A Chandra X-Ray study of the dense globular cluster Terzan 5

Abstract

We report a Chandra ACIS-I observation of the dense globular cluster Terzan 5. The previously known transient low-mass X-ray binary (LMXB) EXO 1745-248 in the cluster entered a rare high state during our 2000 August observation, complicating the analysis. Nevertheless, nine additional sources clearly associated with the cluster are also detected, ranging from L-X(0.5-2.5 keV) = 5.6 x 10(32) down to 8.6 x 10(31) ergs s(-1). Their X-ray colors, luminosities, and spectral fitting indicate that five of them are probably cataclysmic variables and four are likely quiescent LMXBs containing neutron stars. We estimate the total number of sources between L-X(0.5-2.5 keV) = 10(32) and 10(33) ergs s(-1) as 11.4(-1.8)(+4.7) by the use of artificial point source tests and note that the numbers of X-ray sources are similar to those detected in NGC 6440. The improved X-ray position allowed us to identify a plausible infrared counterpart to EXO 1745-248 on our 1998 Hubble Space Telescope NICMOS images. This blue star (F110W = 18.48, F187W = 17.30) lies within 0."2 of the bore-sighted LMXB position. Simultaneous Rossi X-Ray Timing Explorer spectra, combined with the Chandra spectrum, indicate that EXO 1745-248 is an ultracompact binary system and show a strong broad 6.55 keV iron line and an 8 keV smeared reflection edge.