4U 1957+11: a persistent low-mass X-ray binary and black hole candidate in the high state?

We report on several pointed Rossi X-ray Timing Explorer observations of the enigmatic low-mass X-ray binary (LMXB) 4U 1957+11 at different X-ray luminosities. The luminosity of the source varied by more than a factor of 4 on time-scales of months to years. The spectrum of the source tends to become harder when its luminosity increases. Only very weak  (1–2  per cent rms amplitude,  0.001–10 Hz  ,  2–60 keV)  rapid X-ray variability was observed during the observations. A comparison of the spectral and temporal behaviour of 4U 1957+11 with other X-ray binary systems, in particular LMC X-3, indicates that 4U 1957+11 is likely to be a persistent LMXB harbouring a black hole and it is persistently in the black hole high state. If confirmed, it would be the only such system known.     

Application of a relativistic accretion disc model to X-ray spectra of LMC X-1 and GRO J1655-40

We present a general relativistic accretion disc model and its application to the soft-state X-ray spectra of black hole binaries. The model assumes a flat, optically thick disc around a rotating Kerr black hole. The disc locally radiates away the dissipated energy as a blackbody. Special and general relativistic effects influencing photons emitted by the disc are taken into account. The emerging spectrum, as seen by a distant observer, is parametrized by the black hole mass and spin, the accretion rate, the disc inclination angle and the inner disc radius.
We fit the ASCA soft-state X-ray spectra of LMC X-1 and GRO J1655-40 by this model. We find that, having additional limits on the black hole mass and inclination angle from optical/UV observations, we can constrain the black hole spin from X-ray data. In LMC X-1 the constraint is weak, and we can only rule out the maximally rotating black hole. In GRO J1655-40 we can limit the spin much better, and we find 0.68 a 0.88 . Accretion discs in both sources are radiation-pressure dominated. We do not find Compton reflection features in the spectra of any of these objects.     

A model of magnetically induced disc–corona for black hole binaries

We propose a model of magnetic connection (MC) of a black hole with its surrounding accretion disc based on large-scale magnetic field. The MC gives rise to transport of energy and angular momentum between the black hole and the disc, and the closed field lines pipe the hot matter evaporated from the disc, and shape it in the corona above the disc to form a magnetically induced disc–corona system, in which the corona has the same configuration as the large-scale magnetic field. We numerically solve the dynamic equations in the context of the Kerr metric, in which the large-scale magnetic field is determined by dynamo process and equipartition between magnetic pressure and gas pressure. Thus we can obtain a global solution rather than assuming the distribution of large-scale magnetic field beforehand. The main MC effects lie in three aspects. (1) The rotational energy of a fast-spinning black hole can be extracted, enhancing the dissipation in the accretion disc, (2) the closed field lines provide a natural channel for corona matter escaping from disc and finally falling into black hole and (3) the scope of the corona can be bounded by the conservation of magnetic flux. We simulate the high-energy spectra of this system by using Monte Carlo method, and find that the relative hardness of the spectra decreases as accretion rate or black hole spin a _* increases. We fit the typical X-ray spectra of three black hole binaries  (GRO J1655−40, XTE 1118+480 and GX 339−4)  in the low/hard or very high state.     

Thermal synchrotron radiation and its Comptonization in compact X-ray sources

We investigate the process of synchrotron radiation from thermal electrons at semirelativistic and relativistic temperatures. We find an analytic expression for the emission coefficient for random magnetic fields with an accuracy significantly higher than those derived previously. We also present analytic approximations to the synchrotron turnover frequency, treat Comptonization of self-absorbed synchrotron radiation, and give simple expressions for the spectral shape and the emitted power. We also consider modifications of the above results by bremsstrahlung.
We then study the importance of Comptonization of thermal synchrotron radiation in compact X-ray sources. We first consider emission from hot accretion flows and active coronae above optically thick accretion discs in black hole binaries and active galactic nuclei (AGNs). We find that for plausible values of the magnetic field strength, this radiative process is negligible in luminous sources, except for those with hardest X-ray spectra and stellar masses. Increasing the black hole mass results in a further reduction of the maximum Eddington ratio from this process. Then, X-ray spectra of intermediate-luminosity sources, e.g. low-luminosity AGNs, can be explained by synchrotron Comptonization only if they come from hot accretion flows, and X-ray spectra of very weak sources are always dominated by bremsstrahlung. On the other hand, synchrotron Comptonization can account for power-law X-ray spectra observed in the low states of sources around weakly magnetized neutron stars.     

