Physics of Neutron Stars - 2011

Activity: Lecture and oral contribution

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INTEGRAL observations of long X-ray bursts Type I X-ray bursts are thermonuclear explosions in the surface layers of a neutron star. They are generally characterized by an exponential decay phase, which duration ranges typically between a few seconds and a couple of minutes, and are explained by the unstable nuclear burning of a mixture of helium and hydrogen. However, some bursts have occasionally been observed with decay times of a few tens of minutes. Because of their duration and energy release these rare long bursts appear as intermediate between the above-mentioned short X-ray bursts and exceptional superbursts that last several hours and are thought to arise from carbon shell ashes in the layers below the surface of the neutron star. Thanks to the wide eld of view of the JEM-X coded-mask X-ray monitor aboard the INTEGRAL satellite many X-ray sources are simultaneously observed in the 3-35 keV energy range, that makes it possible to monitor several X-ray bursters in one shot and/or the occurrence of rare events. So far, a good number of intermediate long bursts have been detected by INTEGRAL, and the mechanisms up to high energies of these unusual events have been investigated. With this research we aim to inquire the relationship between nuclear ignition processes, burning regimes, and accretion states of the binary system, that lead up to long bursts. In particular, a handful of long bursts have been observed that exhibit dual decay phases with an initial spike similar to a normal short burst; we discuss the possibility for such twofold bursts to be some kind of link between dierent burning regimes. Depending on the composition of the accreted material, intermediate long bursts may be explained by either the unstable burning of a large pile of mixed hydrogen and helium, or the ignition of a thick pure helium layer. The latter case is particularly expected at very low accretion rates, which seem to be prevalent in ultra-compact binaries; it may also provide an opportunity to study the transition from a hydrogen-rich bursting regime to a pure helium regime. On February 13, 2011, INTEGRAL observed its first superburst, from the transient source SAX J1747.0{2853 (see ATel 3183). At the time of writing, only very preliminary observation results are available, but analysis results from this spectacular event will also be presented at the conference.
Place: Sct. Petersburg

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Keywords

  • Astrophysics
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ID: 2373957