Chris Done

Modelling the behaviour of accretion flows in X-ray binaries

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.

Is the soft excess in active galactic nuclei real

We systematically analyse all publicly available XMM-Newton spectra of radioquiet PG quasars. The soft X-ray excess in these objects is well modelled by an additional, cool, Compton scattering region. However, the remarkably constant temperature derived for this component over the whole sample requires a puzzling fine tuning of the parameters. Instead, we propose that the soft excess is an artifact of strong, relativistically smeared, partially ionized absorption. The strong jump in opacity at ∼0.7 keV from ovii, oviii and iron can lead to an apparent soft excess below this energy, which is trivially constant since it depends on atomic processes. This can have a dramatic effect on the derived spectrum, which has implications for fitting the relativistic smearing of the reflected iron line emission from the disc.

The ultraluminous state

We revisit the question of the nature of ultraluminous X-ray sources (ULXs) through a detailed investigation of their spectral shape, using the h ighest quality X-ray data available in the XMM-Newton public archives ( � 10, 000 counts in their EPIC spectrum). We confirm that simple spectral models commonly used for the analys is and interpretation of ULXs (power-law continuum and multi-colour disc blackbody models) are inadequate in the face of such high quality data. Instead we find two near ubiquitous features in the spectrum: a soft excess and a roll-over in the spectrum at energies above 3 keV. We investigate a range of more physical models to describe these data. Slim discs which include radiation trapping (approximated by a p-free disc model) do not adequately fit the data, and several o bjects give unphysically high disc temperatures (kTin > 3 keV). Instead, disc plus Comptonised corona models fit the data well, but the derived corona is cool, and op tically thick (� � 5 30). This is unlike the � � 1 coronae seen in Galactic binaries, ruling out models where ULXs are powered by sub-Eddington accretion onto an intermediate mass black hole despite many objects having apparently cool disc temperatures. We argue that these observed disc temperatures are not a good indicator of the black hole mass as the powerful, optically thick corona drains energy from the inner disc, and obscures it. We estimate the intrinsic (corona-less) disc temperature, and demonstrate that in most cases it lies in th e regime of stellar mass black holes. These objects have spectra which range from those similar to the highest mass accretion rate states in Galactic binaries (a single peak at 2‐3 ke V), to those which clearly have two peaks, one at energies below 1 keV (from the outer, unComptonised disc) and one above 3 keV (from the Comptonised, inner disc). However, a few ULXs have a significantly cooler corrected disc temperature; we suggest that these are the most extreme stellar mass black hole accretors, in which a massive wind completely envelopes the inner disc regions, creating a cool photosphere. We conclude that ULXs provide us with an observational template for the transition between Eddington and super-Eddington accretion flows, with the latter occupying a new ultraluminousaccretion state.

The 1989 May outburst of the soft X-ray transient GS 2023+338 (V404 Cyg)

We reanalyse archival Ginga data of the soft X-ray transient source GS 20231338 covering the beginning of its 1989 May outburst. The source showed a number of rather unusual features: very high and apparently saturated luminosity, dramatic flux and spectral variability (often on ,1 s time-scale), and generally very hard spectrum, with no obvious soft thermal component characteristic for soft/high state. We describe the spectrum obtained at the maximum of flux and we demonstrate that it is very different from spectra of other soft X-ray transients at similar luminosity. We confirm previous suggestions that the dramatic variability was the result of heavy and strongly variable photoelectric absorption. We also demonstrate that for a short time the spectrum of the source did look like a typical soft/high state spectrum but that this coincided with very

