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Register a new userModelling the behaviour of accretion flows in X-ray binaries
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(abridged) 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 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 and neutron stars. 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. These systems can also show very different spectra at these high luminosities, in which the disc spectrum 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 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 1s, and the growth of the first black holes in the Early Universe.
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The properties of wind accretion in symbiotic X-ray binaries (SyXBs) consisting of red-giant and magnetized neutron star (NS) are discussed. The spin-up/spin-down torques applied to NS are derived based on a hydrodynamic theory of quasi-spherical accretion onto magnetized NSs. In this model, a settling subsonic accretion proceeds through a hot shell formed around the NS magnetosphere. The accretion rate onto the NS is determined by the ability of the plasma to enter the magnetosphere.Due to large Reynolds numbers in the shell, the interaction of the rotating magnetosphere with plasma initiates a subsonic turbulence. The convective motions are capable of carrying the angular momentum through the shell. We carry out a population synthesis of SyXBs in the Galaxy with account for the spin evolution of magnetized NS. The Galactic number of SyXBs with bright (M_v<1) low-mass red-giant companion is found to be from sim 40 to 120, and their birthrate is sim 5times 10^{-5}-10^{-4} per year. According to our model, among known SyXBs, Sct X-1 and IRXS J180431.1-273932 are wind-fed accretors. GX 1+4 lies in the transition from the wind-fed SyXBs to SyXBs in which the giants overflow their Roche lobe. The model successfully reproduces very long NS spins (such as in IGR J16358-4724 and 4U 1954+31) without the need to invoke very strong magnetic fields.
When the matter from a companion star is accreted towards the central compact accretor, i.e. a black hole (BH) or a neutron star (NS), an accretion disc and a jet outflow will form, providing bight X-ray and radio emission, which is known as X-ray binaries (XRBs). In the low/hard state, there exist disc-jet couplings in XRBs, but it remains uncertain whether the jet power comes from the disc or the central accretor. Moreover, BHXRBs have different properties compared with NSXRBs: quiescent BHXRBs are typically two to three orders of magnitude less luminous than NSXRBs in X-ray, whereas BHXRBs are more radio loud than NSXRBs. In observations, an empirical correlation has been established between radio and X-ray luminosity, $L_{rm R} propto L_{rm X}^b$, where $bsim 0.7$ for BHXRBs and $b sim 1.4$ for non-pulsating NSXRBs. However, there are some outliers of BHXRBs showing unusually steep correlation as NSXRBs at higher luminosities. In this work, under the assumption that the origin of jet power is related to the internal energy of the inner disc, we apply our magnetized, radiatively efficient thin disc model and the well-known radiatively inefficient accretion flow model to NSXRBs and BHXRBs. We find that the observed radio/X-ray correlations in XRBs can be well understood by the disc-jet couplings.
From hot, tenuous gas dominated by Compton processes, to warm, photoionized emission-line regions, to cold, optically thick fluorescing matter, accreting gas flows in X-ray binaries span a huge portion of the parameter space accessible to astrophysical plasmas. The coexistence of such diverse states of material within small volumes (10^33-10^36 cm^3) leaves X-ray spectroscopists with a challenging set of problems, since all such matter produces various X-ray spectral signatures when exposed to hard X rays. Emission-line regions in X-ray binaries are characterized by high radiation energy densities, relatively high particle densities, and velocities ~1000 km/s. In this article, we describe some recent efforts to generate detailed X-ray line spectra from models of X-ray binaries, whose aims are to reproduce spectra acquired with the ASCA, Chandra, and XMM-Newton observatories. With emphasis on the global nature of X-ray line emission in these systems, the article includes separate treatments of high-mass and low-mass systems, as well as summaries of continuum spectroscopy
We search the literature for reports on the spectral properties of neutron-star low-mass X-ray binaries when they have accretion luminosities between 1E34 and 1E36 ergs/s. We found that in this luminosity range the photon index (obtained from fitting a simple absorbed power-law in the 0.5-10 keV range) increases with decreasing 0.5-10 keV X-ray luminosity (i.e., the spectrum softens). Such behaviour has been reported before for individual sources, but here we demonstrate that very likely most (if not all) neutron-star systems behave in a similar manner and possibly even follow a universal relation. When comparing the neutron-star systems with black-hole systems, it is clear that most black-hole binaries have significantly harder spectra at luminosities of 1E34 - 1E35 erg/s. Despite a limited number of data points, there are indications that these spectral differences also extend to the 1E35 - 1E36 erg/s range. This observed difference between the neutron-star binaries and black-hole ones suggests that the spectral properties (between 0.5-10 keV) at 1E34 - 1E35 erg/s can be used to tentatively determine the nature of the accretor in unclassified X-ray binaries. We discuss our results in the context of properties of the accretion flow at low luminosities and we suggest that the observed spectral differences likely arise from the neutron-star surface becoming dominantly visible in the X-ray spectra. We also suggest that both the thermal component and the non-thermal component might be caused by low-level accretion onto the neutron-star surface for luminosities below a few times 1E34 erg/s.
Precessing accretion discs have long been suggested as explanations for the long periods observed in a variety of X-ray binaries, most notably Her X-1/HZ Her. We show that an instability of the discs response to the radiation reaction force from the illumination by the central source can cause the disc to tilt out of the orbital plane and precess in something like the required manner. The rate of precession and disc tilt obtained for realistic values of system parameters compare favourably with the known body of data on X-ray binaries with long periods. We explore other possible types of behaviour than steadily precessing discs that might be observable in systems with somewhat different parameters. At high luminosities, the inner disc tilts through more than 90 degrees, i.e. it rotates counter to the usual direction, which may explain the torque reversals in systems such as 4U 1626-67.
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