Journal Article

A superburst candidate in EXO 1745—248 as a challenge to thermonuclear ignition models

D. Altamirano, L. Keek, A. Cumming, G. R. Sivakoff, C. O. Heinke, R. Wijnands, N. Degenaar, J. Homan and D. Pooley

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 426, issue 2, pages 927-934
Published in print October 2012 | ISSN: 0035-8711
Published online October 2012 | e-ISSN: 1365-2966 | DOI:
A superburst candidate in EXO 1745—248 as a challenge to thermonuclear ignition models

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We report on Chandra, Rossi X-ray Timing Explorer (RXTE), Swift/Burst Alert Telescope (BAT) and MAXI observations of an X-ray flare of ∼1 d and subsequent outburst of a transient X-ray source observed in 2011 October–November in the globular cluster Terzan 5. We show that the source is the same as the transient that was active in 2000, i.e. the neutron star low-mass X-ray binary EXO 1745−248. For the X-ray flare we estimate a 6–11 h exponential decay time and a radiated energy of 2–9 × 1042 erg. These properties, together with strong evidence of decreasing blackbody temperature during the flare decay, are fully consistent with what is expected for a thermonuclear superburst. We use the most recent superburst models and estimate an ignition column depth of ≈1012 g cm−2 and an energy release between 0.1 × 1018 and 2 × 1018 erg g−1, also consistent with expected superburst values. We conclude therefore that the flare was most probably a superburst. We discuss our results in the context of theoretical models and find that even when assuming a few days of low-level accretion before the superburst onset (which is more than what is suggested by the data), the observations of this superburst are very challenging for current superburst ignition models.

Keywords: accretion, accretion discs; binaries: close; stars: individual: EXO 1745−248; stars: neutron; X-rays: binaries; X-rays: stars

Journal Article.  6390 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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