X-ray light curve of the Rapid Burster in a very active Type II burst state
The Rapid Burster (MXB 1730-335) is a recurrent, transient low mass X-ray binary discovered in 1976. The source shows activity for a few weeks, two to three times per year. It is located at a distance of 8 kpc in a globular cluster (Liller 1) in the Galactic centre region.
Low mass X-ray binaries are systems where a compact object, either a neutron star or a black hole, accretes material from a relatively low mass companion star. In this case the compact object is a neutron star.
As the name indicates the source is an X-ray burster. However, X-ray bursts are divided into two classes, not particularly informatively named Type-I and Type-II.
Type-I bursts are thermonuclear flashes in the material accreted on the surface of a neutron star. Currently 88 Galactic systems showing Type-I bursts are known (five of these systems were discovered to be Type-I X-ray bursters by INTEGRAL). One of them is the Rapid Burster which occasionally shows Type-I bursts.
However, the Rapid Burster is also the prototype of the much rarer Type-II burster, where the burst mechanism is quite different. A Type-II burst is due to a instability in the accretion disc, and thus derives its energy from the strong gravitational field near the neutron star.
The Rapid Burster is the only known source to exhibit both Type-I and Type-II bursts. During one of its active periods the bursting pattern evolves, normally starting with Type-I bursts, followed by both Type-I and Type-II bursts, and eventually towards the end going through a phase of very regular, frequent, short Type-II bursts.
The Rapid Burster was observed to be active during INTEGRAL observation of the Galactic centre region in February-March 2008 (see related publication by Kuulkers et al., ATel #1398). The image shows light curves in the 3-15 keV X-ray band from the JEM-X monitor:
Provided by the JEM-X team at the National Space Institute DTU, Copenhagen, data courtesy of Marco Tavani, INAF-IASF Roma.