350 research outputs found
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
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
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