On the complex disc–corona interactions in the soft spectral states of soft X-ray transients

Accreting black holes show a complex and diverse behaviour in their soft spectral states. Although these spectra are dominated by a soft, thermal component which almost certainly arises from an accretion disc, there is also a hard X-ray tail indicating that some fraction of the accretion power is instead dissipated in hot, optically thin coronal material. During such states, best observed in the early outburst of soft X-ray transients, the ratio of power dissipated in the hot corona to that in the disc can vary from ∼ 0 (pure disc accretion) to ∼ 1 (equal power in each). Here we present results of spectral analyses of a number of sources, demonstrating the presence of complex features in their energy spectra. Our main findings are: (1) the soft components are not properly described by a thermal emission from accretion discs: they are appreciably broader than can be described by disc blackbody models even including relativistic effects, and (2) the spectral features near     commonly seen in such spectra can be well described by reprocessing of hard X-rays by optically thick, highly ionized, relativistically moving plasma.     

Simultaneous X-ray/optical observations of GX 9+9 (4U 1728−16)

We report on the results of the first simultaneous X-ray ( RXTE ) and optical [South African Astronomical Observatory (SAAO)] observations of the luminous low-mass X-ray binary (LMXB) GX 9+9 in 1999 August. The high-speed optical photometry revealed an orbital period of 4.1958 h and confirmed previous observations, but with greater precision. No X-ray modulation was found at the orbital period. On shorter time-scales, a possible 1.4-h variability was found in the optical light curves which might be related to the MHz quasi-periodic oscillations seen in other LMXBs. We do not find any significant X-ray/optical correlation in the light curves. In X-rays, the colour–colour and hardness-intensity diagrams indicate that the source shows characteristics of an atoll source in the upper banana state, with a correlation between intensity and spectral hardness. Time-resolved X-ray spectroscopy suggests that two-component spectral models give a reasonable fit to the X-ray emission. Such models consist of a blackbody component which can be interpreted as the emission from an optically thick accretion disc or an optically thick boundary layer, and a hard Comptonized component for an extended corona.     

Frequency-resolved spectroscopy of Cyg X-1: fast variability of the reflected emission in the soft state

Using RXTE /PCA data, we study the fast variability of the reflected emission in the soft spectral state of Cyg X-1 by means of Fourier frequency-resolved spectroscopy. We find that the rms amplitude of variations of the reflected emission has the same frequency dependence as the primary radiation down to time-scales of ≲30–50 ms. This might indicate that the reflected flux reproduces, with nearly flat response, variations of the primary emission. Such behaviour differs notably from that of the hard spectral state, in which variations of the reflected flux are significantly suppressed in comparison with the primary emission, on time-scales shorter than ∼0.5–1 s.
If related to the finite light-crossing time of the reflector, these results suggest that the characteristic size of the reflector, presumably an optically thick accretion disc, in the hard spectral state is larger by a factor of ≳5–10 than in the soft spectral state. Modelling the transfer function of the disc, we estimate the inner radius of the accretion disc to be R _in∼100 R _g in the hard state and R _in≲10 R _g in the soft state for a 10-M_⊙ black hole.     

Magnetic flares and the optical variability of the X-ray transient XTE J1118+480

The simultaneous presence of a strong quasi-periodic oscillation, of period ∼10 s, in the optical and X-ray light curves of the X-ray transient XTE J1118+480 suggests that a significant fraction of the optical flux originates from the inner part of the accretion flow, where most of the X-rays are produced. We present a model of magnetic flares in an accretion disc corona where thermal cyclo-synchrotron emission contributes significantly to the optical emission, while the X-rays are produced by inverse Compton scattering of the soft photons produced by dissipation in the underlying disc and by the synchrotron process itself. Given the observational constraints, we estimate the values for the coronal temperature, optical depth and magnetic field intensity, as well as the accretion rate for the source. Within our model we predict a correlation between optical and hard X-ray variability and an anticorrelation between optical and soft X-rays. We also expect optical variability on flaring time-scales (∼tens of ms), with a power-density spectrum similar to that observed in the X-ray band. Finally, we use both the available optical/extreme-ultraviolet/X-ray spectral energy distribution and the low-frequency time variability to discuss limits on the inner radius of the optically thick disc.     

