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.

We present a 7 yr timing study of the 2.5 ms X-ray pulsar SAX J1808.4À3658, an X-ray transient with a recurrence time of %2 yr, using data from the Rossi X-Ray Timing Explorer covering four transient outbursts (1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005). We verify that the 401 Hz pulsation traces the spin frequency fundamental and not a harmonic. Substantial pulse shape variability, both stochastic and systematic, was observed during each outburst. Analysis of the systematic pulse shape changes suggests that, as an outburst dims, the X-ray ''hot spot'' on the pulsar surface drifts longitudinally and a second hot spot may appear. The overall pulse shape variability limits the ability to measure spin frequency evolution within a given X-ray outburst (and calls previous˙measurements of this source into question), with typical upper limits of j˙j P 2:5 ; 10 À14 Hz s À1 (2 ). However, combining data from all the outbursts shows with high (6 ) significance that the pulsar is undergoing long-term spin down at a rate˙¼ (À5:6 AE 2:0) ; 10 À16 Hz s À1 , with most of the spin evolution occurring during X-ray quiescence. We discuss the possible contributions of magnetic propeller torques, magnetic dipole radiation, and gravitational radiation to the measured spin down, setting an upper limit of B < 1:5 ; 10 8 G for the pulsar's surface dipole magnetic field and Q/I < 5 ; 10 À9 for the fractional mass quadrupole moment. We also measured an orbital period derivative ofṖ orb ¼ (3:5 AE 0:2) ; 10 À12 s s À1 . This surprisingly largė P orb is reminiscent of the large and quasi-cyclic orbital period variation observed in the so-called black widow millisecond radio pulsars, which further strengthens previous speculation that SAX J1808.4À3658 may turn on as a radio pulsar during quiescence. In an appendix we derive an improved (0:15 00 ) source position from optical data.

The Rossi X-ray Timing Explorer has observed five outbursts from the transient 2.5 ms accretionpowered pulsar SAX J1808.4−3658 during 1998-2008. We present a pulse timing study of the most recent outburst and compare it with the previous timing solutions. The spin frequency of the source continues to decrease at a rate of (−5.5 ± 1.2) × 10 −18 Hz s −1 , which is consistent with the previously determined spin derivative. The spin-down occurs mostly during quiescence, and it is most likely due to the magnetic dipole torque from a B = 1.5 × 10 8 G dipolar field at the neutron star surface. We also find that the 2 hr binary orbital period is increasing at a rate of (3.80 ± 0.06) × 10 −12 s s −1 , also consistent with previous measurements. It remains uncertain whether this orbital change reflects secular evolution or short-term variability.