A population study of type II bursts in the Rapid Burster

Abstract

Type II bursts are thought to arise from instabilities in the accretion flow onto a neutron star in an X-ray binary. Despite having been known for almost 40 years, no model can yet satisfactorily account for all their properties. To shed light on the nature of this phenomenon and provide a reference for future theoretical work, we study the entire sample of Rossi X-ray Timing Explorer data of type II bursts from the Rapid Burster (MXB 1730-335). We find that type II bursts are Eddington-limited in flux, that a larger amount of energy goes in the bursts than in the persistent emission, that type II bursts can be as short as 0.130 s, and that the distribution of recurrence times drops abruptly below 15-18 s. We highlight the complicated feedback between type II bursts and the NS surface thermonuclear explosions known as type I bursts, and between type II bursts and the persistent emission. We review a number of models for type II bursts. While no model can reproduce all the observed burst properties and explain the source uniqueness, models involving a gating role for the magnetic field come closest to matching the properties of our sample. The uniqueness of the source may be explained by a special combination of magnetic field strength, stellar spin period and alignment between the magnetic field and the spin axis.

Accretion Disks and Coronae in the X-Ray Flashlight

“…from the photosphere, disk, or corona), perhaps similar to the wind-like outflows that have been tentatively inferred from emission/absorption features in other X-ray bursts (e.g. Ballantyne and Everett 2005;in 't Zand and Weinberg 2010;. There is one reported case of the detection of narrow absorption features in the stacked X-ray spectrum of a number of X-ray bursts from the quasi-persistent LMXB EXO 0748-676, which were interpreted as gravitationally-redshifted lines from the neutron star photosphere (Cottam et al 2002).…”

“…In two sources with these strong variations, 4U 1820-30 and IGR J17062-6143, emission/absorption features were seen in the X-ray burst spectra, which may be related to the unusual flux variations ( The large-amplitude flux variability is likely linked to the material that is ejected from the photosphere during the superexpansion (Paczynski and Proszynski 1986;Ballantyne and Everett 2005;Weinberg et al 2006;in 't Zand et al 2011;. This is further discussed in Sect.…”

“…Tracing the evolution of the reflection features for 4U 1820-30 suggested that the inner disk receded during the X-ray burst (Ballantyne and Strohmayer 2004). To explain this, it was proposed that the inner part of the accretion flow was evacuated during the X-ray burst (Ballantyne and Everett 2005, see also Sect. 5.1).…”

“…The disappearance of the QPO signal could possibly be due to draining of the accretion disk (accompanied by a temporary enhancement in the accretion rate up to a factor of ≈ 6), but Peille et al (2014) pointed out that the (viscous) time to restore a depleted disk is > 100 s, whereas for most X-ray bursts the QPO was detected already ≈ 20-30 s after the X-ray burst onset. Therefore, these authors instead favored an interpretation in which the X-ray burst emission irradiates and heats the inner accretion disk, causing it to puff up (see also Ballantyne and Everett 2005). In Sect.…”

“…5.1.2). Although a thermallydriven disk wind should be ineffective in the inner disk region due to the strong gravity of the NS, the case of a radiatively-driven outflow from a disk during an X-ray burst was investigated by Ballantyne and Everett (2005). The computations suggested that the mass-outflow rate through the wind did not exceed the accretion rate through the disk, therefore the inner ≈ 100 r g (≈ 220 km) of the disk around 4U 1820-30 could not be emptied simply by a wind.…”