Multicolour observations of V404 Cyg with ULTRACAM

We present high-time-resolution multicolour observations of the quiescent soft X-ray transient V404 Cyg obtained with ULTRACAM. Superimposed on the ellipsoidal modulation of the secondary star are large flares on time-scales of a few hours, as well as several distinct rapid flares on time-scales of tens of minutes. The rapid flares, most of which show further variability and unresolved peaks, cover shorter time-scales than those reported in previous observations. The power density spectrum of the 5-s time-resolution data shows a quasi-periodic oscillation (QPO) feature at 0.78 mHz (=21.5 min). Assuming this periodicity represents the Keplerian period at the transition between the thin and advective disc regions, we determine the transition radius. We discuss the possible origins for the QPO feature in the context of the advection-dominated accretion flow model.
We determine the colour of the large flares and find that the i '-band flux per unit frequency interval is larger than that in the g ' band. The colour is consistent with optically thin gas with a temperature of ∼8000 K arising from a region with an equivalent blackbody radius of at least  2 R_⊙  , which covers 3 per cent of the surface of the accretion disc. Our timing and spectral analysis results support the idea that the rapid flares (i.e. the QPO feature) most likely arise from regions near the transition radius.     

The aperiodic broad-band X-ray variability of Cygnus X-3

We study the soft X-ray variability of Cygnus X-3. By combining data from the All-Sky Monitor and Proportional Counter Array instruments on the RXTE satellite with EXOSAT /Medium Energy (ME) detector observations, we are able to analyse the power density spectrum (PDS) of the source from 10⁻⁹ to 0.1 Hz, thus covering time-scales from seconds to years. As the data on the longer time-scales are unevenly sampled, we combine traditional power spectral techniques with simulations to analyse the variability in this range. The PDS at higher frequencies  (≳10⁻³ Hz)  are for the first time compared for all states of this source. We find that it is for all states well described by a power law, with index  ∼−2  in the soft states and a tendency for a less steep power law in the hard state. At longer time-scales, we study the effect of the state transitions on the PDS, and find that the variability below  ∼10⁻⁷ Hz  is dominated by the transitions. Furthermore, we find no correlation between the length of a high/soft-state episode and the time since the previous high/soft state. On intermediate time-scales, we find evidence for a break in the PDS at time-scales of the order of the orbital period. This may be interpreted as evidence for the existence of a tidal resonance in the accretion disc around the compact object, and constraining the mass ratio to   M ₂/ M ₁≲ 0.3  .     

The flux-dependent amplitude of broadband noise variability in X-ray binaries and active galaxies

Standard shot-noise models, which seek to explain the broadband noise variability that characterizes the X-ray light curves of X-ray binaries and active galaxies, predict that the power spectrum of the X-ray light curve is stationary (i.e. constant amplitude and shape) on short time-scales. We show that the broadband noise power spectra of the black hole candidate Cyg X-1 and the accreting millisecond pulsar SAX J1808.4−3658 are intrinsically non-stationary, in that rms variability scales linearly with flux. Flux-selected power spectra confirm that this effect is due to changes in power-spectral amplitude and not shape. The light curves of three Seyfert galaxies are also consistent with a linear relationship between rms variability and flux, suggesting that it is an intrinsic feature of the broadband noise variability in compact accreting systems over more than six decades of central object mass. The rms variability responds to flux variations on all measured time-scales, raising fundamental difficulties for shot-noise models which seek to explain this result by invoking variations in the shot parameters. We suggest that models should be explored where the longest time-scale variations are fundamental and precede the variations on shorter time-scales. Possible models which can explain the linear rms-flux relation include the fractal break-up of large coronal flares, or the propagation of fluctuations in mass accretion rate through the accretion disc. The linear relationship between rms variability and flux in Cyg X-1 and SAX J1808.4−3658 is offset on the flux axis, suggesting the presence of a second, constant-flux component to the light curve which contributes ∼25 per cent of the total flux. The spectrum of this constant component is similar to the total spectrum, suggesting that it may correspond to quiet, non-varying regions in the X-ray emitting corona.     

