Evidence of heavy-element ashes in thermonuclear X-ray bursts with photospheric superexpansion - Astrophysics > High Energy Astrophysical PhenomenaReport as inadecuate




Evidence of heavy-element ashes in thermonuclear X-ray bursts with photospheric superexpansion - Astrophysics > High Energy Astrophysical Phenomena - Download this document for free, or read online. Document in PDF available to download.

Abstract: A small subset of thermonuclear X-ray bursts on neutron stars exhibit such astrong photospheric expansion that for a few seconds the photosphere is locatedat a radius r ph >~ 1000 km. Such `superexpansions- imply a large and rapidenergy release, a feature characteristic of pure He burst models. Previouscalculations have shown that during a pure He burst, the freshly synthesizedheavy-element ashes of burning can be ejected in a strong radiative wind andproduce significant spectral absorption features. We search the burst datacatalogs and literature and find 32 superexpansion bursts. We find that thesebursts exhibit the following interesting features: 1 At least 31 are fromcandidate ultracompact X-ray binaries in which the neutron star accreteshydrogen-deficient fuel, suggesting that these bursts indeed ignite in ahelium-rich layer. 2 In 2 bursts we detect strong absorption edges during theexpansion phase. The edge energies and depths are consistent with the H-like orHe-like edge of iron-peak elements with abundances greater than 100 timessolar, suggesting that we are seeing the exposed ashes of nuclear burning. 3The superexpansion phase is always followed by a moderate expansion phaseduring which r ph ~ 30 km and the luminosity is near the Eddington limit. 4The decay time of the bursts, t d, ranges from short approximately 10 s tointermediate >~ 1000 s. However, despite the large range of t d, the durationof the superexpansion is always a few seconds, independent of t d. By contrast,the duration of the moderate expansion is always of order t d. 5 Thephotospheric radii r ph during the moderate expansion phase are much smallerthan steady state wind models predict. We show that this may be furtherindication that the wind contains highly non-solar abundances of heavyelements.



Author: J.J.M. in 't Zand SRON, N.N. Weinberg UC Berkeley

Source: https://arxiv.org/







Related documents