INTEGRAL Picture Of the Month
May 2008

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The Rapid Burster observed 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 2-3 times per year. It is located at a distance of 8 kpc in a globular cluster (Liller 1) in the Galactic center 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 (5 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 an instability in the accretion disc, and thus derives it energy from the strong gravity 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 is normally changing from Type-I bursts in the beginning, then Type-I and Type-II bursts, and eventually towards the end a phase of very regular, frequent, short Type-II bursts.

The Rapid Burster was observed to be active during INTEGRAL observation of the Galactic Center region in February-March 2008 (Kuulkers et al., ATEL #1398). Here we show light curves in the 3-15 keV X-ray band from the JEM-X monitor. The upper panel of the plot shows a 3 hour period starting at 04:47, March 13, 2008, where the Rapid Burster did its best to live up to its popular name, firing off 318 Type-II bursts in rapid succession. In order to see finer details, the middle panel shows a 6x zoom to show 44 bursts in 30 minutes (red, 4 second binning), and in the lower panel we zoom 6 times further to show 8 bursts in 5 minutes (blue, 2 second binning).

Provided by the JEM-X team at the National Space Institute DTU, Copenhagen, data courtesy of Marco Tavani, INAF-IASF Roma.

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