Discovery of Photon Index Saturation in the Black Hole Binary GRS 1915+105

Abstract

We present a study of the correlations between spectral, timing properties and mass accretion rate observed in X-rays from the Galactic Black Hole (BH) binary GRS 1915+105 during the transition between hard and soft states. We analyze all transition episodes from this source observed with Rossi X-ray Timing Explorer (RXTE), coordinated with Ryle Radio Telescope (RT) observations. We show that broad-band energy spectra of GRS 1915+105 during all these spectral states can be adequately presented by two Bulk Motion Comptonization (BMC) components: a hard component (BMC1, photon index Γ 1 = 1.7 − 3.0) with turnover at high energies and soft thermal component (BMC2, Γ 2 = 2.7−4.2) with characteristic color temperature ≤ 1 keV, and the redskewed iron line (LAOR) component. We also present observable correlations between the index and the normalization of the disk "seed" component. The use of "seed" disk normalization, which is presumably proportional to mass accretion rate in the disk, is crucial to establish the index saturation effect during the transition to the soft state. We discovered the photon index saturation of the soft and hard spectral components at values of < ∼ 4.2 and 3 respectively. We present a physical model which explains the indexseed photon normalization correlations. We argue that the index saturation effect of the hard component (BMC1) is due to the soft photon Comptonization in the converging inflow close to BH and that of soft component is due to matter accumulation in the transition layer when mass accretion rate increases. Furthermore

On the stability of the thermal Comptonization index in neutron star low-mass X-ray binaries in their different spectral states

“…The index saturation observed in BH sources as they move to the HSS finds a natural explanation in the framework of a bulk-dominated scenario (see mathematical proof of this statement in e.g. Titarchuk & Seifina 2009). Moreover, when the sources are in the LHS the points in the Γ vs. N disk /QPO diagram form a plateau around Γ ∼ 1.5 before the rising phase.…”

The complex behaviour of the microquasar GRS 1915+105 in theρclass observed withBeppoSAX

“…Once again, we found that the best-fit spectral distributions, particularly those of the P-1 and P-2 intervals, were not acceptable, in term of either the χ 2 values or the large residuals. The use of an equivalent column density, N H = 5×10 22 cm −2 , as the one already considered by Titarchuk & Seifina (2009), provided only a marginal improvement to the fits. The spectra from the SLT-1 and FDT intervals could be modeled by a combination of two bmcs, the χ 2 distribution of the spectra were compatible at the 2σ level with the expected ones, and the spectral parameters of the first bmc are well-defined with an approximately stable temperature (kT 1 keV), photon index (Γ = 2.8), and Comptonised fraction ( f = 0.5).…”

“…iv) -Comptonization from bulk-motion matter: the model (bmc in XSPEC) was developed by Titarchuk et al (1997) to describe the emission of accreting matter onto a black hole and takes into account the Comptonization of soft photons by a hot gas and the gain in energy originating from the dynamic bulk motion. The combination of two bmc components, which was used by Titarchuk & Seifina (2009) to fit many RXTE spectra of GRS 1915+105, is characterized by the black-body temperature of the soft photon source, a spectral (energy) index, and an illumination parameter related to the fraction of the bulk-motion flow irradiated by the thermal photon source. We left the temperatures of the two bmc as free parameters, because fixing them to the value used by Titarchuk & Seifina (2009) for the RXTE data (1 keV) did not provide acceptable fits to all spectra.…”

“…The combination of two bmc components, which was used by Titarchuk & Seifina (2009) to fit many RXTE spectra of GRS 1915+105, is characterized by the black-body temperature of the soft photon source, a spectral (energy) index, and an illumination parameter related to the fraction of the bulk-motion flow irradiated by the thermal photon source. We left the temperatures of the two bmc as free parameters, because fixing them to the value used by Titarchuk & Seifina (2009) for the RXTE data (1 keV) did not provide acceptable fits to all spectra. An iron line with a Gaussian shape was added to the model, together with an high energy cut-off for one of the two bmc components.…”

“…The detection of a fluorescence Kα iron line from GRS 1915+105 has been reported on several occasions (Martocchia et al 2002;Zdziarski et al 2005;Martocchia et al 2006;Ueda et al 2010) and, more recently, by Titarchuk & Seifina (2009). In particular, Martocchia et al (2002) from the detection of a strong line found evidence of relativistic effects in a black hole environment.…”