Superburst with outburst from EXO 1745-248 in Terzan 5 with MAXI

Abstract

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 d. 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 × 1042 erg in 2-20 keV, assuming an 8.7 kpc distance. The sphere radius of the blackbody and its luminosity are estimated to be 6.2 km and 1.1 × 1038ergs -1, respectively, from a spectral fitting at the flux peak. Those e-folding time, temperature, softening, fluence, and radius are typical of superbursts from low-mass X-ray binaries. The superburst was followed by an outburst 28 hours after the superburst onset. The outburst lasted for 5 d and the fluence was 4.3 × 1042 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 the surface of the companion star by the superburst would be another possibility. © 2012. Astronomical Society of Japan.