Oscillations during thermonuclear x-ray bursts

Abstract

High amplitude, nearly coherent X-ray brightness oscillations during thermonuclear X-ray bursts were discovered with the Rossi X-ray Timing Explorer (RXTE) in early 1996. Spectral and timing evidence strongly supports the conclusion that these oscillations are caused by rotational modulation of the burst emission and that they reveal the spin frequency of neutron stars in low mass X-ray binaries, a long sought goal of X-ray astronomy. Studies carried out over the past year have led to the discovery of burst oscillations in four new sources, bringing to ten the number with confirmed burst oscillations. I review the status of our knowledge of these oscillations and indicate how they can be used to probe the physics of neutron stars. For a few burst oscillation sources it has been proposed that the strongest and most ubiquitous frequency is actually the first overtone of the spin frequency and hence that two nearly antipodal hot spots are present on the neutron star. This inference has important implications for both the physics of thermonuclear burning as well as the mass -radius relation for neutron stars, so its confirmation is crucial. I discuss recent attempts to confirm this hypothesis for 4U 1636-53, the source for which a signal at the putative fundamental (290 Hz) has been claimed. INTRODUCTIONSince its launch in December, 1995 NASA's Rossi X-ray Timing Explorer has provided astronomers with a fundamental new view of neutron stars, and in particular those which are accreting in binary systems. Since the discovery of rapidly rotating neutron stars as millisecond radio pulsars it has been suspected that neutron stars in low mass X-ray binaries (LMXB) are spun up to millisecond periods by the capture of angular momentum via mass transfer from an accretion disk. Efforts to confirm this hypothesis by detecting millisecond X-ray pulsars in LMXBs went unrewarded for many years. This situation changed dramatically with the advent of RXTE. Within a few months of its launch RXTE observations had provided strong evidence suggesting that neutron stars in LMXB are spinning with frequencies ≥ 300 Hz. These first indications came with the discovery of high frequency (millisecond) X-ray brightness oscillations, "burst oscillations," during thermonuclear (Type I) X-ray bursts from several neutron star LMXB systems (see Strohmayer et al. 1996;Smith, Morgan & Bradt 1997;Zhang et al. 1996).As of this writing burst oscillation detections have been claimed for a total of ten different LMXB systems, with four of these only appearing in the past few months (see Strohmayer et

Light curves and polarization of accretion- and nuclear-powered millisecond pulsars

“…However, this method is indirect. Discovery of millisecond coherent pulsations during X-ray bursts in 13 low-mass X-ray binaries (so called nuclearpowered millisecond pulsars, see reviews by Strohmayer 2001;Strohmayer & Bildsten 2003) and in the persistent emission of five sources (accretion-powered millisecond pulsars, see review by Wijnands 2004) opens a completely new range of possibilities. The emission in these cases is produced at the surface of a rapidly spinning neutron star and the observed pulse profiles and oscillation amplitudes are strongly affected by special as well as general relativistic effects (Braje et al 2000;Weinberg et al 2001;Poutanen & Gierliński 2003, hereafter PG03).…”

An RXTE Archival Search for Coherent X-ray Pulsations in LMXB 4U 1820-30

“…In 5 LMXBs, coherent pulsations are observed (Wijnands & van der Klis 1998;Markwardt et al 2002;Galloway et al 2002;Markwardt et al 2003a;. In 11 LMXBs burst oscillations are observed (Strohmayer 2001;Strohmayer et al 2003). In 2 of these LMXBs, both coherent pulsations and burst oscillations are observed (Chakrabarty et al 2003;Strohmayer et al 2003).…”

Magnetic Fields of Neutron Stars in X-Ray Binaries

“…At the moment we know 15 systems of this class in which the NSs create persistent or intermittent X-ray pulsations with periods of a few milliseconds (see, e.g., Patruno & Watts 2012) and several sources which show millisecond pulsations during type I bursts (e.g. Strohmayer 2001). These findings show that the NSs in LMXBs have spin periods much shorter than those in HMXBs.…”