Implications of kHz QPOs for the spin frequencies and magnetic fields of neutron stars: new results from Circinus X-1

Detection of paired kilohertz quasi-periodic oscillations (kHz QPOs) in the X-ray emission of a compact object is compelling evidence that the object is an accreting neutron star. In many neutron stars, the stellar spin rate is equal or roughly equal to Delta-nu, the frequency separation of the QPO pair, or to 2Delta-nu. Hence, if the mechanism that produces the kilohertz QPOs is similar in all stars, measurement of Delta-nu can provide an estimate of the star's spin rate. The involvement of the stellar spin in producing Delta-nu indicates that the magnetic fields of these stars are dynamically important. We focus here on the implications of the paired kHz QPOs recently discovered in the low-mass X-ray binary (LMXB) system Cir X-1 (Boutloukos et al. 2006). The kHz QPOs discovered in Cir X-1 are generally similar to those seen in other stars, establishing that the compact object in the Cir X-1 system is a neutron star. However, the frequency nu-u of its upper kHz QPO is up to a factor of three smaller than is typical, and Delta-nu varies by about a factor 2 (167 Hz, the largest variation so far observed). Periodic oscillations have not yet been detected from Cir X-1, so its spin rate has not yet been measured directly. The low values of nu-u and the large variation of Delta-nu challenge current models of the generation of kHz QPOs. Improving our understanding of Cir X-1 will improve our knowledge of the spin rates and magnetic fields of all neutron stars.Comment: 3 pages, 3 figures, a table with all known accreting millisecond X-ray pulsars up to October 2007. To appear in "40 Years of Pulsars, Millisecond Pulsars, Magnetars and More" conference proceeding

Origin of intermittent accretion-powered X-ray oscillations in neutron stars with millisecond spin periods

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 about 10 degrees away can increase the fractional rms amplitude from less than about 0.5 percent, 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 accretion-powered 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.Comment: 5 pages, 1 figure; includes additional discussion and updated references; accepted for publication in ApJ

A model for the waveform behavior of accreting millisecond pulsars: Nearly aligned magnetic fields and moving emission regions

We investigate further a model of the accreting millisecond X-ray pulsars we proposed earlier. In this model, the X-ray-emitting regions of these pulsars are near their spin axes but move. This is to be expected if the magnetic poles of these stars are close to their spin axes, so that accreting gas is channeled there. As the accretion rate and the structure of the inner disk vary, gas is channeled along different field lines to different locations on the stellar surface, causing the X-ray-emitting areas to move. We show that this "nearly aligned moving spot model" can explain many properties of the accreting millisecond X-ray pulsars, including their generally low oscillation amplitudes and nearly sinusoidal waveforms; the variability of their pulse amplitudes, shapes, and phases; the correlations in this variability; and the similarity of the accretion- and nuclear-powered pulse shapes and phases in some. It may also explain why accretion-powered millisecond pulsars are difficult to detect, why some are intermittent, and why all detected so far are transients. This model can be tested by comparing with observations the waveform changes it predicts, including the changes with accretion rate.Comment: 21 pages, 6 figures; includes 3 new sections, 14 additional pages, 4 additional figures with 11 new plots, and additional references; accepted for publication in Ap