The 1997 hard state outburst of the X-ray transient GS 1354-64 / BW Cir

Abstract

We present observations of the 1997 outburst of the X-ray transient GS 1354-64 (BW Cir) at X-ray, optical and, for the first time, radio wavelengths; this includes upper limits to the linear and circular polarisation of the radio data. The X-ray outburst was unusual in that the source remained in the low/hard X-ray state throughout; the X-ray peak was also preceded by at least one optical outburst, suggesting that it was an `outside-in' outburst - similar to those observed in dwarf novae systems, although possibly taking place on a viscous timescale in this case. It therefore indicates that the optical emission was not dominated by the reprocessing of X-rays but that instead we see the instability directly. While the radio source was too faint to detect any extended structure, spectral analysis of the radio data and a comparison with other similar systems suggests that mass ejections, probably in the form of a jet, took place and that the emitted synchrotron spectrum may have extended as far as infrared wavelengths. Finally, we compare this 1997 outburst of GS 1354-64 with possible previous outbursts and also with other hard state objects, both transient and persistent. It appears that a set of characteristics -- such as a weak, flat spectrum radio jet, a mHz QPO increasing in frequency, a surprisingly high optical:X-ray luminosity ratio and the observed optical peak preceding the X-ray peak -- may be common to all hard state X-ray transients.