We review the properties and behavior of 20 X-ray binaries that contain a dynamically-confirmed black hole, 17 of which are transient systems. During the past decade, many of these transien...

/pdf/x-ray-properties-of-black-hole-binaries-2udxi1i1oe.pdf

X-Ray Properties of Black-Hole Binaries

We present a unified semiquantitative model for the disc–jet coupling in black hole X-ray binary systems. In the process we have compiled observational aspects from the existing literature, as well as performing new analyses. We argue that during the rising phase of a black hole transient outburst the steady jet known to be associated with the canonical ‘low/hard’ state persists while the X-ray spectrum initially softens. Subsequently, the jet becomes unstable and an optically thin radio outburst is always associated with the soft X-ray peak at the end of this phase of softening. This peak corresponds to a ‘soft very high state’ or ‘steep power-law’ state. Softer X-ray states are not associated with ‘core’ radio emission. We further demonstrate quantitatively that the transient jets associated with these optically thin events are considerably more relativistic than those in the ‘low/hard’ X-ray state. This in turn implies that, as the disc makes its collapse inwards, the jet Lorentz factor rapidly increases, resulting in an internal shock in the outflow, which is the cause of the observed optically thin radio emission. We provide simple estimates for the efficiency of such a shock in the collision of a fast jet with a previously generated outflow that is only mildly relativistic. In addition, we estimate the jet power for a number of such transient events as a function of X-ray luminosity, and find them to be comparable to an extrapolation of the functions estimated for the ‘low/hard’ state jets. The normalization may be larger, however, which may suggest a contribution from some other power source such as black hole spin, for the transient jets. Finally, we attempt to fit these results together into a coherent semiquantitative model for the disc–jet coupling in all black hole X-ray binary systems.

/pdf/towards-a-unified-model-for-black-hole-x-ray-binary-jets-2jy6aae9i3.pdf

Towards a unified model for black hole X-ray binary jets

We describe new optically thin solutions for rotating accretion flows around black holes and neutron stars. These solutions are advection dominated, so that most of the viscously dissipated energy is advected radially with the flow. We model the accreting gas as a two temperature plasma and include cooling by bremsstrahlung, synchrotron, and Comptonization. We obtain electron temperatures $T_e\sim 10^{8.5}-10^{10}$K. The new solutions are present only for mass accretion rates $\dot M$ less than a critical rate $\dot M_{crit}$ which we calculate as a function of radius $R$ and viscosity parameter $\alpha$. For $\dot M<\dot M_{crit}$ we show that there are three equilibrium branches of solutions. One of the branches corresponds to a cool optically thick flow which is the well-known thin disk solution of Shakura \& Sunyaev (1973). Another branch corresponds to a hot optically thin flow, discovered originally by Shapiro, Lightman \& Eardley (1976, SLE). This solution is thermally unstable. The third branch corresponds to our new advection-dominated solution. This solution is hotter and more optically thin than the SLE solution, but is viscously and thermally stable. It is related to the ion torus model of Rees et al. (1982) and may potentially explain the hard X-ray and $\gamma$-ray emission from X-ray binaries and active galactic nuclei.

/pdf/advection-dominated-accretion-underfed-black-holes-and-cc41v8tdhl.pdf

Advection-Dominated Accretion: Underfed Black Holes and Neutron Stars

We review how the recent increase in X-ray and radio data from black hole and neutron star binaries can be merged together with theoretical advances to give a coherent picture of the physics of the accretion flow in strong gravity. Both long term X-ray light curves, X-ray spectra, the rapid X-ray variability and the radio jet behaviour are consistent with a model where a standard outer accretion disc is truncated at low luminosities, being replaced by a hot, inner flow which also acts as the launching site of the jet. Decreasing the disc truncation radius leads to softer spectra, as well as higher frequencies (including quasi periodic oscillations, QPOs) in the power spectra, and a faster jet. The collapse of the hot flow when the disc reaches the last stable orbit triggers the dramatic decrease in radio flux, as well as giving a qualitative (and often quantitative) explanation for the major hard–soft spectral transition seen in black holes. The neutron stars are also consistent with the same models, but with an additional component due to their surface, giving implicit evidence for the event horizon in black holes. We review claims of observational data which conflict with this picture, but show that these can also be consistent with the truncated disc model. We also review suggested alternative models for the accretion flow which do not involve a truncated disc. The most successful of these converge on a similar geometry, where there is a transition at some radius larger than the last stable orbit between a standard disc and an inner, jet dominated region, with the X-ray source associated with a mildly relativistic outflow, beamed away from the disc. However, the observed uniformity of properties between black holes at different inclinations suggests that even weak beaming of the X-ray emission may be constrained by the data. After collapse of the hot inner flow, the spectrum in black hole systems can be dominated by the disc emission. Its behaviour is consistent with the existence of a last stable orbit, and such data can be used to estimate the black hole spin. By contrast, these systems can also show very different spectra at these high luminosities, in which the disc spectrum (and probably structure) is strongly distorted by Comptonization. The structure of the accretion flow becomes increasingly uncertain as the luminosity approaches (and exceeds) the Eddington luminosity, though there is growing evidence that winds may play an important role. We stress that these high Eddington fraction flows are key to understanding many disparate and currently very active fields such as ULX, Narrow Line Seyfert 1’s, and the growth of the first black holes in the Early Universe.