High-energy X-ray spectra of Seyferts and Unification schemes for active galactic nuclei

The Unified Model of active galactic nuclei (AGN) predicts that the sole difference between type 1 and 2 Seyfert galaxies nuclei is the viewing angle with respect to an obscuring structure around the nucleus. High-energy photons above 20 keV are not affected by this absorption if the column is Compton thin, so their 30–100 keV spectra should be the same. However, the observed spectra at high energies appear to show a systematic difference, with type 1 Seyfert galaxies having Γ∼ 2.1 whereas type 2 Seyfert galaxies are harder with Γ∼ 1.9. We estimate the mass and the accretion rate of Seyferts detected in these high-energy samples, and show that they span a wide range in   L / L _Edd  . Both black hole binary systems and AGN show a correlation between spectral softness and Eddington fraction, so these samples are probably heterogeneous, spanning a range of intrinsic spectral indices which are hidden in individual objects by poor signal-to-noise ratio. However, the mean Eddington fraction for the type 1 Seyfert galaxies is higher than for the type 2 Seyfert galaxies, so the samples are consistent with this being the origin of the softer spectra seen in type 1 Seyfert galaxies. We stress that high-energy spectra alone are not necessarily a clean test of Unification schemes, but that the intrinsic nuclear properties should also change with   L / L _Edd  .     

On the influence of resonant absorption on the iron emission‐line profiles from accreting black holes

The fluorescent iron K α emission-line profile provides an excellent probe of the innermost regions of active galactic nuclei. Fe  xxv and Fe  xxvi in diffuse plasma above the accretion disc can affect the X-ray spectrum by iron K α resonant absorption. This in turn can influence the interpretation of the data and the estimation of the accretion disc and black hole parameters. We embark on a fully relativistic computation of this effect and calculate the iron line profile in the framework of a specific model in which rotating, highly ionized and resonantly absorbing plasma occurs close to the black hole. This can explain the features seen in the iron K α line profile recently obtained by Nandra et al. for the type 1 Seyfert galaxy NGC 3516. We show that the redshift of this feature can be mainly gravitational in origin and accounted for without the need to invoke fast accretion of matter on to the black hole. New X-ray satellites such as XMM , ASTRO-E and Chandra provide excellent opportunities to test the model against high-quality observational data.     

High-frequency X-ray variability as a mass estimator of stellar and supermassive black holes

There is increasing evidence that supermassive black holes in active galactic nuclei (AGN) are scaled-up versions of Galactic black holes. We show that the amplitude of high-frequency X-ray variability in the hard spectral state is inversely proportional to the black hole mass over eight orders of magnitude. We have analysed all available hard-state data from RXTE of seven Galactic black holes. Their power density spectra change dramatically from observation to observation, except for the high-frequency (≳10 Hz) tail, which seems to have a universal shape, roughly represented by a power law of index −2. The amplitude of the tail,   C _M   (extrapolated to 1 Hz), remains approximately constant for a given source, regardless of the luminosity, unlike the break or quasi-periodic oscillation frequencies, which are usually strongly correlated with luminosity. Comparison with a moderate-luminosity sample of AGN shows that the amplitude of the tail is a simple function of black hole mass,   C _M = C / M   , where   C ≈ 1.25 M_⊙ Hz⁻¹  . This makes   C _M   a robust estimator of the black hole mass which is easy to apply to low- to moderate-luminosity supermassive black holes. The high-frequency tail with its universal shape is an invariant feature of a black hole and, possibly, an imprint of the last stable orbit.     

Hard X-ray emission from elliptical galaxies

We report the detection of hard X-ray emission components in the spectra of six nearby, giant elliptical galaxies observed with the ASCA satellite. The systems studied, which exhibit strong dynamical evidence for supermassive black holes in their nuclei, are M87, NGC 1399 and NGC 4696 (the dominant galaxies of the Virgo, Fornax and Centaurus clusters, respectively) and NGC 4472, 4636 and 4649 (three further giant ellipticals in the Virgo cluster). The ASCA data for all six sources provide clear evidence for hard emission components, which can be parametrized by power-law models with photon indices in the range Γ=0.6–1.5 (mean value 1.2) and intrinsic 1–10 keV luminosities of 2×10⁴⁰–2×10⁴² erg s⁻¹. Our results imply the identification of a new class of accreting X-ray source, with X-ray spectra significantly harder than those of binary X-ray sources, Seyfert nuclei or low-luminosity active galactic nuclei, and bolometric luminosities relatively dominated by their X-ray emission. We discuss various possible origins for the hard X-ray emission and argue that it is most likely to be due to accretion on to the central supermassive black holes, via low radiative efficiency accretion flows coupled with strong outflows. In the case of M87, our detected power-law flux is in good agreement with a previously reported measurement from ROSAT High Resolution Imager observations, which were able to resolve the jet from the nuclear X-ray emission components. We confirm previous results showing that the use of multiphase models in the analysis of the ASCA data leads to determinations of approximately solar emission-weighted metallicities for the X-ray gas in the galaxies. We also present results on the individual element abundances in NGC 4636.     

