4. Transient behavior
4U 1630-47 has the shortest outburst recurrence interval of any known BHXT. The large number of observed outbursts provides a unique data set by which to study the BHXT outburst mechanism. The nature of these outbursts is still uncertain (see e.g. Lasota 1996), although recently an accretion disk instability trigger has been favored above the mass transfer instability model (see Narayan et al. 1996; van Paradijs 1996). Van Paradijs & Verbunt (1984) proposed that outbursts in low-mass X-ray binaries and dwarf novae may have a similar origin. Based on the similarity between the recurrence intervals of SU UMa outbursts and the timescales of the quasi-periodic intensity cycles seen from a number of persistent low-mass X-ray binaries, Priedhorsky (1986) and Priedhorsky & Holt (1987) suggested that these two types of systems are more suited to comparative studies. SU UMa systems often show superoutbursts in addition to normal outbursts (see e.g. Warner 1995). These outbursts are stronger and have longer durations and recurrence times than the normal ones. During superoutbursts optical variations with periods slightly longer than the orbital period may be observed (superhumps). The detection of superhumps during some BHXT outbursts has strengthened this similarity (Charles et al. 1991; Bailyn 1992; Kato et al. 1995). A further similarity between BHXTs and SU UMa systems was proposed by Kuulkers et al. (1996a) who noticed that some extreme members of the SU UMa class, the "tremendous outburst amplitude dwarf novae", or TOADs, have the following characteristics very similar to those of BHXTs: (a) long quiescent intervals between large-amplitude outbursts; (b) no intervening normal outbursts; (c) optical outburst lightcurves exhibiting both "glitches" and superhumps; (d) very small mass ratios; and (e) very low mass transfer rates during quiescence (Kuulkers et al. 1996a, and references therein). The similarity in properties may indicate that the underlying mechanism which produces TOAD and BHXT outbursts is the same.
The 4U 1630-47 outburst recurrence interval of 1.6 yrs is shorter than those of other BHXTs of 10-50 yrs and those of typical TOADs, which have recurrence times of up to decades. The 4U 1630-47 outburst recurrence interval is more similar to those of the superoutbursts of the shorter recurrence outbursting TOADs, such as SW UMa ( 1-4 years; Howell et al. 1995). Interestingly, the superoutburst recurrence intervals of individual SU UMa sources vary by 10% (Vogt 1980), which is similar to the variation in the 4U 1630-47 outburst times of 8%.
© European Southern Observatory (ESO) 1997
Online publication: July 3, 1998