Modelling the behaviour of accretion flows in x-ray binaries.

We derive the luminosity‐temperature relation for the supercritically accreting black holes (BHs) and compare it to the data on ultraluminous X-ray sources (ULXs). At super-Eddington accretion rates, an outflow forms within the spherization radius. We construct the accretion disc model accounting for the advection and the outflow, and compute characteristic disc temperatures. The bolometric luminosity exceeds the Eddington luminosity LEdd by a logarithmic factor 1 + 0. 6l n˙ m (where ˙ m is the accretion rate in Eddington units) and the wind kinetic luminosity is close to LEdd. The apparent luminosity for the face-on observer is 2‐7 times higher because of geometrical beaming. Such an observer has a direct view of the inner hot accretion disc, which has a peak temperature Tmax of a few keV in stellar mass BHs. The emitted spectrum extends as a power law FE ∝ E −1 down to the temperature at the spherization radius Tsp ≈ ˙ m −1/2 keV. We associate T max with a few keV spectral components and Tsp with the soft, 0.1‐0.2 keV components observed in ULXs. An edge-on observer sees only the soft emission from the extended envelope, with the photosphere radius exceeding the spherization radius by orders of magnitude. The dependence of the photosphere temperature on luminosity is consistent with that observed in the super-Eddington accreting BHs SS 433 and V4641 Sgr. Strong outflows combined with the large intrinsic X-ray luminosity of the central BH explain naturally the presence of the photoionized nebulae around ULXs. An excellent agreement between the model and the observational data strongly argues in favour of ULXs being supercritically accreting, stellar mass BHs similar to SS 433, but viewed close to the symmetric axis.

Supercritically accreting stellar mass black holes as ultraluminous X-ray sources

GRS 1915+105 was observed by the Oriented Scintillation Spectroscopy Experiment (OSSE) aboard the Compton Gamma Ray Observatory nine times in 1995-2000, and eight of those observations were simultaneous with those by the Rossi X-Ray Timing Explorer (RXTE). We present an analysis of all of the OSSE data and of two RXTE-OSSE spectra with the lowest and highest X-ray fluxes. The OSSE data show a power-law-like spectrum extending up to 600 keV without any break. We interpret this emission as strong evidence for the presence of nonthermal electrons in the source. The broadband spectra cannot be described by either thermal or bulk-motion Comptonization, whereas they are well described by Comptonization in hybrid thermal/nonthermal plasmas.

OSSE and RXTE Observations of GRS 1915+105: Evidence for Nonthermal Comptonization

We investigate the association between the radio plateau states and the large superluminal flares in GRS 1915+105 and propose a qualitative scenario to explain this association. To investigate the properties of the source during a superluminal flare, we present Giant Metrewave Radio Telescope observations during a radio flare that turned out to be a preplateau flare, as shown by the contemporaneous Ryle Telescope observations. A major superluminal ejection was observed at the end of this plateau state (as described by V. Dhawan et al.), associated with highly variable X-ray emission showing X-ray soft dips. This episode thus has all three types of radio emission: a preplateau flare, a plateau state, and superluminal jets. We analyze all the available Rossi X-Ray Timing Explorer (RXTE) Proportional Counter Array data during this episode and show that (1) the preflare plateau state consists of a three-component X-ray spectrum that includes a multicolor disk-blackbody, a Comptonized component, and a power law and (2) the Compton cloud, which is responsible for the Comptonizing component, is ejected away during the X-ray soft dips. We investigate all the available monitoring data on this source, identify several candidate superluminal flare events, and analyze the contemporaneous RXTE pointed observations. We detect a strong correlation between the average X-ray flux during the plateau state and the total energy emitted in radio during the subsequent radio flare. We find that the sequence of events is similar for all large radio flares, with a fast rise and exponential decay morphology. Based on these results, we propose a qualitative scenario in which the separating ejecta during the superluminal flares are the interaction of the matter blob ejected during the X-ray soft dips with the steady jet already established during the plateau state. This picture can explain all the types of radio emission observed from this source in terms of its X-ray-emission characteristics.

