Astron. Astrophys. 319, 855-862 (1997)

1. Introduction

4U 1630-47 is an X-ray transient that was discovered by Uhuru (Jones et al. 1976), with the first recorded outburst in 1969 detected by VELA 5B (Priedhorsky 1986). The first five outbursts were observed by a combination of the VELA 5B, Uhuru, OSO 7, and Ariel V satellites and seemed to occur every 600 day (Jones et al. 1976; Priedhorsky 1986). The peak 2-10 keV luminosity is 2  erg s^-1 (for an assumed distance of 10 kpc) with the lightcurve often exhibiting a characteristic rapid rise and exponential decay. Typical e -folding rise and decay times vary between 1-15 and 20-130 days, respectively (Chen et al. submitted). A strict outburst periodicity was apparently ruled out by an extended 1977 outburst which started 70 days later than predicted and may have lasted for up to six months (Kaluzienski et al. 1978; Sims & Watson 1978). In 1984 the source underwent another outburst (Tanaka et al. 1984), the decay of which was observed by EXOSAT (Parmar et al. 1986; hereafter P86). Recently, an archival search revealed the presence of two previously unreported outbursts (Parmar et al. 1995; hereafter P95). The first was observed by the Einstein Solid State Spectrometer (SSS) in 1979 and the second by the ROSAT Position Sensitive Proportional Counter (PSPC) in 1992. The times of these outbursts are consistent with the previously reported 600 day outburst recurrence interval, allowing P95 to constrain the recurrence interval to be  days. Subsequent ROSAT observations in 1993 did not detect 4U 1630-47 providing stringent upper-limits to any 0.2-2.4 keV quiescent emission (P95).

No bursts or periodicities have been detected from 4U 1630-47 and close to the peak of the 1984 outburst the source exhibited irregular intensity variability with a characteristic timescale of 20 s (P86; Kuulkers et al. 1996b). During this outburst the 1-40 keV EXOSAT spectrum could be modeled by an absorbed soft 1 keV Wien-like component with a high-energy power-law tail. As the outburst decayed, both the low-energy absorption and the relative contribution of the soft component compared to that of the power-law decreased. The ultra-soft spectral shape at low energies, the change in relative contributions of the two spectral components, and the lack of detected pulsations or bursts are all indicative of a black hole nature for the compact object. Similar behavior is seen from other X-ray transients such as A 0620-00, GS 1124-684 and GS 2023+338 which are believed to contain black holes on the basis of their dynamically derived masses (e.g. McClintock 1992; Cowley 1994). There are now at least 15 similar black hole X-ray transient (BHXT) systems (White 1994; Tanaka & Lewin 1995) of which five are known to recur (see references in P95). The other BHXT systems have been observed only once during the 25 years of satellite observations. This suggests a typical recurrence timescale of 10-50 years. The more prolific outburst activity of 4U 1630-47 is therefore unusual.

We report the results of four Ginga and one ASCA observation of 4U 1630-47. The Ginga observations provide further confirmation of the 600 day period, while the more recent ASCA results imply a more complex outburst behavior. In addition, 4U 1630-47 was twice observed by the Einstein High Resolution Imager (HRI). During one observation the source was in outburst, while during the second a faint source was detected at a position coincident with 4U 1630-47.

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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