Theory and Observations of Type I X-Ray Bursts from Neutron Stars

I review our understanding of the thermonuclear instabilities on accreting neutron stars that produce Type I X-Ray bursts. I emphasize those observational and theoretical aspects that should interest the broad audience of this meeting. The easily accessible timescales of the bursts (durations of tens of seconds and recurrence times of hours to days) allow for a very stringent comparison to theory. The largest discrepancy (which was found with EXOSAT observations) is the accretion rate dependence of the Type I burst properties. Bursts become less frequent and energetic as the global accretion rate increases, just the opposite of what the spherical theory predicts. I present a resolution of this issue by taking seriously the observed dependence of the burning area on the global accretion rate, which implies that as the accretion rate increases, the accretion rate per unit area decreases. This resurrects the unsolved problem of knowing where the freshly accreted material accumulates on the star, equally relevant to the likely signs of rotation during the bursts summarized by Swank at this meeting. I close by highlighting the Type I bursts from GS 1826-238 that were found with BeppoSAX and RXTE. Their energetics, recurrence times and temporal profiles clearly indicate that hydrogen is being burned during these bursts, most likely by the rapid-proton (rp) process.Comment: 10 pages, 2 figures. to appear in "Cosmic Explosions", proceeding of the 10th Annual October Astrophysics Conference (ed. S.S. Holt and W. W. Zhang

The Angular Momentum of Accreting Neutron Stars

I review the rotation measurements of accreting neutron stars. Many of the highly magnetic accreting X-ray pulsars have been continuously observed with the Burst and Transient Source Experiment (BATSE) aboard the Compton Gamma-Ray Observatory (CGRO) since April 1991. These observations show that the accretion torque exerted on many disk-fed accreting X-ray pulsars changes sign on a monthly to yearly timescale. This results in alternating periods of spin-up and spin-down with nearly the same torques, leading to little net angular momentum gained by accretion. I also summarize recent discoveries with the Rossi X-Ray Timing Explorer (RXTE) of periodicities during Type I X-ray bursts. These seem to indicate that many of the rapidly accreting and weakly magnetic neutron stars in our galaxy are rotating at frequencies greater than 250 Hertz. Most remarkable is that they all rotate within a rather narrow range of frequencies.Comment: 10 pages,2 .ps figures, LaTeX, uses aipproc.sty, to appear in "Accretion Processes in Astrophysical Systems: Some Like It Hot", eds. S. S. Holt and T. Kallma

SPIN-ORBIT INTERACTION IN NEUTRON STAR/MAIN SEQUENCE BINARIES AND IMPLICATIONS FOR PULSAR TIMING

The spin-induced quadrupole moment of a rapidly rotating star changes the orbital dynamics in a binary system, giving rise to advance (or regression) of periastron and precession of the orbital plane. We show that these effects are important in the recently discovered radio pulsar/main sequence star binary system PSR J0045$-$7319, and can reliably account for the observed peculiar timing residuals. Precise measurements of the apsidal motion and orbital plane precession can yield valuable information on the internal structure and rotation of the star. The detection of orbital precession implies that the spin of the companion star is not aligned with the orbital angular momentum, and suggests that the supernova gave the pulsar a kick out of the original orbital plane. Tidal excitation of g-mode oscillations in the PSR J0045$-$7319 system induces an orbital period change of order $|\Delta P_{\rm orb}/P_{\rm orb}|\sim 10^{-6}$ at each periastron passage, but the secular trend depends on the radiative damping time of the g-modes. We also discuss the spin-orbit coupling effects for the accreting X-ray pulsars and the other known radio pulsar/main sequence binary, PSR B1259$-$63.Comment: 12 pages, 2 figures. Plain Te