Optical spectroscopy of GX 339−4 during the high–soft and low–hard states – II. Line ionization and emission region

We have carried out observations of the X-ray transient GX 339−4 during its high–soft and low–hard X-ray spectral states. Our high-resolution spectroscopic observation in 1999 April suggests that the H α line has a single-peaked profile in the low–hard state as speculated in our previous paper. The He  ii λ 4686 line, however, has a double-peaked profile in both the high–soft and low–hard states. This suggests that the line-emission mechanism is different in the two states. Our interpretation is that double-peaked lines are emitted from a temperature-inversion layer on the accretion disc surface when it is irradiatively heated by soft X-rays. Single-peaked lines may be emitted from outflow/wind matter driven by hard X-ray heating. We have constructed a simple plane-parallel model and we use it to illustrate that a temperature-inversion layer can be formed at the disc surface under X-ray illumination. We also discuss the conditions required for the formation of temperature inversion and line emission. Based on the velocity separations measured for the double-peaked lines in the high–soft state, we propose that GX 339−4 is a low-inclination binary system. The orbital inclination is about 15° if the orbital period is 14.8 h.     

ASCA and ROSAT observations of the Seyfert 1 galaxy RX J0437.4−4711

Results of ASCA and ROSAT observations of the Seyfert 1 galaxy RX J0437.4−4711 are presented. The X-ray continuum spectrum can be described by the sum of a power law with photon index 2.15 ± 0.04 and a soft emission component characterized by a blackbody with temperature 29 ± 2 eV. The total luminosity of the soft component is larger than that of the power-law component if the power law is cut off around a few hundred keV. A weak absorption edge with τ = 0.26 ± 0.13 at the rest-frame energy of E  = 0.83 ± 0.05 keV and an Fe Kα line with EW = 430 ± 220 eV at an energy E  = 6.47 ± 0.15 keV are also detected. The X-ray flux showed a 47 per cent increase between two ASCA observations 4 months apart, but no spectral variability was seen. We argue that reprocessing of the hard X-ray emission cannot produce all the soft X-ray emission, since the total luminosity of the soft component is larger than that of the integrated power-law component. Similarities with some stellar black hole candidates are briefly discussed.     

Studying the X-ray hysteresis in GX 339−4: the disc and iron line over one decade

We report on a comprehensive and consistent investigation into the X-ray emission from GX 339−4. All public observations in the 11 year RXTE archive were analysed. Three different types of model – single power law, broken power law and a disc + power law – were fitted to investigate the evolution of the disc, along with a fixed Gaussian component at 6.4 keV to investigate any iron line in the spectrum. We show that the relative variation in flux and X-ray colour between the two best sampled outbursts are very similar. The decay of the disc temperature during the outburst is clearly seen in the soft state. The expected decay is   S _Disc∝ T ⁴  ; we measure   T ^4.75±0.23  . This implies that the inner disc radius is approximately constant in the soft state. We also show a significant anticorrelation between the iron line equivalent width (EW) and the X-ray flux in the soft state while in the hard state the EW is independent of the flux. This results in hysteresis in the relation between X-ray flux and both line flux and EW. To compare the X-ray binary outburst to the behaviour seen in active galactic nuclei (AGN), we construct a disc fraction luminosity diagram for GX 339−4, the first for an X-ray binary. The shape qualitatively matches that produced for AGN. Linking this with the radio emission from GX 339−4 the change in radio spectrum between the disc and power-law-dominated states is clearly visible.     

