Globular cluster X-ray sources

Abstract

Two new bright X-ray sources in globular clusters, and many less luminous ones in globular clusters and in old open clusters, have been discovered with ROSAT. Accurate positions obtained with ROSAT help identification with optical objects, which however is still very incomplete in globular clusters. One dim globular cluster source has been identified with a recycled radio pulsar; several others may be cataclysmic variables. The four brightest X-ray sources identified in the old open cluster M 67 are puzzling, as we do not understand why they emit X-rays. In comparison with the two old open clusters studied so far, globular clusters are remarkably underluminous in X-rays. FirstI am grateful to the organizers of this meeting for the opportunity to contribute to this celebration of the 65 th birthday of Joachim Trümper. During my years (1985)(1986)(1987)(1988)(1989) at the Max Planck Institut für extraterrestrische Physik, I have always felt very much at home, as if an 'honorary German', and I thank Joachim and my other colleagues for this. I was impressed with the width and depth of Joachim Trümper's interests, and thoroughly enjoyed my many discussions with him.The deadline for the first Announcement of Opportunity for observing with ROSAT was on a Sunday evening, at midnight (in checking the date, I find it was on my birthday!). Joachim Trümper was in his office that night, to sign proposals until just before the deadline, or almost, as 11 copies needed to be made for submission. Some of the proposals he signed that night contributed to the results discussed in this review.

Disc reflection and a possible disc wind during a soft X-ray state in the neutron star low-mass X-ray binary 1RXS J180408.9–342058

“…The tentative classification scheme of Sidoli et al (2001) gained credence through testing for an UCXB nature by alternative methods such as the relation of van Paradijs & McClintock (1994) discussed above, and the composition of the accreted matter inferred from thermonuclear X-ray bursts (see Verbunt & Lewin 2006). Its validity has also been strengthened by a number of additional sources (e.g., Gierliński & Poutanen 2005;Falanga et al 2005;Fiocchi et al 2008).…”

Ultra-compact (X-ray) binaries

“…Five of the 12 systems with known orbital periods reside in GCs, an even larger enhancement compared to the disc than for longerperiod X-ray binaries. For reviews of X-ray sources in globular clusters see ; Verbunt & Lewin (2005). The shortest period system 4U 1820-30, which with an orbital period of 11 min for a long time has been the shortest period binary known and is the most well-known source of this class.…”

“…Four more systems are identified based on similarities with known systems in either their X-ray spectra (Juett et al, 2001) or as a result of their optical faintness in outburst, as expected for the small accretion discs in these systems (van Paradijs & McClintock, 1994). Six systems are found in a globular cluster (see Kuulkers et al, 2003;Verbunt & Lewin, 2005, for reviews of globular cluster X-ray sources).…”

Joint Discussion 6 Neutron stars and black holes in star clusters

“…See Verbunt & Lewin (2006) for a fuller (but dated) review, and the next section for some XMM-Newton results. The best-studied cluster (and the main focus in this section) is 47 Tucanae (47 Tuc), which has been well observed with Chandra(detecting 300 X-ray sources), HST in the optical and UV, and Parkes for pulsar timing observations (detecting 22 pulsars; see Sec.…”