Transient outbursts have been discovered by various X-ray satellites, in particular those with large field-of-view detectors on board, e.g., Ariel V, Tenma, Ginga, Granat, CGRO and more recently RXTE, BeppoSAX.
Among —300 XBs known to date, roughly half of them are classified as transient sources. The transients are further divided into two different classes: the HMXB transients and the LMXB transients. Almost all HMXB transients are recurrent X-ray pulsars, in which a strongly magnetized neutron star in an eccentric orbit periodically encounters a stellar wind zone around a massive companion. They are not relevant to this chapter.
Of about 150 LMXBs known to date , one half are transients. These LMXB transients are characterized by episodic X-ray outbursts without a fixed periodicity. They are in a quiescent state for most of the time, and occasionally undergo dramatic X-ray outbursts. During an outburst, they show a soft X-ray spectrum (softer than the X-ray pulsars) characteristic of high-luminosity LMXBs, as explained in Sect. 16.4.3. These are called "soft X-ray transients," or sometimes "X-ray novae." The soft X-ray transients are a subset of LMXBs containing either a weakly-magnetized neutron star or a black hole. Many soft X-ray transients exhibit recurrent outbursts with intervals ranging from a few years to tens of years or even longer (e.g., Table 16.1). Perhaps all soft X-ray transients are recurrent. X-ray outbursts are accompanied by optical outbursts and sometimes radio jets that allow identification of the optical counterparts. For reviews, see  and .
As shown in the preceding section, 17 of the secure black-hole XBs are LMXBs and are all soft X-ray transients. In addition, —20 more soft X-ray transients are suspected to contain black holes (black hole candidates; see ) based on their X-ray and other properties, discussed in Sect. 16.4.3. This fact indicates that blackhole binaries occupy a major fraction of soft X-ray transients. The observed outburst rate allows a rough estimation of the total number of black-hole binaries existing in our Galaxy. Though the average recurrence period is largely uncertain, a modest estimate gives a few hundred, and could possibly be much more (e.g., [65, 78]).
Figure 16.1 shows X-ray light curves of the outbursts of four black-hole binaries, which exhibit monotonous exponential decays except for intermediate increases. However, many other outbursts show much more complex light curves (see [10,39]. In most of the outbursts, the source brightens to an X-ray luminosity Lx — 1038 erg s-1or sometimes even —1039 erg s-1at the outburst peak. The source returns to quiescence usually after a few months. There are exceptional cases: GRS 1915+105 has remained bright since the initial outburst in 1992. GX339-4 repeats frequent outbursts but has never decayed into the quiescent state. Radio outbursts, sometimes
superluminal jets, are often observed at the X-ray outbreak, hence nicknamed "microquasars" (e.g., ).
An X-ray outburst is triggered by a sudden onset of the accretion flow onto the compact object, followed by the decay as the accretion rate gradually diminishes. Because the accretion rate changes over orders of magnitude through the decay, soft X-ray transients are extremely useful for studying the physics of mass accretion and X-ray properties as a function of the accretion rate. In fact, much of our current knowledge described in the following sections has been obtained from the studies of soft X-ray transients.
Whether a low-mass binary remains persistently X-ray active or not seems to depend on the accretion rate. Apparently, there is a minimum accretion rate Mmin below which X-ray emission is practically turned off. (Notice the abrupt fall in the last part of decay in Fig. 16.1) The available data indicate that Mmin — 1016 g s"1 corresponding to Lx — 1036 erg s"1. In fact, there exist few persistent sources of Lx C 1036 erg s"1. The long quiescence of soft X-ray transients between outbursts is probably because M < Mmin.
A soft X-ray transient outbursts are generally explained by the disk instability model originally developed for dwarf novae. The concept is as follows: An accretion disk has two stable states; a cool neutral state and a hot fully-ionized state. The quiescent state corresponds to the cool state where viscosity is too small to allow a stable accretion flow. As matter from the companion accumulates in the outer disk, the surface density and temperature increase gradually. When the surface density reaches a certain critical value, the disk jumps to the hot state because of a thermal instability. This triggers an accretion flow into the inner part of the disk, causing an X-ray outburst. When the surface density drops below another critical value, the disk returns to the cool quiescent state.
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