We have assembled a sample of 1187 thermonuclear (type I) X-ray bursts from observations of 48 accreting neutron stars by the Rossi X-ray Timing Explorer, spanning more than 10 years. The sample contains examples of two of the three theoretical ignition regimes (confirmed via comparisons with numerical models) and likely examples of the third. We present a detailed analysis of the variation of the burst profiles, energetics, recurrence times, presence of photospheric radius expansion, and presence of burst oscillations, as a function of accretion rate. We estimated the distance for 35 sources exhibiting radius-expansion bursts, and found that the peak flux of such bursts varies typically by 13%. We classified sources into two main groups based on the burst properties: (1) both long and short bursts (indicating mixed H/ He accretion), and (2) consistently short bursts (primarily He accretion), and we calculated the mean burst rate as a function of accretion rate for the two groups. The decrease in burst rate observed at >0:06Ṁ Edd (k2 ; 10 37 ergs s À1) is associated with a transition in the persistent spectral state and (as has been suggested previously) may be related to the increasing role of steady He burning. We found many examples of bursts with recurrence times <30 minutes, including burst triplets and even quadruplets. We describe the oscillation amplitudes for 13 of the 16 burst oscillation sources, as well as the stages and properties of the bursts in which the oscillations are detected. The burst properties are correlated with the burst oscillation frequency; sources spinning at <400 Hz generally have consistently short bursts, while the more rapidly spinning systems have both long and short bursts. This correlation suggests either that shear-mediated mixing dominates the burst properties, or alternatively that the nature of the mass donor (and hence the evolutionary history) has an influence on the long-term spin evolution.

Millisecond pulsars are neutron stars that are thought to have been spun-up by mass accretion from a stellar companion.1 It is unknown whether there is a natural brake for this process, or if it continues until the centrifugal breakup limit is reached at submillisecond periods. Many neutron stars that are accreting mass from a companion star exhibit thermonuclear X-ray bursts that last tens of seconds, caused by unstable nuclear burning on their surfaces.2 Millisecond-period brightness oscillations during bursts from ten neutron stars (as distinct from other rapid X-ray variability that is also observed 3,4 ) are thought to measure the stellar spin, 2,5 but direct proof of a rotational origin has been lacking. Here, we report the detection of burst oscillations at the known spin frequency of an accreting millisecond pulsar, and we show that these oscillations always have the same rotational phase. This firmly establishes burst oscillations as nuclear-powered pulsations tracing the spin of accreting neutron stars, corroborating earlier evidence.5,6 The distribution of spin frequencies of the 11 nuclear-powered pulsars cuts off well below the breakup frequency for most neutron star models, supporting theoretical predictions that gravitational radiation losses can limit accretion torques in spinning up millisecond pulsars. 7-9The millisecond oscillations observed during X-ray bursts are not perfectly coherent, but usually drift in frequency by several hertz over the course of a burst, generally reaching an asymptotic maximium frequency that is repeatable in a given neutron star.2 This frequency drift has been interepreted as arising from angular momentum conservation in a decoupled surface burning layer that expands and contracts during the burst, so that the asymptotic frequency is the stellar spin frequency.10,11 A puzzle in this picture is why the oscillation persists late in the burst, well after the nuclear burning has spread over the entire star. Also, in most of these neutron stars, unexplained pairs of kilohertz quasi-periodic oscillations (kHz QPOs) are also observed in the non-burst X-ray emission, with the QPO separation frequency approximately equal to either the burst oscillation frequency or half this value, We have observed the transient X-ray source SAX J1808.4−3658, which has been detected in four outbursts since its discovery

We have identified the third known accretion-powered millisecond pulsar, XTE J0929−314, with the Rossi X-Ray Timing Explorer. The source is a faint, high-Galactic-latitude X-ray transient (d 5 kpc) that was in outburst during 2002 April-June. The 185 Hz (5.4 ms) pulsation had a fractional rms amplitude of 3-7% and was generally broad and sinusoidal, although occasionally double-peaked. The hard X-ray pulses arrived up to 770 µs earlier than the soft X-ray pulses. The pulsar was spinning down at an average rate ofν = (−9.2 ± 0.4) × 10 −14 Hz s −1 ; the spin-down torque may arise from magnetic coupling to the accretion disk, a magnetohydrodynamic wind, or gravitational radiation from the rapidly spinning pulsar. The pulsations were modulated by a 43.6 min ultracompact binary orbit, yielding the smallest measured mass function (2.7 × 10 −7 M ⊙ ) of any stellar binary. The binary parameters imply a ≃ 0.01M ⊙ white dwarf donor and a moderately high inclination. We note that all three known accreting millisecond pulsars are X-ray transients in very close binaries with extremely low mass transfer rates. This is an important clue to the physics governing whether or not persistent millisecond pulsations are detected in low-mass X-ray binaries.

The standard approach for time-resolved X-ray spectral analysis of thermonuclear bursts involves subtraction of the pre-burst emission as background. This approach implicitly assumes that the persistent flux remains constant throughout the burst. We reanalyzed 332 photospheric radius expansion bursts observed from 40 sources by the Rossi X-ray Timing Explorer, introducing a multiplicative factor f a to the persistent emission contribution in our spectral fits. We found that for the majority of spectra the best-fit value of f a is significantly greater than 1, suggesting that the persistent emission typically increases during a burst. Elevated f a values were not found solely during the radius expansion interval of the burst, but were also measured in the cooling tail. The modified model results in a lower average value of the χ 2 fit statistic, indicating superior spectral fits, but not yet to the level of formal statistical consistency for all the spectra. We interpret the elevated f a values as an increase of the mass accretion rate onto the neutron star during the burst, likely arising from the effects of Poynting-Robertson drag on the disk material. We measured an inverse correlation of f a with the persistent flux, consistent with theoretical models of the disc response. We suggest that this modified approach may provide more accurate burst spectral parameters, as well as offering a probe of the accretion disk structure.