HARD X-RAY EMISSION FROM LOW-MASS X-RAY BINARIES

We report on Rossi X-Ray Timing Explorer observations of four type I X-ray bursters, namely, 1E 1724-3045, GS 1826-238, SLX 1735-269, and KS 1731-260. The first three were in a low state, with 1-200 keV X-ray luminosities in the range ~0.05-0.1LEdd (LEdd: Eddington luminosity for a neutron star, LEdd = 2.5 × 1038 ergs s-1), whereas KS 1731-260 was in a high state, with luminosity ~0.35LEdd. The low-state sources have very similar power spectra, displaying high-frequency noise up to ~200 Hz. For KS 1731-260, its power spectrum is dominated by noise at frequencies 20 Hz; in addition a quasi-periodic oscillation at 1200 Hz is detected in a segment of the observation. The 1-200 keV spectra of the low-state sources are all consistent with resulting from thermal Comptonization with an electron temperature (kTe) around 25-30 keV. For KS 1731-260, the spectrum is also dominated by thermal Comptonization, but with a much lower kTe ~ 3 keV and no significant hard X-ray emission. With the exception of GS 1826-238, they each have an underlying soft component, carrying at most ~25% of the total 1-200 keV luminosity. For all sources, we have detected an iron Kα line at 6.4 keV (although it is weak and marginal in 1E 1724-3045). A reflection component is present in the spectra of GS 1826-238 and SLX 1735-269, and for both we find that the reflecting medium subtends only a small solid angle (Ω/2π ~ 0.15, 0.28). The origin of the line and the reflection component is most likely to be irradiation of the accretion disk by the X-ray source. We suggest a model in which the region of main energy release, where hard X-rays are produced, would be an optically thin boundary layer merged with an advection-dominated accretion flow (ADAF) and would be responsible for the rapid variability observed. The soft component observed probably represents the unscattered emission from an optically thick accretion disk of variable inner radius. When the accretion rate increases, the inner disk radius shrinks and the strength of the reflected component and associated iron line increase. At the same time, the Comptonization region cools off in response to an increased cooling flux from the accretion disk and from the reprocessed/reflected component, thus leading progressively to a quenching of the hard X-ray emission. If low-state neutron stars (NSs) accrete via ADAFs, the observation of X-ray bursts, indicating that all the accreting matter actually accumulates onto the NS surface, argues against the existence of strong winds from such accretion flows. Finally, we discuss two criteria recently proposed to distinguish between nonquiescent black holes (BHs) and NSs that are not contradicted by existing observations. The first one states that, when thermal Comptonization is responsible for the hard X-ray emission, only BHs have kTe larger than ~50 keV. However, this criterion is weakened by the fact that there are NSs displaying nonattenuated power laws extending up to at least 200 keV, possibly implying nonthermal Comptonization or thermal Comptonization with kTe larger than 50 keV. The second criterion stipulates that only BHs are capable of emitting hard X-ray tails with 20-200 keV luminosities 1.5 × 1037 ergs s-1.

Grand unification of AGN activity in the ΛCDM cosmology

We track the coevolution of supermassive black holes (SMBHs) and their host galaxies through cosmic time. The calculation is embedded in the GALFORM semi-analytic model which simulates the formation and evolution of galaxies in a cold dark matter (CDM) universe. The black hole (BH) and galaxy formation models are coupled: during the evolution of the host galaxy, hot and cold gas are added to the SMBH by flows triggered by halo gas cooling, disc instabilities and galaxy mergers. This builds up the mass and spin of the BH, and the resulting accretion power regulates gas cooling and subsequent star formation. The accretion flow is assumed to form a geometrically thin cool disc when the accretion rate exceeds Graphic, and a geometrically thick, radiatively inefficient hot flow when the accretion rate falls below this value. The resulting quasar optical luminosity function matches observations well, and the mass of the SMBH correlates with the mass of the galaxy bulge as in the observed Mbh–Mbulge relation. The BH spin distribution depends strongly on whether we assume that the gas in any given accretion episode remains in the same plane or it fragments into multiple, randomly aligned accretion episodes due to its self-gravity. We refer to these cases as the ‘prolonged’ and ‘chaotic’ accretion modes, respectively. In the chaotic accretion model there is a clear correlation of spin with SMBH mass (and hence host galaxy bulge mass). Massive BHs (M > 5 × 108 M⊙) are hosted by giant elliptical galaxies and are rapidly spinning, while lower mass BHs are hosted in spiral galaxies and have much lower spin. Using the Blandford–Znajek mechanism for jet production to calculate the jet power, our model reproduces the radio loudness of radio galaxies, low ionization emission regions (LINERS) and Seyferts, suggesting that the jet properties of active galaxy nuclei (AGN) are a natural consequence of both the accretion rate on to and the spin of the central SMBH. This is the first confirmation that a CDM galaxy formation model can reproduce the observed radio phenomenology of AGN.

A periodicity of ∼ 1 hour in X-ray emission from the active galaxy RE J1034+396

Active galactic nuclei and quasars are thought to be scaled-up versions of Galactic black hole binaries, powered by accretion onto supermassive black holes with masses of 106–109 , as opposed to the ∼10  in binaries (here is the solar mass). One example of the similarities between these two types of systems is the characteristic rapid X-ray variability seen from the accretion flow. The power spectrum of this variability in black hole binaries consists of a broad noise with multiple quasi-periodic oscillations superimposed on it. Although the broad noise component has been observed in many active galactic nuclei, there have hitherto been no significant detections of quasi-periodic oscillations. Here we report the discovery of an ∼1-hour X-ray periodicity in a bright active galaxy, RE J1034+396. The signal is highly statistically significant (at the 5.6σ level) and very coherent, with quality factor Q > 16. The X-ray modulation arises from the direct vicinity of the black hole.