On the Origin of the Various Types of Radio Emission in GRS 1915+105

We report X-ray observations of the Galactic X-ray transient source GRS 1915+105 with the pointed proportional counters of the Indian X-ray Astronomy Experiment (IXAE) onboard the Indian satellite IRS-P3, which show remarkable richness in temporal variability. The observations were carried out on 1997 June 12-29 and August 7-10, in the energy range of 2-18 keV and revealed the presence of very intense X-ray bursts. All the observed bursts have a slow exponential rise, a sharp linear decay, and broadly can be put in two classes: irregular and quasi-regular bursts in one class, and regular bursts in the other. The regular bursts are found to have two distinct timescales and to persist over extended durations. There is a strong correlation between the preceding quiescent time and the burst duration for the quasi-regular and irregular bursts. No such correlation is found for the regular bursts. The ratio of average flux during the burst time to the average flux during the quiescent phase is high and variable for the quasi-regular and irregular bursts, while it is low and constant for the regular bursts. We present a comprehensive picture of the various types of bursts observed in GRS 1915+105 in the light of the recent theories of advective accretion disks. We suggest that the peculiar bursts that we have seen are characteristic of the change of state of the source. The source can switch back and forth between the low-hard state and the high-soft state near critical accretion rates in a very short timescale, giving rise to the irregular and quasi-regular bursts. The fast timescale for the transition of the state is explained by invoking the appearance and disappearance of the advective disk in its viscous timescale. The periodicity of the regular bursts is explained by matching the viscous timescale with the cooling timescale of the postshock region. A test of the model is presented using the publicly available 13-60 keV RXTE/PCA data for irregular and regular bursts concurrent with our observations. It is found that the 13-60 keV flux relative to the 2-13 keV flux shows clear evidence for state change between the quiescent phase and the burst phase. The value of this ratio during burst is consistent with the values observed during the high-soft state seen on 1997 August 19, while its value during quiescent phase is consistent with the values observed during the low-hard state seen on 1997 May 8.

/pdf/different-types-of-x-ray-bursts-from-grs-1915-105-and-their-1e2pbt95h1.pdf

Different types of X-ray bursts from GRS 1915+105 and their origin

We present an exhaustive analysis of five broad-band observations of GRS 1915+105 in two variability states, / and w, observed simultaneously by the Proportional Counter Array (PCA) and High-Energy X-ray Timing Experiment (HEXTE) detectors aboard the Rossi X-ray Timing Explorer, and the Oriented Scintillation Spectrometer Experiment (OSSE) detector aboard the Compton Gamma-ray Observatory. We find all the spectra well fitted by Comptonization of disc blackbody photons, with very strong evidence for the presence of a non-thermal electron component in the Comptonizing plasma. Both the energy and the power spectra in the X state are typical of the very high/intermediate state of black hole binaries. The spectrum of the ω state is characterized by a strong blackbody component Comptonized by thermal electrons and a weak non-thermal tail. We then calculate rms spectra (fractional variability as functions of energy) for the PCA data. We accurately model the rms spectra by coherent superposition of variability in the components implied by the spectral fits, namely a less variable blackbody and more variable Comptonization. The latter dominates at high energies, resulting in a flattening of the rms at high energies in most of the data. This is also the case for the spectra of the quasi-periodic oscillations present in the X state. Then, some of our data require a radial dependence of the rms of the disc blackbody. We also study the distance to the source, and find d ≃ 11 kpc as the most likely value, contrary to a recent claim of a much lower value.

GRS 1915+105: the distance, radiative processes and energy‐dependent variability

We investigate the origin of the soft X-ray excess emission from Ark 564 and Mrk 1044. Based on a long XMM-Newton observation of Ark 564, we find clear evidence for time delays such that the variations in the 4-10 kev band lag behind that in the 0.2-0.5 kev band by -->1768 ± 122 s. The full-band power density spectrum (PDS) of Ark 564 has a break at ~ -->1.2 × 1−3 Hz with power-law indices of ~1 and ~3 below and above the break. The hard (3-10 kev) band PDS is stronger and flatter than that in the soft (0.2-0.5) band. Based on a short XMM-Newton observation of Mrk 1044, we find no correlation between the 0.2-0.3 and 5-10 kev bands at zero lag. These observations imply that the soft excess is not the reprocessed hard X-ray emission. The EPIC-pn spectrum of Ark 564 is best described by a complex model consisting of optically thick Comptonization in a cool plasma for the soft excess and a steep power law, modified by two warm absorber media as inferred from the RGS data. The smeared wind and optically thick Comptonization models both describe the spectrum of Mrk 1044 satisfactorily, but the ionized reflection model requires extreme parameters. The data suggest two component coronas—a cool, optically thick corona for the soft excess and a hot corona for the power-law component. The existence of a break in the soft band PDS suggests a compact cool corona that can either be an ionized surface of the inner disk or an inner optically thick region coupled to a truncated disk.

A. R. Rao

Papers

OSSE and RXTE Observations of GRS 1915+105: Evidence for Nonthermal Comptonization

On the Origin of the Various Types of Radio Emission in GRS 1915+105

Different types of X-ray bursts from GRS 1915+105 and their origin

GRS 1915+105: the distance, radiative processes and energy‐dependent variability

AN INVESTIGATION OF THE ORIGIN OF SOFT X-RAY EXCESS EMISSION FROM Ark 564 AND Mrk 1044