SUZAKUOBSERVATION OF GRS 1915+105: EVOLUTION OF ACCRETION DISK STRUCTURE DURING LIMIT-CYCLE OSCILLATION

Abstract

We present results from the Suzaku observation of the microquasar GRS 1915+105 performed during the 2005 October multiwavelength campaign. The data include both stable state (class χ) and limit-cycle oscillation (class θ). Correct interstellar absorption as well as effects of dust scattering are fully taken into account in the spectral analysis. The energy spectra in the 2-120 keV band in both states are all dominated by strong Comptonization of disk photons by an optically thick (τ ≈7-10) and low temperature (T e ≈2-3 keV) hybrid plasmas containing non-thermal electrons produced with 10-60% of the total power input. Absorption lines of highly ionized Fe ions detected during the oscillation indicate that a strong disk wind is developed. The ionization stage of the wind correlates with the X-ray flux, supporting the photoionization origin. The iron-K emission line shows a strong variability during the oscillation; the reflection is strongest during the dip but disappears during the flare. We interpret this as evidence for "self-shielding" that the Comptonizing corona becomes geometrically thick in the flare phase, preventing photons from irradiating the outer disk. The lowtemperature and high luminosity disk emission suggests that the disk structure is similar to that in the very high state of canonical black hole binaries. The spectral variability during the oscillation is explained by the change of the disk geometry and of the physical parameters of Comptonizing corona, particularly the fractional power supplied to the acceleration of non-thermal particles.

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

“…The values of γ min and Γ, which were free to vary during first sets of fitting, were found to maintain almost constant values (γ min : mean 1.3, rms 0.7; Γ: mean 4.6, rms 1.6) in all the observations and in the various segments and were then fixed to their average values. The γ min closely agrees with the results obtained by Zdziarski et al (2005) for the analysis of RXTE+OSSE observations, while, Γ is fixed to a value slightly larger or marginally compatible with those from previous measurements (Zdziarski et al 2005;Ueda et al 2010).…”

“…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.…”

“…The low energy absorption of GRS 1915+105 is complex and variable (Belloni et al 2000;Lee et al 2002;Martocchia et al 2006;Yadav 2006;Ueda et al 2009Ueda et al , 2010van Oers et al 2010). The values of the equivalent column density N H obtained from the analysis of X-ray data ranges between 2 × 10 22 and 16 × 10 22 cm −2 (Greiner et al 1996;Taam et al 1997;Trudolyubov 2001;Martocchia et al 2002;Lee et al 2002;Yadav 2006;van Oers et al 2010) and are generally higher than the values derived from the 21 cm hydrogen line emission along the line of sight (4.7 ± 0.2 × 10 22 cm −2 ; Chaty et al 1996) (≥1.8×10 22 cm −2 ; Dickey & Lockman 1990).…”

“…The source has been extensively observed in the X-ray band (Fender & Belloni 2004;Rodriguez et al 2008;Ueda et al 2010;Vierdayanti et al 2010). The X-ray emission is characterized by strong variability with alternating bright bursts and quiet phases.…”

“…The phenomenological power-law model is used to describe a Comptonization component from an optically thick corona above the disk with an electron population containing either thermal or non-thermal particles (Zdziarski et al 2001) and where energy can be also gained from the gas bulk motion (Titarchuk et al 1997). In addition to the main spectral components, lower intensity features have occasionally been detected: a Compton reflection from the disk (Zdziarski et al 2001;Ueda et al 2009), an iron line present only in a few observations (Feroci et al 1999;Martocchia et al 2002Martocchia et al , 2006Ueda et al 2010).…”