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 good long look at the black hole candidates LMC X-1 and LMC X-3

LMC X-1 and LMC X-3 are the only known persistent stellar-mass black-hole candidates that have almost always shown spectra that are dominated by a soft, thermal component. We present here results from 170-ks-long Rossi X-ray Timing Explorer ( RXTE ) observations of these objects, taken in 1996 December, where their spectra can be described by a disc blackbody plus an additional soft     high-energy power law (detected up to energies of 50 keV in LMC X-3). These observations, as well as archival Advanced Satellite for Cosmology and Astrophysics ( ASCA ) observations, constrain any narrow Fe line present in the spectra to have an equivalent width ≲90 eV. Stronger, broad lines (≈150 eV EW,     are permitted. We also study the variability of LMC X-1. Its X-ray power spectral density (PSD) is approximately proportional to     between 10⁻³ and 0.3 Hz with a root-mean-square (rms) variability of ≈7 per cent. At energies >5 keV, the PSD shows evidence of a break at     possibly indicating an outer disc radius of ≲1000  GM c ² in this likely wind-fed system. Furthermore, the coherence function     a measure of the degree of linear correlation between variability in the >5 keV band and variability in the lower energy bands, is extremely low (≲50 per cent). We discuss the implications of these observations for the mechanisms that might be producing the soft and hard X-rays in these systems.     

The 'off' state of GX 339–4

We report BeppoSAX and optical observations of the black hole candidate GX 339–4 during its X-ray 'off' state in 1999. The broad-band (0.8–50 keV) X-ray emission can be fitted by a single power law with spectral index, α ∼1.6. The observed luminosity is 6.6×10³³ erg s⁻¹ in the 0.5–10 keV band, which is at the higher end of the flux distribution of black hole soft X-ray transients in quiescence, comparable to that seen in GS 2023+338 and 4U 1630–47. An optical observation just before the BeppoSAX observation shows the source to be very faint at these wavelengths as well ( B =20.1, V =19.2). By comparing with previously reported 'off' and low states (LS), we conclude that the 'off' state is actually an extension of the LS, i.e. an LS at lower intensities. We propose that accretion models such as the advection-dominated accretion flows are able to explain the observed properties in such a state.     

Optical spectroscopy of GX 339−4 during the high–soft and low–hard states – I

We carried out spectroscopic observations of the candidate black hole binary GX 339−4 during its low–hard and high–soft X-ray states. We have found that the spectrum is dominated by emission lines of neutral elements with asymmetric, round-topped profiles in the low–hard state. In the high–soft state, however, the emission lines from both neutral and ionized elements have unambiguously resolved double-peaked profiles. The detection of double-peaked emission lines in the high–soft state, with a larger peak separation for higher ionization lines, indicates the presence of an irradiatively heated accretion disc. The round-topped lines in the low–hard state are probably caused by a dense matter outflow from an inflated non-Keplerian accretion disc. Our data do not show velocity modulations of the line centres caused by the orbital motion of the compact object, neither do the line basewidths show substantial variations in each observational epoch. There are no detectable absorption lines from the companion star. All these features are consistent with those of a system with a low-mass companion star and low orbital inclination.     

Long-term optical and X-ray variability of the Seyfert galaxy Markarian 79

We present the results of concurrent X-ray and optical monitoring of the Seyfert 1 galaxy Mrk 79 over a period of more than 5 yr. We find that on short to medium time-scales (days to a few tens of days) the 2–10 keV X-ray and optical u - and V -band fluxes are significantly correlated, with a delay between the bands consistent with 0 d. We show that most of these variations may be well reproduced by a model where the short-term optical variations originate from reprocessing of X-rays by an optically thick accretion disc. The optical light curves, however, also display long time-scale variations over thousands of days, which are not present in the X-ray light curve. These optical variations must originate from an independent variability mechanism and we show that they can be produced by variations in the (geometrically) thin disc accretion rate as well as by varying reprocessed fractions through changes in the location of the X-ray corona.     

