Do the Spectra of Soft X‐Ray Transients Reveal Bulk‐Motion Inflow Phenomenon?

Abstract

We present our analysis of the high-energy radiation from black hole (BH) transients, using archival data obtained primarily with the Rossi X-ray Timing Explorer (RXTE), and a comprehensive test of the bulk motion Comptonization (BMC) model for the high-soft state continuum. The emergent spectra of over 30 separate measurements of GRO J1655-40, GRS 1915+105, GRS 1739 1630-47 XTE J1755-32, and EXO 1846-031 X-ray sources are successfully fitted by the BMC model, which has been derived from basic physical principles in previous work. This in turn provides direct physical insight into the innermost observable regions where matter impinging upon the event horizon can effectively be directly viewed. The BMC model is characterized by three parameters: the disk color temperature, a geometric factor related to the illumination of the black hole (BH) site by the disk and a spectral index related to the efficiency of the bulk motion upscattering. For the case of GRO J1655-40, where there are distance and mass determinations, a self consistency check of the BMC model has been made, in particular, the assumption regarding the dominance of gravitational forces over the pressure forces within the inner few Schwarzschild radii. We have also examined the time behavior of these parameters which can provide information on the source structure. Using our inferred model parameters: color temperature, spectral index and an absolute normalization we present new, independently derived, constraints on the black hole mass, mass accretion rate and the distance for the aforementioned sources. Also notable is the relationship between the color temperature and flux, which for GRO J1655-40 is entirely distinct from a simple T 4 dependence, and strikingly consistent with the disk model we have invoked -standard Shakura-Sunyaev's disk with the modification to the electron scattering. This -3provides insight into the origin of the seed soft photons, and allows us to impose an important estimation of the hardness parameter, T h , which is the ratio of the color temperature to the effective temperature -we find T h ≃ 2.6, higher than previous estimates used in the literature. Subject headings: accretion -black hole physics -binaries: close -radiation mechanisms: nonthermal -Compton and inverse Compton-relativitystars: individual (GRO J1655-40, GRS 1915+105, GRS 1739-278, 4U 1630-47 XTE J1755-32, and EXO 1846-031)

The X-ray spectral evolution of Cygnus X-2 in the framework of bulk Comptonization

“…The scenario where both thermal and bulk Comptonization contribute to explaining the LMXB spectral evolution is gaining strong theoretical (Titarchuk et al 1996Laurent & Titarchuk 1999, 2001) and observational support (Shrader & Titarchuk 1998;Borozdin et al 1999;P06;F07;F08;this work). This is also true in the case of accretion-powered X-ray pulsars for which a theoretical model based on thermal and bulk Comptonization occurring in accreting shocked gas was developed by Becker & Wolff (2007).…”

Polarimetric observations of GRB 011211

“…However, the analysis of the broad-band spectrum taken on Dec. 12.3 (∼1 day before our polarization measurement), including data in the optical (Holland et al 2002) and X-ray bands (Reeves et al 2002;Borozdin & Trudolyubov 2002), requires the presence of a spectral break at about ν b ∼ 10 15 Hz, very close to the optical band (Fig. 4).…”

XMM–Newtonobservations of the ultraluminous nuclear X-ray source in M 33

“…We tried also the bulk motion Comptonization model (bmc model in xspec, Laurent & Titarchuk 1999), which has been used successfully to fit the soft state of several Galactic black hole candidates (Borozdin et al 1999), but also some ULX, with M 33 X-8 among them (Schrader & Titarchuk 2003); however, in the present case, the fit gives unphysical results, with pegged parameters. We therefore do not mention this in Table 2.…”