G. Miniutti

A light bending model for the X-ray temporal and spectral properties of accreting black holes

Some of the X-ray temporal and spectral properties of accreting black holes represent a challenge for current theoretical models. In particular, uncorrelated variability between direct continuum and reflection components (including the iron line, if present) has been reported in many cases. Here, we explore a light bending model in which we assume a primary source of X-rays located close to a central, maximally rotating Kerr black hole and illuminating both the observer at infinity and the accretion disc. We show that, due to strong light bending, the observed flux can vary by more than one order of magnitude as the height of the primary source above the accretion disc varies, even if its intrinsic luminosity is constant. We identify three different regimes in which the reflection-dominated component (and the iron line) is correlated, anticorrelated or almost independent with respect to the direct continuum. These regimes correspond to low, high and intermediate flux states of the X-ray source. As a general rule, the reflection component varies with much smaller amplitude than the continuum. X-ray observations of the Seyfert galaxy MCG-6-30-15 and of the Galactic black hole candidate XTE Jl 650-500 reveal that a series of predictions of our model is actually observed; the consistent behaviour of the iron line flux and equivalent width with respect to the direct continuum, as well as the increase of the relative strength of disc reflection as the flux drops, all match very well our predictions. The iron line profile is predicted to be narrower in high flux states and broader in (reflection-dominated) low flux states, in fairly good agreement with observations of the best-studied case of MCG-6-30-15. Observations of some other narrow-line Seyfert 1 galaxies (e.g. NGC 4051) also seem to support our model, which may explain what are otherwise puzzling characteristics of some sources. We also show that beaming along the equatorial plane can enhance the re-emission of narrow reflection features from distant material during low flux states providing a possible contribution to the observed X-ray Baldwin effect.

Broad line emission from iron K- and L-shell transitions in the active galaxy 1H 0707-495

Since the 1995 discovery of the broad iron K-line emission from the Seyfert galaxy MCG–6-30-15 (ref. 1), broad iron K lines have been found in emission from several other Seyfert galaxies, from accreting stellar-mass black holes and even from accreting neutron stars. The iron K line is prominent in the reflection spectrum created by the hard-X-ray continuum irradiating dense accreting matter. Relativistic distortion of the line makes it sensitive to the strong gravity and spin of the black hole. The accompanying iron L-line emission should be detectable when the iron abundance is high. Here we report the presence of both iron K and iron L emission in the spectrum of the narrow-line Seyfert 1 galaxy 1H 0707-495. The bright iron L emission has enabled us to detect a reverberation lag of about 30 s between the direct X-ray continuum and its reflection from matter falling into the black hole. The observed reverberation timescale is comparable to the light-crossing time of the innermost radii around a supermassive black hole. The combination of spectral and timing data on 1H 0707-495 provides strong evidence that we are witnessing emission from matter within a gravitational radius, or a fraction of a light minute, from the event horizon of a rapidly spinning, massive black hole.

Broad iron L line and X-ray reverberation in 1H0707-495

A detailed analysis of a long XMM-Newton observation of the narrow-line type 1 Seyfert galaxy 1H0707-495 is presented, including spectral fitting, spectral variability and timing studies. The two main features in the spectrum are the drop at ?7?keV and a complex excess below 1?keV. These are well described by two broad, K and L, iron lines. Alternative models based on absorption, although they may fit the high-energy drop, cannot account for the 1?keV complexity and the spectrum as a whole. Spectral variability shows that the spectrum is composed of at least two components, which are interpreted as a power law dominating between 1-4?keV and a reflection component outside this range. The high count rate at the iron L energies has enabled us to measure a significant soft lag of ?30 s between 0.3-1 and 1-4?keV, meaning that the direct hard emission leads the reflected emissions. We interpret the lag as a reverberation signal originating within a few gravitational radii of the black hole.

STELLAR-MASS BLACK HOLE SPIN CONSTRAINTS FROM DISK REFLECTION AND CONTINUUM MODELING

Accretion disk reflection spectra, including broad iron emission lines, bear the imprints of the strong Doppler shifts and gravitational redshifts close to black holes. The extremity of these shifts depends on the proximity of the innermost stable circular orbit to the black hole, and that orbit is determined by the black hole spin parameter. Modeling relativistic spectral features, then, gives a means of estimating black hole spin. We report on the results of fits made to archival X-ray spectra of stellar-mass black holes and black hole candidates, selected for strong disk reflection features. Following recent work, these spectra were fit with reflection models and disk continuum emission models (where required) in which black hole spin is a free parameter. Although our results must be regarded as preliminary, we find evidence for a broad range of black hole spin parameters in our sample. The black holes with the most relativistic radio jets are found to have high spin parameters, though jets are observed in a black hole with a low spin parameter. For those sources with constrained binary system parameters, we examine the distribution of spin parameters versus black hole mass, binary mass ratio, and orbital period. We discuss the results within the context of black hole creation events, relativistic jet production, and efforts to probe the innermost relativistic regime around black holes.