Black hole masses from power density spectra: determinations and consequences

We analyse the scaling of the X-ray power density spectra with the mass of the black hole in the examples of Cyg X-1 and the Seyfert 1 galaxy NGC 5548. We show that the high-frequency tail of the power density spectrum can be successfully used for the determination of the black hole mass. We determine the masses of the black holes in six broad-line Seyfert 1 galaxies, five narrow-line Seyfert 1 galaxies and two quasi-stellar objects (QSOs) using the available power density spectra. The proposed scaling is clearly appropriate for other Seyfert galaxies and QSOs. In all but one of the normal Seyferts, the resulting luminosity to Eddington luminosity ratio is smaller than 0.15, with the source MCG -6-15-30 being an exception. The applicability of the same scaling to a narrow-line Seyfert 1 is less clear and there may be a systematic shift between the power spectra of NLS1 and S1 galaxies of the same mass, leading to underestimation of the black hole mass. However, both the method based on variability and the method based on spectral fitting show that those galaxies have relatively low masses and a high luminosity to Eddington luminosity ratio, supporting the view of those objects as analogues of galactic sources in their high, soft or very high state, based on the overall spectral shape. The bulge masses of their host galaxies are similar to that of normal Seyfert galaxies, so they do not follow the black hole mass–bulge mass relation for Seyfert galaxies, being evolutionarily less advanced, as suggested by Mathur. The bulge mass–black hole mass relation in our sample is consistent with being linear, with the black hole to bulge ratio ∼0.03 per cent, similar to Wandel and Laor for low-mass objects, but significantly shifted from the relation of Magorrian et al. and McLure & Dunlop.     

Linking jet emission and X-ray properties in the peculiar neutron star X-ray binary Circinus X-1

We present the results of simultaneous X-ray and radio observations of the peculiar Z-type neutron star X-ray binary Cir X-1, observed with the Rossi X-ray Timing Explorer satellite and the Australia Telescope Compact Array in 2000 October and 2002 December. We identify typical Z-source behaviour in the power density spectra as well as characteristic Z patterns drawn in an X-ray hardness–intensity diagram. Power spectra typical of bright atoll sources have also been identified at orbital phases after the periastron passage, while orbital phases before the periastron passage are characterized by power spectra that are typical neither of Z nor of atoll sources. We investigate the coupling between the X-ray and the radio properties, focusing on three orbital phases when an enhancement of the radio flux density has been detected, to test the link between the inflow (X-ray) and the outflow (radio jet) to/from the compact object. In two out of three cases, we associate the presence of the radio jet to a spectral transition in the X-rays, although the transition does not precede the radio flare, as detected in other Z sources. An analogous behaviour has recently been found in the black hole candidate GX 339-4. In the third case, the radio light curve shows a similar shape to the X-ray light curve. We discuss our results in the context of jet models, considering also black hole candidates.     

A short review of relativistic iron lines from stellar‐mass black holes

In this contribution, I briefly review recent progress in detecting and measuring the properties of relativistic iron lines observed in stellar‐mass black hole systems, and the aspects of these lines that are most relevant to studies of similar lines in Seyfert‐1 AGN. In particular, the lines observed in stellar‐mass black holes are not complicated by complex low‐energy absorption or partial‐covering of the central engine, and strong lines are largely independent of the model used to fit the underlying broad‐band continuum flux. Indeed, relativistic iron lines are the most robust diagnostic of black hole spin that is presently available to observers, with specific advantages over the systematics–plagued disk continuum. If accretion onto stellar‐mass black holes simply scales with mass, then the widespread nature of lines in stellar‐mass black holes may indicate that lines should be common in Seyfert‐1 AGN, though perhaps harder to detect. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Observational effects of the Kerr metric

Strong gravity effects should have crucial impact on structure and radiative properties of an accretion flow surrounding a black hole. We discuss several observational consequences of such effects. (i) We note that the hard X-ray spectra of Seyfert galaxies, which appear to be intrinsically harder when observed at higher inclination angles, may be most naturally explained by radiative properties of plasmas in the Kerr metric. (ii) We indicate bending of photon trajectories to the equatorial plane, which is a distinct property of rapidly rotating black holes, as the most feasible effect underlying reduced variability of the Fe Kα line observed in several objects. (iii) Both the extreme Fe line profile and the variability pattern (observed, e.g., in a Seyfert galaxy MCG–6-30-15) independently indicate that a primary hard X-ray source must be located within a few gravitational radii from the Kerr black hole. We indicate a hot inner corona as the most likely model of such a source.     

