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

JJM in't Zand, MH van Kerkwijk, D Pooley, F Verbunt, Rudy Adam Dirk Wijnands, WHG Lewin

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39 Citations (Scopus)


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(-12)(+31) km and an unabsorbed bolometric luminosity for the neutron star atmosphere of (2.1 +/- 0.8) x 10(23) ergs s(-1) that is consistent with predictions for a cooling neutron star.

Original languageEnglish
Pages (from-to)pp.L41-L44.
Number of pages4
JournalAstrophysical Journal
Issue number1
Publication statusPublished - 10 Dec 2001


  • globular clusters : individual (NGC 6440)
  • stars : neutron
  • X-rays : binaries


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