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 GS1354-64 (BW Cir) at X-ray, optical and, for the first time, radiowavelengths. The X-ray outburst was unusual in that the sourceremained in the low/hard X-ray state throughout; the X-ray peak wasalso preceded by at least one optical outburst, suggesting thatit was an `outside-in' outburst – similar to those observed in dwarfnovae systems, although possibly taking place on a viscous timescalein this case. It therefore indicates that the optical emission wasnot dominated by the reprocessing of X-rays but that instead wesee the instability directly. While the radio source was too faint todetect any extended structure, spectral analysis of the radio data anda comparison with other similar systems suggests that mass ejections,probably in the form of a jet, took place and that the emittedsynchrotron spectrum may have extended as far as infraredwavelengths. Finally, we compare this 1997 outburst of GS 1354-64with possible previous outbursts and also with other hard stateobjects, both transient and persistent. It appears that a set ofcharacteristics – such as a weak, flat spectrum radio jet, a mHz QPOincreasing in frequency, a surprisingly high optical:X-ray luminosityratio and the observed optical peak preceding the X-ray peak – may becommon to all hard state X-ray transients.