GRO J1655−40: Early Stages of the 2005 Outburst

Abstract

The black hole X-ray binary transient GRO J1655-40 underwent an outburst beginning in early 2005. We present the results of our multiwavelength observational campaign to study the early outburst spectral and temporal evolution, which combines data from X-ray (RXTE and INTEGRAL), radio (VLA), and optical (ROTSE and SMARTS) instruments. During the reported period, the source left quiescence and went through four major accreting black hole states: low-hard, hard intermediate, soft intermediate, and high-soft. We investigated dipping behavior in the RXTE band and compare our results to the 1996-1997 case, when the source was predominantly in the high-soft state, finding significant differences. We consider the evolution of the low-frequency quasi-periodic oscillations and find that the frequency strongly correlates with the spectral characteristics, before shutting off prior to the transition to the high-soft state. We model the broadband high-energy spectrum in the context of empirical models, as well as more physically motivated thermal and bulk motion Comptonization and Compton reflection models. RXTE and INTEGRAL data together support a statistically significant high-energy cutoff in the energy spectrum at ≈100-200 keV during the low-hard state. The RXTE data alone also show it very significantly during the transition, but not in the high-soft state spectra. We consider radio, optical, and X-ray connections in the context of possible synchrotron and synchrotron self-Compton origins of X-ray emission in low-hard and intermediate states. In this outburst of GRO J1655-40, the radio flux does not rise strongly with the X-ray flux.