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ARCHIVAL OBSERVATIONS OF THE ULTRA-SOFT X-RAY TRANSIENT 4U 1630-47

We report on archival observations of the ultra-soft X-ray transient 4U 1630−47. In 1979 February, 1989 March, and 1994 September, 4U 1630−47 was observed in outburst by the Einstein High Resolution Imager (HRI), the Ginga Large Area Counter (LAC) and the ASCA Gas Imaging Spectrometers, respectively. In 1987 October, the Ginga LAC observed the region of sky containing 4U 1630−47 and a faint source was detected. While it is possible that this emission originates from another object within the LAC field of view, its properties are consistent with an observation towards the end of an outburst. The times of the Ginga outbursts are consistent with the known ∼600 day ephemeris. However, the ASCA observation and observations of the subsequent outburst by the Rossi X-ray Timing Explorer, indicate that these outbursts started 100–150 days later than predicted. This delay confirms the occurrence of more complex outburst behavior from 4U 1630−47. Finally, an observation by the imaging Einstein HRI in 1980 February, approximately mid-way between expected outburst times, revealed a faint source at a position consistent with 4U 1630−47. The measured source intensity is higher than the previous best upper-limit. This implies that (a) the quiescent intensity is variable by a factor ∼10, or (b) that the 1979 outburst had an extremely long duration, or (c) that the emission originates from previously undetected inter-outburst activity.