Low-frequency quasi-periodic oscillations spectra and Lense–Thirring precession

We show that the low frequency QPO seen in the power density sp ectra of black hole binaries (and neutron stars) can be explained by Lense-Thirring prec ession. This has been proposed many times in the past, and simple, single radius models can q u litatively match the observed increase in QPO frequency by decreasing a characteristic ra dius, as predicted by the truncated disc models. However, this also predicts that the frequency is strongly dependent on spin, and gives a maximum frequency at the last stable orbit which is ge nerally much higher than the remarkably constant maximum frequency at ∼ 10 Hz observed in all black hole binaries. The key aspect of our model which makes it match these observatio ns is that the precession is of a radially extended region of the hot inner flow. The outer r adius is set by the truncation radius of the disc as above, but the inner radius lies well out side of the last stable orbit at the point where numerical simulations show that the density dro ps ff sharply for a misaligned flow. Physically motivated analytic estimates for this inne r radius show that it increases with a∗, decreasing the expected frequency in a way which almost com pletely cancels the expected increase with spin, and ties the maximum predicted frequenc y to around 10 Hz for all a∗. This is the first QPO model which explains both frequencies and spe ctrum in the context of a well established geometry for the accretion flow.

Testing Accretion Disk Theory in Black Hole X-Ray Binaries

WepresentresultsfromspectralmodelingofthreeblackholeX-raybinaries:LMCX-3,GROJ1655� 40,andXTE J1550� 564. Using a sample of disk-dominated observations, we fit the data with a range of spectral models that in- cludes a simple multitemperature blackbody (DISKBB), a relativistic accretion disk model based on color-corrected blackbodies (KERRBB), and a relativistic model based on non-LTE atmosphere models within anprescription (BHSPEC). BHSPEC provides the best fit for a BeppoSAXobservation of LMC X-3, which has the broadest energy coverage of our sample. It also provides the best fit for multiple epochs of Rossi X-Ray Timing Explorer (RXTE) data in this source, except at the very highest luminosity (L/LEdd k0:7), where additional physics must be coming into play.BHSPECisalsothebest-fitmodelformultiepochRXTEobservationsofGROJ1655� 40andXTEJ1550� 564, although the best-fit inclination of the inner disk differs from the binary inclination. All our fits prefer � ¼ 0:01 to � ¼ 0:1, in apparent disagreement with the large stresses inferred from the rapid rise times observed in outbursts of these two sources. In all three sources our fits imply moderate black hole spins (a� � 0:1 0:8), but this is sensitive to the reliability of independent measurements of these system parameters and to the physical assumptions that un- derly our spectral models. Subject headingg accretion, accretion disks — black hole physics — X-rays: binaries

The average X-ray/gamma-ray spectra of Seyfert galaxies from Ginga and OSSE and the origin of the cosmic X-ray background

Abstract : We have obtained the first average 2-500 keV spectra of Seyfert galaxies, using the data from Ginga and GRO OSSE. Our sample contains 3 classes of objects with markedly different spectra: radio-quiet Seyfert 1s and 2s, and radio-loud Seyfert 1s. The average radio-quiet Seyfert 1 spectrum is well- fitted by a power law continuum with the energy spectral index approximately equal 0.9, a Compton reflection component corresponding to a ~ 2x covering solid angle, and ionized absorption. There is a high-energy cutoff in the incident power law continuum: the e-folding energy is E(sub c) approximately equal 0.6 (sup +0.8, sub -0.3)MeV. The simplest model that describes this spectrum is Comptonization in a relativistic optically-thin thermal corona above the surface of an accretion disk. Radio-quiet Seyfert 2s show strong neutral absorption, and there is an indication that their X-ray power laws are intrinsically harder, although the Seyfert 1 spectrum with = 0.9 and strong reflection cannot be ruled out by the data. Finally, the radio-loud Seyfert spectrum has alpha approximately equal 0.7, moderate neutral absorption, E (sub c) = 0.4(sup +0.7, sub -0.2) MeV, and no or little Compton reflection. This is incompatible with the radio-quiet Seyfert 1 spectrum, and probably indicating that the X-rays are beamed away from the accretion disk in these objects. The average spectra of Seyferts integrated over redshift with a power law evolution can explain the hard X-ray spectrum of the cosmic background. The hump at ~ 30 keV in that spectrum is due to the dominant contribution of Seyfert 2s.