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

R Wijnands, C O Heinke, D Pooley, P D Edmonds, W H G Lewin, J E Grindlay, P G Jonker, J M Miller

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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 similar to2 x 10(33) ergs s(-1). This is similar to several other neutron-star transients observed in their quiescent states. However, the quiescent X-ray 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 that the low-luminosity X-ray sources in globular clusters are classified.

Original languageEnglish
Pages (from-to)883-890
Number of pages8
JournalAstrophysical Journal Letters
Publication statusPublished - 10 Jan 2005


  • accretion, accretion disks
  • globular clusters : individual (Terzan 5)
  • stars : individual (EXO 1745-248)
  • X-rays : stars
  • SAX J1808.4-3658
  • CORE
  • NGC-6440


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