The lack of variability of the iron line in MCG-6-30-15: general relativistic effects

ABSTRACT The spectrum and variability of the Seyfert galaxy MCG–6-30-15 can be decomposedinto two apparently disconnected components: a highly variable power law and analmost constant component which contains a broad and strong iron line. We explorea possible explanation of the puzzling lack of variability of the iron line, by assumingthat the variations of the power law component are due to changes in the height ofthe primary source in the near vicinity of a rotating black hole. Due to the bendingof light in the strong field of the central black hole, the apparent brightness of thepower-law component can vary by about a factor 4 according to its position, while thetotal iron line flux variability is less than 20 per cent. This behaviour is obtained ifthe primary source is located within 3–4 gravitational radii (r g ) from the rotation axiswith a variable height of between ∼ 3 and 8 r g . These results revive the possibility thatfuture X–ray observations of MCG–6-30-15 can map out the strong gravity regime ofaccreting black holes.Key words: accretion discs — black hole physics — line: profiles — X-rays: general

X-ray reflection in the narrow-line Seyfert 1 galaxy 1H 0707-495

We apply a reflection-dominated model to the second XMM-Newton observation of the Narrow Line Seyfert 1 galaxy 1H 0707-495. As in the first XMM-Newton observation a sharp spectral drop is detected with energy that has shifted from 7 keV to 7.5 keV in two years. The drop is interpreted in terms of relativistically blurred ionised reflection from the accretion disc, while the energy shift can be accounted for by changes in the ionisation state and, more importantly, emissivity profile on the disc. A flatter emissivity profile during the second higher flux observation reduces gravitational redshift effects, therefore shifting the edge to higher energy. Remarkably, ionised disc reflection and the associated power law continuum provide a reasonable description of the broadband spectrum, including the soft excess. Considering both observations, the spectral variability in 1H 0707-495 appears to be due to the interplay between these two spectral components. The reflection component in the second observation is significantly less variable than the power law. Changes of the emissivity profile, spectral shape and variability properties (such as the rms spectrum) within the two observations are all consistent with a recently proposed model in which relativistic effects in the very inner regions of the nucleus play a major role.

Discovery of a relation between black hole mass and soft X-ray time lags in active galactic nuclei

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.

A systematic look at the very high and low/hard state of GX 339-4 : constraining the black hole spin with a new reflection model

We present a systematic study of GX 339−4 in both its very high and low hard states from simultaneous observations made with XMM‐Newton and RXTE in 2002 and 2004. The X-ray spectra of both these extreme states exhibit strong reflection signatures, with a broad, skewed Fe Kα line clearly visible above the continuum. Using a newly developed, self-consistent reflection model which implicitly includes the blackbody radiation of the disc as well as the effect of Comptonization, blurred with a relativistic line function, we were able to infer the spin parameter of GX 339−4 to be 0.935 ± 0.01 (statistical) ±0.01 (systematic) at 90 per cent confidence. We find that both states are consistent with an ionized thin accretion disc extending to the innermost stable circular orbit around the rapidly spinning black hole.

A prominent accretion disk in the low-hard state of the black hole candidate SWIFT J1753.5-0127

We report on simultaneous XMM-Newton and RXTE observations of the stellar mass black hole candidate SWIFT J1753.5-0127. The source was observed in the low-hard state, during the decline of a hard outburst. The inner accretion disk is commonly assumed to be radially truncated in the low-hard state, and it has been suggested that this property may be tied to the production of steady, compact jets. Fits to the X-ray spectra of SWIFT J1753.5-0127 with a number of simple models clearly reveal a cool (kT 0.2 keV) accretion disk. The disk component is required at more than the 8 σ level of confidence. Although estimates of inner disk radii based on continuum spectroscopy are subject to considerable uncertainty, fits with a number of models suggest that the disk is observed at or close to the innermost stable circular orbit. Recently, an observation of GX 339-4 revealed a disk extending to the innermost stable circular orbit at LX/LEdd 0.05; our results from SWIFT J1753.5-0127 may extend this finding down to LX/LEdd 0.003(d/8.5 kpc)2(M/10 M☉). We discuss our results within the context of low-luminosity accretion flow models and disk-jet connections.

Flux and energy modulation of redshifted iron emission in NGC 3516: implications for the black hole mass

We report on the tentative detection of the modulation of a transient, redshifted Fe K emission feature in the X-ray spectrum of the Seyfert galaxy NGC 3516. The detection of the spectral feature at 6.1 keV, in addition to a stable 6.4-keV line, has been reported previously. We find, on reanalysing the XMM—Newton data, that the feature varies systematically in flux at intervals of 25 ks. The peak moves in energy between 5.7 and 6.5 keV. The spectral evolution of the feature agrees with Fe K emission arising from a spot on the accretion disc, illuminated by a corotating flare located at a radius of (7—16) rg, modulated by Doppler and gravitational effects as the flare orbits around the black hole. Combining the orbital time-scale and the location of the orbiting flare, the mass of the black hole is estimated to be (1—5) ◊ 10 7 M , which is in good agreement with values obtained from other techniques. Ke yw ords: line: profiles — relativity — galaxies: active — galaxies: individual: NGC 3516 — X-rays: galaxies.