Oscillations during thermonuclear X-ray bursts: A new probe of neutron stars

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

Radial modes of levitating atmospheres around Eddington luminosity neutron stars

“…So for any given mode, assuming that the polytropic index of the atmosphere remains the same, frequency of any of the eigenmodes would increase with time if the luminosity of the source decreases with time. This is similar to what is usually observed in the decay phase of the X-ray bursts (Strohmayer 2001;Strohmayer & Bildsten 2006).…”

“…X-ray bursters exhibit high frequency (300 -600 Hz) 'burst oscillations' (Strohmayer 2001), which may be related to the Eddington-luminosity of the sources. In bright Xray bursts, neutron star often reaches near Eddington-limit ⋆ E-mail: deepika@camk.edu.pl † E-mail: wlodek@camk.edu.pl luminosities during which the dominant radiation pressure drives the photospheric layers off the neutron star; this phenomenon is termed as photospheric radius expansion (PRE).…”