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D. K. Galloway, D. Chakrabarty (MIT), M. P. Muno (UCLA), D. Psaltis (U. Arizona), A. Cumming (UC Santa Cruz)
I present results from a project using archival Rossi X-ray Timing Explorer data to study thermonuclear (type-I) X-ray bursts. These bursts occur due to unstable ignition of accumulated H/He fuel on the surface of accreting neutron stars in binary systems. To date we have accumulated 820 bursts from 41 of the more than 80 known burst sources. I will summarise the properties of the sample, and variations in the properties of the bursts as a function of accretion rate, as well as giving details of some of the more exciting results from the study.
The data show that photospheric radius-expansion (PRE) bursts, which are thought to reach the Eddington limit, do not all reach the same peak flux for a given source. Instead, the peak fluxes of the PRE bursts vary by (typically) 10-20%, although in 4U 1636-536 the peak fluxes vary by a factor of 4. In 4U 1728-34 we found a correlation between the persistent flux and the peak PRE burst flux, which we attribute to reprocessing of the flux in a warped, precessing accretion disk. However, this mechanism cannot account for variations in the peak fluxes of PRE bursts from all sources.
Measurements of the burst energetics and recurrence times are possible using RXTE with much improved precision compared to previous instruments. In the "Clocked Burster", GS 1826-24, hydrogen burns during the bursts via the rapid-proton (rp) process, of particular attention in recent years via theoretical and modelling studies. The burst energies and the variation of alpha (the ratio of persistent to burst fluence) with accretion rate strongly suggests solar atmospheric metallicity, although this appears inconsistent with the corresponding variation of the recurrence time. Possible explanations include extra heating between the burst, or a change in the fraction of the neutron star over which accretion takes place. I will also present more recent results from other sources with which we are gradually probing a wide range of burst regimes.
This work was supported in part by the NASA Long Term Space Astrophysics program under grant NAG 5-9184.
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.