A classification of the X-ray and radio states of Cyg X-3 and their long-term correlations

We present a detailed classification of the X-ray states of Cyg X-3 based on the spectral shape and a new classification of the radio states based on the long-term correlated behaviour of the radio and soft X-ray light curves. We find a sequence of correlations, starting with a positive correlation between the radio and soft X-ray fluxes in the hard spectral state, changing to a negative one at the transition to soft spectral states. The temporal evolution can be in either direction on that sequence, unless the source goes into a very weak radio state, from which it can return only following a major radio flare. The flare decline is via relatively bright radio states, which results in a hysteresis loop on the flux–flux diagram. We also study the hard X-ray light curve, and find its overall anticorrelation with the soft X-rays. During major radio flares, the radio flux responds exponentially to the level of a hard X-ray high-energy tail. We also specify the detailed correspondence between the radio states and the X-ray spectral states. We compare our results to those of black hole and neutron star binaries. Except for the effect of strong absorption and the energy of the high-energy break in the hard state, the X-ray spectral states of Cyg X-3 closely correspond to the canonical X-ray states of black hole binaries. Also, the radio/X-ray correlation closely corresponds to that found in black hole binaries, but it significantly differs from that in neutron star binaries. Overall, our results strongly support the presence of a black hole in Cyg X-3.     

Investigating a fluctuating-accretion model for the spectral-timing properties of accreting black hole systems

The fluctuating-accretion model of Lyubarskii and its extension by Kotov, Churazov & Gilfanov seek to explain the spectral-timing properties of the X-ray variability of accreting black holes in terms of inward-propagating mass accretion fluctuations produced at a broad range of radii. The fluctuations modulate the X-ray emitting region as they move inwards and can produce temporal-frequency-dependent lags between energy bands, and energy-dependent power spectral densities (PSDs) as a result of the different emissivity profiles, which may be expected at different X-ray energies. Here, we use a simple numerical implementation to investigate in detail the X-ray spectral-timing properties of the model and their relation to several physically interesting parameters, namely the emissivity profile in different energy bands, the geometrical thickness and viscosity parameter of the accretion flow, the strength of damping on the fluctuations and the temporal coherence (measured by the 'quality factor', Q ) of the fluctuations introduced at each radius. We find that a geometrically thick flow with large viscosity parameter is favoured, and we confirm that the predicted lags are quite robust to changes in the emissivity profile and physical parameters of the accretion flow, which may help to explain the similarity of the lag spectra in the low/hard and high/soft states of Cyg X-1. We also demonstrate the model regime where the light curves in different energy bands are highly spectrally coherent. We compare model predictions directly to X-ray data from the narrow line Seyfert 1 galaxy NGC 4051 and the black hole X-ray binary (BHXRB) Cyg X-1 in its high/soft state, and we show that this general scheme can reproduce simultaneously the time lags and energy-dependence of the PSD.     

The quiescent spectral energy distribution of V404 Cyg

We present a multiwavelength study of the black hole X-ray binary V404 Cyg in quiescence, focusing upon the spectral energy distribution (SED). Radio, optical, ultraviolet (UV) and X-ray coverage is simultaneous. We supplement the SED with additional non-simultaneous data in the optical through infrared where necessary. The compiled SED is the most complete available for this, the X-ray and radio brightest quiescent black hole system. We find no need for a substantial contribution from accretion light from the near-UV to the near-IR, and in particular the weak UV emission constrains published spectral models for V404 Cyg. We confirm that no plausible companion spectrum and interstellar extinction can fully explain the mid-IR, however, and an infrared (IR) excess from a jet or cool disc appears to be required. The X-ray spectrum is consistent with a  Γ∼ 2  power law as found by all other studies to date. There is no evidence for any variation in the hardness over a range of a factor of 10 in luminosity. The radio flux is consistent with a flat spectrum (in   f _ν  ). The break frequency between a flat and optically thin spectrum most likely occurs in the mid or far-IR, but is not strongly constrained by these data. We find the radio to be substantially variable but with no clear correlation with X-ray variability.     

