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 \pm 0.8 ) \times 10 ^ { 33 } erg s ^ { -1 } which is consistent with predictions for a cooling neutron star .