The 2005 outburst of GRO J1655-40: spectral evolution of the rise, as observed by Swift

Abstract

We present Swift observations of the black hole X-ray transient, GRO J1655-40, during the recent outburst. With its multiwavelength capabilities and flexible scheduling, Swift is extremely well-suited for monitoring the spectral evolution of such an event. GRO J1655-40 was observed on 20 occasions and data were obtained by all instruments for the majority of epochs. X-ray spectroscopy revealed spectral shapes consistent with the ``canonical'' low/hard, high/soft and very high states at various epochs. The soft X-ray source (0.3-10 keV) rose from quiescence and entered the low/hard state, when an iron emission line was detected. The soft X-ray source then softened and decayed, before beginning a slow rebrightening and then spending $\sim 3$ weeks in the very high state. The hard X-rays (14-150 keV) behaved similarly but their peaks preceded those of the soft X-rays by up to a few days; in addition, the average hard X-ray flux remained approximately constant during the slow soft X-ray rebrightening, increasing suddenly as the source entered the very high state. These observations indicate (and confirm previous suggestions) that the low/hard state is key to improving our understanding of the outburst trigger and mechanism. The optical/ultraviolet lightcurve behaved very differently from that of the X-rays; this might suggest that the soft X-ray lightcurve is actually a composite of the two known spectral components, one gradually increasing with the optical/ultraviolet emission (accretion disc) and the other following the behaviour of the hard X-rays (jet and/or corona).