Identification of the optical and quiescent counterparts to the bright X-ray transient in NGC 6440

Abstract

After 3 years of quiescence, the globular cluster NGC 6440 exhibited a bright transient X-ray source turning on in August 2001, as noted with the RXTE All-Sky Monitor. We carried out a short target of opportunity observation with the Chandra X-ray Observatory and are able to associate the transient with the brightest of 24 X-ray sources detected during quiescence in July 2000 with Chandra. Furthermore, we securely identify the optical counterpart and determine that the 1998 X-ray outburst in NGC 6440 was from the same object. This is the first time that an optical counterpart to a transient in a globular cluster is securely identified. Since the transient is a type I X-ray burster, it is established that the compact accretor is a neutron star. Thus, this transient provides an ideal case to study the quiescent emission in the optical and X-ray of a transiently accreting neutron star while knowing the distance and reddening accurately. One model that fits the quiescent spectrum is an absorbed power law plus neutron star hydrogen atmosphere model. We find an intrinsic neutron star radius of 17_{-12}^{+31} km and an unabsorbed bolometric luminosity for the neutron star atmosphere of (2.1+/-0.8)E33 erg/s which is consistent with predictions for a cooling neutron star.Comment: Accepted for publication in ApJ Letter