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Post Info TOPIC: EXO 1745-248
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RE: EXO 1745-248
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Title: A superburst candidate in EXO 1745--248 as a challenge to thermonuclear ignition models
Authors: D. Altamirano, L. Keek, A. Cumming, G. R. Sivakoff, C. O. Heinke, R. Wijnands, N. Degenaar, J. Homan, D. Pooley

We report on Chandra, RXTE, Swift/BAT and MAXI observations of a ~1 day X-ray flare and subsequent outburst of a transient X-ray source observed in October-November 2011 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 hr exponential decay time and a radiated energy of 2-9 x 10^42 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 ~10^12 g cm^-2 and an energy release between 0.1-2 x 10^18 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.

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Blobrana


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Title: Superburst with Outburst from EXO 1745-248 in Terzan 5 with MAXI
Authors: Motoko Serino (1), Tatehiro Mihara (1), Masaru Matsuoka (1), Satoshi Nakahira (1), Mutsumi Sugizaki (1), Yoshihiro Ueda (2), Nobuyuki Kawai (3), Shiro Ueno (4) ((1) RIKEN, (2) Kyoto University, (3) Tokyo Institute of Technology, (4) JAXA)

MAXI/GSC detected a superburst from EXO 1745-248 in the globular cluster Terzan 5 on 2011 October 24. The GSC light curve shows an exponential decay with an e-folding time of 0.3 day. The spectra are consistent with the blackbody radiation, whose temperature is 2.2 keV and 1.2 keV at MJD 55858.56 and 55859.20, respectively. The fluence is 1.4 x 10^{42} erg in 2-20 keV assuming 8.7 kpc distance. The sphere radius of the blackbody and its luminosity are estimated to be 6.2 km and 1.1 x 10^{38} erg s^{-1}, respectively, from the spectral fitting at the flux peak. Those e-folding time, temperature, softening, fluence, and radius are typical of superbursts from the low-mass X-ray binaries. The superburst was followed by an outburst 28 hours after the superburst onset. The outburst lasted for 5 days and the fluence was 4.3 x 10^{42} erg. The instability of the accretion disk caused by the superburst would be an explanation for the outburst, whereas the mass accretion of the matter evaporated from surface of the companion star by the superburst would be another possibility.

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