lags, seven have not been previously reported in the literature, thus this work significantly increases the number of known sources with a soft/negative lag. The characteristic time-scales of the soft/negative lag are relatively short (with typical frequencies and amplitudes of ν ∼ 0.07-4 × 10−3 Hz and τ ∼ 10-600 s, respectively), and show a highly significant ( 4σ) correlation with the black hole mass. The measured correlations indicate that soft lags are systematically shifted to lower frequencies and higher absolute amplitudes as the mass of the source increases. To first approximation, all the sources in the sample are consistent with having similar mass-scaled lag properties. These results strongly suggest the existence of a mass-scaling law for the soft/negative lag, that holds for AGN spanning a large range of masses (about 2.5 orders of magnitude), thus supporting the idea that soft lags originate in the innermost regions of AGN and are powerful tools for testing their physics and geometry.
NGC 4151 is the brightest Seyfert 1 nucleus in X-rays. It was the first object to show short time delays in the Fe K band, which were attributed to relativistic reverberation, providing a new tool ...for probing regions at the black hole scale. Here we report the results of a large XMM-Newton campaign in 2015 to study these short delays further. Analyzing high-quality data that span timescales between hours and decades, we find that neutral and ionized absorption contributes significantly to the spectral shape. Accounting for their effects, we find no evidence for a relativistic reflection component, contrary to early work. Energy-dependent lags are significantly measured in the new data, but with an energy profile that does not resemble a broad iron line, in contrast to the old data. The complex lag-energy spectra, along with the lack of strong evidence for a relativistic spectral component, suggest that the energy-dependent lags are produced by absorption effects. The long-term spectral variations provide new details on the variability of the narrow Fe K line. We find that its variations are correlated with, and delayed with respect to, the primary X-ray continuum. We measure a delay of days, implying an origin in the inner broad-line region (BLR). The delay is half the Hβ line delay, suggesting a geometry that differs slightly from the optical BLR.
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 XMM–Newton archive above a nominal rms variability and exposure level (a total of 43 sources). Approximately 50 per cent of sources 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 time-scales, 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 versus 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.
We use archival XMM-Newton observations of Ark 564 and Mrk 335 to calculate the frequency-dependent time lags for these two well-studied sources. We discover high-frequency Fe K lags in both sources, ...indicating that the red wing of the line precedes the rest-frame energy by roughly 100 and 150 s for Ark 564 and Mrk 335, respectively. Including these two new sources, Fe K reverberation lags have been observed in seven Seyfert galaxies. We examine the low-frequency lag-energy spectrum, which is smooth, and shows no feature of reverberation, as would be expected if the low-frequency lags were produced by distant reflection off circumnuclear material. The clear differences in the low- and high-frequency lag-energy spectra indicate that the lags are produced by two distinct physical processes. Finally, we find that the amplitude of the Fe K lag scales with black hole mass for these seven sources, consistent with a relativistic reflection model where the lag is the light travel delay associated with reflection of continuum photons off the inner disc.
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 as a pronounced dip in the reverberation lag at 3 keV. 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 time-scale 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 time-scale 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.
Recent X-ray observations have enabled the study of reverberation delays in active galactic nuclei (AGN) for the first time. All the detections so far are in sources with a strong soft excess, and ...the measured delay is between the hard (1-3 keV) direct continuum and the soft excess (0.5-1 keV), interpreted as the reflection continuum smeared by relativistic effects. There is however an inherent ambiguity in identifying and studying the details of the lines in the soft excess. Here we report the first detection of reverberation in the iron K band in any AGN. Using XMM-Newton observations of NGC 4151, we find delays of the order of 2000 s on time-scales of 105 s between the
keV band and the
and
keV bands, with a broad lag profile resembling a relativistically broadened iron line. The peak of the lag spectra shifts to lower energies at higher frequencies, consistent with the red wing of the line being emitted at smaller radii, as expected from reflection off the inner accretion disc. This is a first detection of a broad iron line using timing studies.
We present the drastic transformation of the X-ray properties of the active galactic nucleus (AGN) 1ES 1927+654, following a changing-look event. After the optical/ultraviolet outburst the power-law ...component, produced in the X-ray corona, disappeared, and the spectrum of 1ES 1927+65 instead became dominated by a blackbody component (kT ∼ 80-120 eV). This implies that the X-ray corona, ubiquitously found in AGNs, was destroyed in the event. Our dense ∼450 days long X-ray monitoring shows that the source is extremely variable in the X-ray band. On long timescales the source varies up to ∼4 dex in ∼100 days, while on short timescales up to ∼2 dex in ∼8 hr. The luminosity of the source is found to first show a strong dip down to , and then a constant increase in luminosity to levels exceeding the pre-outburst level 300 days after the optical event detection, rising up asymptotically to . As the X-ray luminosity of the source increases, the X-ray corona is recreated, and a very steep power-law component (Γ 3) reappears, and dominates the emission for 0.3-2 keV luminosities , ∼300 days after the beginning of the event. We discuss possible origins of this event, and speculate that our observations could be explained by the interaction between the accretion flow and debris from a tidally disrupted star. Our results show that changing-look events can be associated with dramatic and rapid transformations of the innermost regions of accreting supermassive black holes.
Swift intensive accretion disk reverberation mapping of four AGN yielded light curves sampled ∼200-350 times in 0.3-10 keV X-ray and six UV/optical bands. Uniform reduction and cross-correlation ...analysis of these data sets yields three main results: (1) The X-ray/UV correlations are much weaker than those within the UV/optical, posing severe problems for the lamp-post reprocessing model in which variations in a central X-ray corona drive and power those in the surrounding accretion disk. (2) The UV/optical interband lags are generally consistent with as predicted by the centrally illuminated thin accretion disk model. While the average interband lags are somewhat larger than predicted, these results alone are not inconsistent with the thin disk model given the large systematic uncertainties involved. (3) The one exception is the U band lags, which are on average a factor of ∼2.2 larger than predicted from the surrounding band data and fits. This excess appears to be due to diffuse continuum emission from the broad-line region (BLR). The precise mixing of disk and BLR components cannot be determined from these data alone. The lags in different AGN appear to scale with mass or luminosity. We also find that there are systematic differences between the uncertainties derived by JAVELIN versus more standard lag measurement techniques, with JAVELIN reporting smaller uncertainties by a factor of 2.5 on average. In order to be conservative only standard techniques were used in the analyses reported herein.
After 26 yr in outburst, the black hole X-ray binary GRS 1915+105 dimmed considerably in early 2018; its flux dropped sharply in mid-2019, and it has remained faint ever since. This faint period, the ..."obscured state," is punctuated by occasional X-ray flares, many of which have been observed by NICER as part of our regular monitoring program. Here we present detailed time-resolved spectroscopy of one bright flare, whose spectrum shows evidence of high column density partial covering absorption and extremely deep absorption lines (equivalent widths over 100 eV in some cases). We study the time-dependent ionization of the obscuring gas with xstar, ultimately attributing the absorption to a radially stratified absorber of density ∼10^(12)–10^(13) per cu.cm at ∼few ×10^(11) cm from the black hole. We argue that a vertically extended outer disk could explain this obscuration. We discuss several scenarios to explain the obscured state, including massive outflows, an increase in the mass accretion rate, and changes in the outer disk that herald the approach of quiescence, but none are entirely satisfactory. Alternative explanations, such as obscuration by the accretion stream impact point, may be testable with current or future data.
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