The hard quiescent spectrum of the neutron - star X - ray transient EXO 1745 – 248 in the globular cluster Terzan 5

We present a Chandra observation of the globular cluster Terzan 5 during times when the neutron-star X-ray transient EXO 1745–248 located in this cluster was in its quiescent state. We detected the quiescent system with a (0.5–10 keV) luminosity of ∼ 2 × 10 ergs s. This is similar to several other neutronstar transients observed in their quiescent states. However, the quiescent Xray spectrum of EXO 1745–248 was dominated by a hard power-law component instead of the soft component that usually dominates the quiescent emission of other neutron-star X-ray transients. This soft component could not conclusively be detected in EXO 1745–248 and we conclude that it contributed at most 10% of the quiescent flux in the energy range 0.5–10 keV. EXO 1745–248 is only the second known neutron-star transient whose quiescent spectrum is dominated by the hard component (SAX J1808.4–3658 is the other one). We discuss possible explanations for this unusual behavior of EXO 1745–248, its relationship to other quiescent neutron-star systems, and the impact of our results on understanding quiescent X-ray binaries. We also discuss the implications of our results on the way the low-luminosity X-ray sources in globular clusters are classified. Subject headings: accretion, accretion disks — stars: individual (EXO 1745– 248)— X-rays: stars – globular clusters: individual(Terzan 5) School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife, KY16 9SS, Scotland, UK 2 Present address: Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ, Amsterdam, The Netherlands; [email protected] Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02139, USA Center for Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA Institute of Astronomy, Madingley Road, CB3 0HA, Cambridge, UK