X-ray Spectral Analysis of the Steady States of GRS 1915+105

Charith S. Peris, Ronald A. Remillard, James F. Steiner, Saeqa D. Vrtilek, Peggy Varniere, Jerome Rodriguez, Guy Pooley

Published 2015-09-29Version 1

We report on the X-ray spectral behavior within the steady states of GRS 1915+105. Our work is based on a vast data set obtained using the Proportional Counter Array on the Rossi X-ray Timing Explorer (RXTE/PCA) during the course of its entire mission (1996-2012). We also utilized 15 GHz radio data obtained using the Ryle Telescope from 1995 to 2006. The steady observations within the X-ray data set naturally separated into two regions in the color-color diagram and we refer to them as steady-soft and steady-hard. GRS 1915+105 displays significant curvature in the coronal component in both the soft and hard data within the RXTE/PCA bandpass. We fit both steady-soft and steady-hard observations with a model comprised of 'simplcut' in tandem with a multicolor disk model 'ezdiskbb' with the steady-soft observations requiring a slightly more complex overall model. A majority of the steady-soft observations displays a roughly constant inner disk radius, reminiscent of canonical soft state black hole binaries. On the other hand, the steady-hard observations display an evolving disk truncation, which is correlated to the mass accretion rate through the disk. The disk flux and coronal flux are strongly correlated in steady-hard observations and very weakly correlated in the steady-soft observations. Within the steady-hard observations we observe two particular circumstances when there are correlations between the coronal X-ray flux and the radio flux with log slopes \eta=0.68+/-0.35 and \eta=1.12+/-0.13. They are consistent with the upper and lower tracks of Gallo et al. (2012), respectively. A comparison of model parameters to the state definitions show that almost all steady-soft observations match the criteria of either thermal or steep power law state. A large portion (80 %) of the steady-hard observations matches the hard state criteria when the disk fraction constraint is neglected.