Origin of Intermittent Accretion-Powered X-Ray Oscillations in Neutron Stars With Millisecond Spin Periods

Abstract

We have shown previously that many of the properties of persistent accretion-powered millisecond pulsars can be understood if their X-ray emitting areas are near their spin axes and move as the accretion rate and structure of the inner disk vary. Here we show that this "nearly aligned moving spot model" may also explain the intermittent accretion-powered pulsations that have been detected in three weakly magnetic accreting neutron stars. We show that movement of the emitting area from very close to the spin axis to ∼ 10 • away can increase the fractional rms amplitude from 0.5%, which is usually undetectable with current instruments, to a few percent, which is easily detectable. The second harmonic of the spin frequency usually would not be detected, in agreement with observations. The model produces intermittently detectable oscillations for a range of emitting area sizes and beaming patterns, stellar masses and radii, and viewing directions. Intermittent oscillations are more likely in stars that are more compact. In addition to explaining the sudden appearance of accretion-powered millisecond oscillations in some neutron stars with millisecond spin periods, the model explains why accretion-powered millisecond oscillations are relatively rare and predicts that the persistent accretionpowered millisecond oscillations of other stars may become undetectable for brief intervals. It suggests why millisecond oscillations are frequently detected during the X-ray bursts of some neutron stars but not others and suggests mechanisms that could explain the occasional temporal association of intermittent accretion-powered oscillations with thermonuclear X-ray bursts.

Broad-band spectral analysis of the accreting millisecond X-ray pulsar SAX J1748.9−2021

“…However, HETE J1900.1-2455 has been also observed pulsating during hard states ) unlike SAX J1748.9-2021 which, we highlight, pulsated only during soft states ). In particular, it has been proposed that intermittency of HETE J1900.1-2455 is due to a magnetic field generally buried under the NS surface (Cumming 2008) or, if the magnetosphere is considered, to a break-through of accreting matter into the magnetic field lines, possibly accompanied by position changes of a hot spot located close to the spin axis of the NS (Romanova et al 2008;Lamb et al 2009). Other hypotheses suggest limitation of the pulse amplitude related to gravitational light bending effects (Özel 2009) or scattering of the coherent pulsation in an optically thick, hot corona around the NS (Titarchuk et al 2002).…”

A Non-Radial Oscillation Mode in an Accreting Millisecond Pulsar?

“…As mentioned in the introduction, AMXPs show small-amplitude X-ray oscillations with periods equal to the spin period of the star. To explain their low modulation amplitudes and nearly sinusoidal waveforms Lamb et al (2009) proposed a model in which X-rays are emitted from a hot spot at the stellar surface and near a magnetic pole that is assumed to be close to the rotation axis of the star. If we assume that this model is correct, then transverse motions induced by the non-radial oscillations at the surface of the star can perturb the hot spot periodically, and these periodicities might be observable in the radiation flux from the star (Numata & Lee 2010).…”

Masses, Radii, and the Equation of State of Neutron Stars

“…It originates from the base of the accretion column, which is then Comptonized in that column (see section 3.3 of Poutanen & Gierlinski 2003). A number of studies have modeled the emission from the X-ray emitting regions at the bases of the accretion columns (Poutanen & Gierliński 2003;Lamb et al 2009). The thermal photons emitted from the surface are Compton scattered in the accretion column, affecting the observed pulse amplitudes.…”

“…The thermal photons emitted from the surface are Compton scattered in the accretion column, affecting the observed pulse amplitudes. Lamb et al (2009) also explored the effect of the location of the X-ray emitting regions and considered models where the base of the accretion column wobble around the spin axes. These models help interpret the AMSP timing data but also explore uncertainties in the mass-radius constraints obtained from fitting the waveforms.…”