Abstract
Context. It has been suggested that the cycles of activity of X-ray binaries (XRB) are triggered by a switch in the dominant disk torque responsible for accretion. As the disk accretion rate increases, the disk innermost regions therefore change from a jet-emitting disk (JED) to a standard accretion disk (SAD). Aims. While JEDs have been proven to successfully reproduce X-ray binary hard states, the existence of an outer cold SAD introduces an extra nonlocal cooling term. We investigate the thermal structure and associated spectra of such a hybrid disk configuration. Methods. We use a two-temperature plasma code, allowing for outside-in computation of the disk local thermal equilibrium with self-consistent advection and optically thin-to-thick transitions in both radiation and gas supported regimes. The nonlocal inverse Compton cooling introduced by the external soft photons is computed by the BELM code. Results. This additional cooling term has a profound influence on JED solutions, allowing a smooth temperature transition from the outer SAD to the inner JED. We explore the full parameter space in disk accretion rate and transition radius, and show that the whole domain in X-ray luminosities and hardness ratios covered by standard XRB cycles is well reproduced by such hybrid disk configurations. Precisely, a reasonable combination of these parameters allows us to reproduce the 3–200 keV spectra of each of five canonical XRB states. Along with these X-ray signatures, JED-SAD configurations also naturally account for the radio emission whenever it is observed. Conclusions. By varying only the radial transition radius and the accretion rate, hybrid disk configurations combining an inner JED and an outer SAD are able to simultaneously reproduce the X-ray spectral states and radio emission of X-ray binaries during their outburst. Adjusting these two parameters, it is then possible to reproduce a full cycle. This will be shown in a forthcoming paper.
Highlights
X-ray binaries (XRBs) display cycles of strong activity, where their luminosity increases by several orders of magnitude and their spectral shape changes drastically on long timescales, before decreasing back to quiescence
In Paper II, we showed that a supersonic accretion with ms = 1.5 allows to reproduce luminous hard states. – b = 0.3: the fraction of the released accretion energy Pacc transferred to the jets has been computed within self-similar models and goes from almost 1 to roughly 0.2 (Ferreira 1997; Petrucci et al 2010)
We have reported in this table the values of the power-law fraction PL f, X-ray luminosity, and spectral index Γ derived from XSPEC fits, as well as the exbpPeeRec/nt(e4dcπorDmad2pνiuoRt)fle,duwxiintdhemnνJsRyit=yus8ai.tn68gG.6EHGqz.H(1azn0,d)F, 8nf.R6am=GHe1zl.y×TFh18e0.6s−eG10Hfl.zuF=xieg1su0hr2ea6sv×9e and 10 illustrate the thermal state radial distribution and the theoretical and faked spectra
Summary
X-ray binaries (XRBs) display cycles of strong activity, where their luminosity increases by several orders of magnitude and their spectral shape changes drastically on long timescales, before decreasing back to quiescence. Petrucci et al (2010) computed the thermal states of a pure JED solution and successfully reproduced the spectral emission, jet power, and jet velocity during hard states of Cygnus X-1 Their calculations were done assuming a one-temperature (1T) plasma, but the necessity of a twotemperature (2T) plasma seems inevitable to cover the large variation of accretion rate expected during an entire outburst SAD-JED local transitions are expected to occur locally on dynamical time scales, typically ∼1 ms Kepler orbital time at 10 Rg, whereas hard-to-soft transitions involve time scales of days or even weeks This implies that, at any given time, the disk must be in some hybrid configuration with some regions emitting jets, while others do not.
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