R. P. Fender

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

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.

Active galactic nuclei as scaled-up Galactic black holes.

A long-standing question is whether active galactic nuclei (AGN) vary like Galactic black hole systems when appropriately scaled up by mass. If so, we can then determine how AGN should behave on cosmological timescales by studying the brighter and much faster varying Galactic systems. As X-ray emission is produced very close to the black holes, it provides one of the best diagnostics of their behaviour. A characteristic timescale—which potentially could tell us about the mass of the black hole—is found in the X-ray variations from both AGN and Galactic black holes, but whether it is physically meaningful to compare the two has been questioned. Here we report that, after correcting for variations in the accretion rate, the timescales can be physically linked, revealing that the accretion process is exactly the same for small and large black holes. Strong support for this linkage comes, perhaps surprisingly, from the permitted optical emission lines in AGN whose widths (in both broad-line AGN and narrow-emission-line Seyfert 1 galaxies) correlate strongly with the characteristic X-ray timescale, exactly as expected from the AGN black hole masses and accretion rates. So AGN really are just scaled-up Galactic black holes.

Correlated X-ray Spectral and Timing Behavior of the Black Hole Candidate XTE J1550-564: A New Interpretation of Black Hole States

We present an analysis of data of the black hole candidate and X-ray transient XTE J1550-564, taken with the Rossi X-Ray Timing Explorer between 1998 November 22 and 1999 May 20. During this period the source went through several different states, which could be divided into soft and hard states based on the relative strength of the high-energy spectral component. These states showed up as distinct branches in the color-color and hardness-intensity diagrams, connecting to form a structure with a comblike topology, the branch corresponding to the soft state forming the spine and the branches corresponding to the various hard states forming the teeth of the comb. The power spectral properties of the source were strongly correlated with its position on the branches. The broadband noise became stronger and changed from power law-like to band-limited, as the spectrum became harder. Three types of quasi-periodic oscillations (QPOs) were found: 1-18 Hz and 102-284 Hz QPOs on the hard branches, and 16-18 Hz QPOs on and near the soft branch. The 1-18 Hz QPOs on the hard branches could be divided into three subtypes. The frequencies of the high- and low-frequency QPOs on the hard branches were correlated with each other and were anticorrelated with spectral hardness. The changes in QPO frequency suggest that the inner disk radius only increases by a factor of 3-4 as the source changes from a soft to a hard state. Our results on XTE J1550-564 strongly favor a two-dimensional description of black hole behavior, where the regions near the spine of the comb in the color-color diagram can be identified with the high state, and the teeth with transitions from the high state, via the intermediate state (which includes the very high state) to the low state, and back. The two physical parameters underlying this two-dimensional behavior vary to a large extent independently and could for example be the accretion rate through the disk and the size of the Comptonizing region causing the hard tail. The difference between the various teeth is then associated with the mass accretion rate through the disk, suggesting that high state ↔ low state transitions can occur at any disk mass accretion rate and that these transitions are primarily caused by another, independent parameter. We discuss how this picture could tie in with the canonical, one-dimensional behavior of black hole candidates that has usually been observed.

GRS 1915+105 and the Disc-Jet Coupling in Accreting Black Hole Systems

▪ Abstract GRS 1915+105—the first stellar-scale, highly relativistic jet source identified—is a key system for our understanding of the disc-jet coupling in accreting black hole systems. Comprehending the coupling between inflow and outflow in this source not only is important for X-ray binary systems but has a broader relevance for studies of active galactic nuclei and gamma-ray bursts. In this paper, we present a detailed review of the observational properties of the system, as established in the decade since its discovery. We attempt to place it in context by a detailed comparison with other sources, and construct a simple model for the disc-jet coupling, which may be more widely applicable to accreting black hole systems.

Quenching of the Radio Jet during the X-Ray High State of GX 339−4

We have observed the black hole candidate X-ray binary GX 339-4 at radio wavelengths before, during, and after the 1998 high/soft X-ray state transition. We find that the radio emission from the system is strongly correlated with the hard X-ray emission and is reduced by a factor of ≥25 during the high/soft state compared with the more usual low/hard state. At the points of state transition, we note brief periods of unusually optically thin radio emission that may correspond to discrete ejection events. We propose that in the low/hard state, black hole X-ray binaries produce a quasi-continuous outflow, that in the high/soft state, this outflow is suppressed, and that state transitions often result in one or more discrete ejection events. Future models for low/hard states, such as advection-dominated solutions, need to take into account the strong outflow of relativistic electrons from the system. We propose that the inferred Comptonizing corona and the base of the jetlike outflow are the same thing, based on the strong correlation between radio and hard X-ray emission in GX 339-4 and other X-ray binaries and on the similarity in inferred location and composition of these two components.

