Identification of the Optical and Quiescent Counterparts to the Bright X-Ray Transient in NGC 6440

After 3 years of quiescence, the globular cluster NGC 6440 exhibited a bright transient X-ray source turning on in 2001 August, as noted with the Rossi X-Ray Timing Explorer 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 2000 July 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⁺³¹_-12 km and an unabsorbed bolometric luminosity for the neutron star atmosphere of (2.1+/-0.8)×10³³ ergs s^-1 that is consistent with predictions for a cooling neutron star.