Relativistic reflection X-ray spectra of accretion disks

We have calculated the relativistic reflection component of the X-ray spectra of accretion disks in active galactic nuclei (AGN). Our calculations have shown that the spectra can be significantly modified by the motion of the accretion flow, and the gravity and rotation of the central black hole. The absorption edges in the spectra suffer severe en- ergy shifts and smearing, and the degree of distortion depends on the system parameters, in particular, the inner radius of the accretion disk and the disk viewing inclination angles. The effects are significant. Fluorescent X-ray emission lines from the inner accretion disk could be a powerful diagnostic of space-time distortion and dynamical relativistic effects near the event horizons of accreting black holes. However, improper treatment of the re- flection component in fitting the X-ray continuum could give rise to spurious line-like features. These features mimic the true fluorescent emission lines and may mask their relativistic signatures. Fully relativistic models for reflection continua together with the emission lines are needed in order to extract black-hole parameters from the AGN X-ray spectra.     

Models of the iron Kα fluorescent line and the Compton Shoulder in irradiated accretion disc spectra

We present a full set of model atmosphere equations for the accretion disc around a supermassive black hole irradiated by a hard X-ray lamp of power-law spectral distribution. Model equations allow for multiple Compton scattering of radiation on free electrons, and for large relative photon–electron energy exchange at the time of scattering. We present spectra in specific intensities integrated over the disc surface. Theoretical outgoing intensity spectra show soft X-ray excess below 1 keV, and distinct Kα and Kβ fluorescent lines of iron. We demonstrate the existence of the Compton Shoulder and claim that it can contribute to the asymmetry and equivalent widths of some observed Fe Kα lines in active galactic nuclei. Our models exhibit the effect of limb-brightening in reflected X-rays.     

Line emission from accretion discs around black holes: the analytic approach

The broad X-ray iron line observed in many active galactic nuclei spectra is thought to originate from the accretion disc surrounding the putative supermassive black hole. We show here how to perform the analytical integration of the geodesic equations that describe the photon trajectories in the general case of a rotating black hole (Kerr metric), in order to write a fast and efficient numerical code for modelling emission line profiles from accretion discs.     

Parallel tracks in infrared versus X-ray emission in black hole X-ray transient outbursts: a hysteresis effect?

We report the discovery of a new hysteresis effect in black hole X-ray binary state transitions, that of the near-infrared (NIR) flux (which most likely originates in the jets) versus X-ray flux. We find, looking at existing data sets, that the IR emission of black hole X-ray transients appears to be weaker in the low/hard state rise of an outburst than the low/hard state decline of an outburst at a given X-ray luminosity. We discuss how this effect may be caused by a shift in the radiative efficiency of the inflowing or outflowing matter, or variations in the disc viscosity or the spectrum/power of the jet. In addition we show that there is a correlation (in slope but not in normalization) between IR and X-ray luminosities on the rise and decline, for all three low-mass black hole X-ray binaries with well-sampled IR and X-ray coverage:   L _NIR∝ L ^0.5–0.7_X  . In the high/soft state this slope is much shallower;   L _NIR∝ L ^0.1–0.2_X  , and we find that the NIR emission in this state is most likely dominated by the viscously heated (as opposed to X-ray heated) accretion disc in all three sources.     

Effects of the stellar wind on X-ray spectra of Cygnus X-3

We study X-ray spectra of Cyg X-3 from BeppoSAX , taking into account absorption and emission in the strong stellar wind of its companion. We find the intrinsic X-ray spectra are well modelled by disc blackbody emission, its upscattering by hot electrons with a hybrid distribution, and by Compton reflection. These spectra are strongly modified by absorption and reprocessing in the stellar wind, which we model using the photoionization code cloudy . The form of the observed spectra implies the wind is composed of two phases. A hot tenuous plasma containing most of the wind mass is required to account for the observed features of very strongly ionized Fe. Small dense cool clumps filling ≲0.01 of the volume are required to absorb the soft X-ray excess, which is emitted by the hot phase but not present in the data. The total mass-loss rate is found to be  (0.6–1.6) × 10⁻⁵ M_⊙ yr⁻¹  . We also discuss the feasibility of the continuum model dominated by Compton reflection, which we find to best describe our data. The intrinsic luminosities of our models suggest that the compact object is a black hole.