Millisecond Oscillations in X-Ray Binaries

Abstract

Key Words neutron stars, black holes, relativity, X-ray binaries, pulsars, quasi-periodic oscillations s Abstract The first millisecond X-ray variability phenomena from accreting compact objects have recently been discovered with the Rossi X-ray Timing Explorer. Three new phenomena are observed from low-mass X-ray binaries containing lowmagnetic-field neutron stars: millisecond pulsations, burst oscillations, and kilohertz quasi-periodic oscillations. Models for these new phenomena involve the neutron star spin and orbital motion close around the neutron star, and rely explicitly on our understanding of strong gravity and dense matter. I review the observations of these new neutron-star phenomena and some possibly related phenomena in black-hole candidates, and describe the attempts to use these observations to perform measurements of fundamental physical interest in these systems.

Rotating Stars in Relativity

“…In the last few years, X-ray observations of accreting sources in LMXBs have revealed a rich phenomenology that is waiting to be interpreted correctly and could lead to significant advances in our understanding of compact objects (see [192, 168, 249]). The most important feature of these sources is the observation of (in most cases) twin kHz quasi-periodic oscillations (QPOs).…”

“…Such bursts show nearly-coherent oscillations in the range 270–620 Hz (see [168, 299] for recent reviews). One interpretation of the burst oscillations is that they are the result of rotational modulation of surface asymmetries during the burst.…”

The rms–flux relations in different branches in Cyg X-2

“…Cyg X-2 is a Z source according to its track through different spectral states on the color-color diagram (CCD) (Hasinger and van der Klis 1989;Hasinger 1990;van der Klis 2000). In general, a Z source has three branches, i.e.…”

“…We call this vertical portion vertical horizontal branch (VHB) (Shirey 1998;Kuulkers et al 1999;Shirey et al 1999). During the evolution of a Z-source, it moves continuously on the Z-track, due to different mass accretion rate (van der Klis 2000). From simultaneous multiband observations, it is thought that the mass accretion rate reaches a minimum at the left end of the HB, increases as the source travels from HB, throughout NB, to FB, and reaches its maximum at the right end of the FB (Hasinger and van der Klis 1989;Hasinger 1990).…”

“…The QPOs that appear in the HB and the upper NB are called HB oscillations with the centroid frequency varying from ∼ 10 Hz to ∼ 70 Hz. The NB oscillations locate in the lower NB with the centroid frequency at ∼ 5-7 Hz (Hasinger and van der Klis 1989;van der Klis 2000;Piraino et al 2002;van der Klis 2004). In addition, some kHz QPOs are observed in Cyg X-2 when the source moves from the left end of the HB to the upper NB (Wijnands et al (1998) and references therein).…”