INTEGRAL and XMM–Newton observations of the X-ray pulsar IGR J16320−4751/AX J1631.9−4752

We report on observations of the X-ray pulsar IGR J16320−4751 (also known as AX J1631.9−4752) performed simultaneously with International Gamma-Ray Astrophysics Laboratory ( INTEGRAL ) and XMM–Newton . We refine the source position and identify the most likely infrared counterpart. Our simultaneous coverage allows us to confirm the presence of X-ray pulsations at ∼1300 s, that we detect above 20 keV with INTEGRAL for the first time. The pulse fraction is consistent with being constant with energy, which is compatible with a model of polar accretion by a pulsar. We study the spectral properties of IGR J16320−4751 during two major periods occurring during the simultaneous coverage with both satellites, namely a flare and a non-flare period. We detect the presence of a narrow 6.4 keV iron line in both periods. The presence of such a feature is typical of supergiant wind accretors such as Vela X-1 or GX 301−2. We inspect the spectral variations with respect to the pulse phase during the non-flare period, and show that the pulse is solely due to variations of the X-ray flux emitted by the source and not due to variations of the spectral parameters. Our results are therefore compatible with the source being a pulsar in a High Mass X-ray Binary. We detect a soft excess appearing in the spectra as a blackbody with a temperature of ∼0.07 keV. We discuss the origin of the X-ray emission in IGR J16320−4751: while the hard X-rays are likely the result of Compton emission produced in the close vicinity of the pulsar, based on energy argument we suggest that the soft excess is likely the emission by a collisionally energized cloud in which the compact object is embedded.     

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.     

RE J2248−511: not all variable, ultrasoft, X-ray AGN have narrow Balmer lines

We present ASCA data on RE J2248−511, extending existing optical and soft X-ray coverage to 10 keV, and monitoring the soft component. These data show that, despite a very strong ultrasoft X-ray excess below 0.3 keV and a soft 0.3–2 keV spectral index in earlier ROSAT data, the hard X-ray spectrum ( α ∼−0.8; 0.6–10 keV) is typical of type 1 active galactic nuclei (AGN), and the soft component has since disappeared. Optical data taken at two different epochs show that the big blue bump is also highly variable. The strength of the ultrasoft X-ray component and the extreme variability in RE J2248−511 are reminiscent of the behaviour observed in many narrow line Seyfert 1s (NLS1s). However, the high-energy end of the ROSAT spectrum, the ASCA spectrum and the Balmer line full widths at half maximum of ∼3000 km s⁻¹ in RE J2248−511 are typical of normal Seyfert 1 AGN.
The change in the soft X-ray spectrum as observed in the ROSAT and ASCA data is consistent with the behaviour of Galactic Black Hole Candidates (GBHCs) as they move from a high to a low state, i.e. a fall in the ultrasoft component and a hardening of the X-ray continuum. This GBHC analogy has also been proposed for NLS1s. Alternatively, the variability may be caused by opacity changes in a hot, optically thin corona which surrounds a cold, dense accretion disc; this was first suggested by Guainazzi et al. for 1H 0419−577, an object which shows remarkably similar properties to RE J2248−511.     

Impact of reverberation in flared accretion discs on temporal characteristics of X-ray binaries

Observations suggest that accretion discs in many X-ray binaries are likely flared. An outer edge of the disc intercepts radiation from the central X-ray source. Part of that radiation is absorbed and re-emitted in the optical/UV spectral ranges. However, a large fraction of that radiation is reflected and appears in the broad-band X-ray spectrum as a Compton reflection bump. This radiation is delayed and variability is somewhat smeared compared with the intrinsic X-ray radiation. We compute response functions for flat and flared accretion discs and for isotropic and anisotropic X-ray sources. A simple approximation for the response function which is valid in the broad range of the disc shapes and inclinations, inner and outer radii, and the plasma bulk velocity is proposed. We also study the impact of the X-ray reprocessing on temporal characteristics of X-ray binaries such as the power spectral density, auto- and cross-correlation functions, and time/phase lags. We propose a reprocessing model which explains the secondary peaks in the phase lag Fourier spectra observed in Cyg X-1 and other Galactic black hole sources. The position of the peaks could be used to determine the size of the accretion disc.