Jets in neutron star X-ray binaries: a comparison with black holes

We present a comprehensive study of the relation between radio and X-ray emission in neutron star (NS) X-ray binaries, use this to infer the general properties of the disc‐jet coupling in such systems and compare the results quantitatively with those already established for black hole (BH) systems. There are clear qualitative similarities between the two classes of object: hard states below about 1 per cent of the Eddington luminosity produce steady jets, while transient jets are associated with outbursting and variable sources at the highest luminosities. However, there are important quantitative differences: the NSs are less radio loud for a given X-ray luminosity (regardless of mass corrections) and they do not appear to show the strong suppression of radio emission in steady soft states that we observe in BH systems. Furthermore, in the hard states, the correlation between radio and X-ray luminosities of the NS systems is steeper than the relation observed in BHs by about a factor of 2. This result strongly suggests that the X-ray emission in the BH systems is radiatively inefficient, with an approximate relation of the form L X ∝ ˙ m 2 , consistent with both advection-dominated models and the jetdominated scenario. In contrast, the jet power in both classes of object scales linearly with accretion rate. This constitutes some of the first observational evidence for the radiatively inefficient scaling of X-ray luminosity with accretion rate in accreting BH systems. Moreover, based on simultaneous radio/X-ray observations of Z-type NSs (the brightest of our Galaxy, always near or at the Eddington accretion rate), we draw a model that can describe the disc‐ jet coupling in such sources, finding a possible association between a particular X-ray state transition [horizontal branch to normal branch] and the emission of transient jets.

A dark jet dominates the power output of the stellar black hole Cygnus X-1

Black holes undergoing accretion are thought to emit the bulk of their power in the X-ray band by releasing the gravitational potential energy of the infalling matter. At the same time, they are capable of producing highly collimated jets of energy and particles flowing out of the system with relativistic velocities. Here we show that the 10-solar-mass (10M[circdot]) black hole in the X-ray binary Cygnus X-1 (refs 3–5) is surrounded by a large-scale (∼5 pc in diameter) ring-like structure that appears to be inflated by the inner radio jet. We estimate that in order to sustain the observed emission of the ring, the jet of Cygnus X-1 has to carry a kinetic power that can be as high as the bolometric X-ray luminosity of the binary system. This result may imply that low-luminosity stellar-mass black holes as a whole dissipate the bulk of the liberated accretion power in the form of ‘dark’, radiatively inefficient relativistic outflows, rather than locally in the X-ray-emitting inflow.

Jet‐dominated states: an alternative to advection across black hole event horizons in ‘quiescent’ X‐ray binaries

We demonstrate that at relatively low mass accretion rates, black hole X-ray binaries should enter ‘jet-dominated’ states, in which the majority of the liberated accretion power is in the form of a (radiatively inefcient) jet and not dissipated as X-rays in the accretion o w. This result follows from the empirically established non-linear relation between radio and X-ray power from hard state black holes, which we assume also to hold for neutron stars. Conservative estimates of the jet power indicate that all ‘quiescent’ black holes should be in this jetdominated regime. In combination with an additional empirical result, namely that black hole X-ray binaries are more ‘radio loud’ than neutron stars, we nd that quiescent neutron stars should be up to two orders of magnitude more luminous in X-rays than the black hole systems, without requiring any signicant advection of energy into a black hole. This ratio is as observed, and such observations should therefore no longer be considered as direct evidence for the existence of black hole event horizons. Furthermore, even if black hole candidates do contain black holes with event horizons, this work demonstrates that there is no requirement for the advection of signicant amounts of accretion energy across the horizon.

A radio‐emitting outflow in the quiescent state of A0620−00: implications for modelling low‐luminosity black hole binaries

Deep observations with the Very Large Array of A0620-00, performed in 2005 August, resulted in the first detection of radio emission from a black hole binary at X-ray luminosities as low as 10-8.5 times the Eddington limit. The measured radio flux density, of 51 +/- 7 ?Jy at 8.5 GHz, is the lowest reported for an X-ray binary system so far, and is interpreted in terms of partially self-absorbed synchrotron emission from outflowing plasma. Making use of the estimated outer accretion rate of A0620-00 in quiescence, we demonstrate that the outflow kinetic power must be energetically comparable to the total accretion power associated with such rate, if it was to reach the black hole with the standard radiative efficiency of 10 per cent. This favours a model for quiescence in which a radiatively inefficient outflow accounts for a sizable fraction of the missing energy, and, in turn, substantially affects the overall dynamics of the accretion flow. Simultaneous observations in the X-ray band, with Chandra, confirm the validity of a non-linear radio/X-ray correlation for hard state black hole binaries down to low quiescent luminosities, thereby contradicting some theoretical expectations. Taking the mass term into account, the A0620-00 data lie on the extrapolation of the so-called Fundamental Plane of black hole activity, which has thus been extended by more than two orders of magnitude in radio and X-ray luminosity. With the addition of the A0620-00 point, the plane relation provides an empirical proof for the scale invariance of the jet-accretion coupling in accreting black holes over the entire parameter space observable with current instrumentation.

Ubiquitous equatorial accretion disc winds in black hole soft states

High-resolution spectra of Galactic black holes (GBHs) reveal the presence of highly ionized absorbers. In one GBH, accreting close to the Eddington limit for more than a decade, a powerful accretion disc wind is observed to be present in softer X-ray states and it has been suggested that it can carry away enough mass and energy to quench the radio jet. Here we report that these winds, which may have mass outflow rates of the order of the inner accretion rate or higher, are a ubiquitous component of the jet-free soft states of all GBHs. We furthermore demonstrate that these winds have an equatorial geometry with opening angles of few tens of degrees, and so are only observed in sources in which the disc is inclined at a large angle to the line of sight. The decrease in Fe xxv/Fe xxvi line ratio with Compton temperature, observed in the soft state, suggests a link between higher wind ionization and harder spectral shapes. Although the physical interaction between the wind, accretion flow and jet is still not fully understood, the mass flux and power of these winds and their presence ubiquitously during the soft X-ray states suggest they are fundamental components of the accretion phenomenon