Globular Cluster X-Ray Sources

Abstract

The discoveries of both steady and bursting X-ray sources in globular clusters have made them as interesting for high energy astrophysics and X-ray astronomy as they were for unlocking galactic structure in Shapley’ s day. Globular clusters are massive (˜ 105M⊙) spherical clusters of primarily low mass (⋞0.8 M⊙) evolved stars and may be the oldest systems in the Galaxy. Thus when several globular clusters were tentatively identified as containing X-ray sources (Giacconiet al. 1974), it was apparent that either dynamical or evolutionary processes must be occurring among the ancient stars in globular clusters to produce the relatively short-lived X-ray sources. The existence of these X-ray sources has prompted considerable discussion as to their origin, since they exist with ˜100 times the probability per unit mass in globular clusters as in the Galaxy as a whole (Katz 1975; Clark 1975). They may be yet another example of X-ray binaries (e. g., Clark 1975; Fabianet al. 1975) or be due to accretion onto a single collapsed object (black hole) at the center of the cluster (Bahcall and Ostrlker 1975; Silk and Arons 1975). At present both of these classes of models are possible and generally consistent with the data, though each has its particular difficulties. We shall review the observational data and then the models in an effort to point out the present balance of evidence for the binaryvs.black hole models as well as the most promising directions for future work. Given the possibly high incidence of X-ray bursters in globular clusters, our discussion will refer to both observations and recent models for bursters. While this review will primarily address the X-ray sources in globular clusters, it is obvious that their eventual understanding will contribute much to studies of globular clusters in general and will prompt future studies in particular areas.

Evidence for ionized hydrogen in the cores of globular clusters

“…Optical flares in the possibly associated accretion disks have been suggested by Katz (1976). For the case of spherical accretion with X-ray heating, flares or shorter bursts may result when a thermal limit accretion rate is exceeded L109 (Grindlay 19776). It is then interesting that the density n ~ 10 6 cm -3 at the radius r ^5 X 10 13 cm implied by the NGC 6624 flare emission measure is approximately that expected for spherical accretion (near the thermal limit) on to a ^lOO M 0 black hole.…”

“…Since the X-ray source (3U 1820-30) in NGC 6624 was apparently not producing bursts at this time (Lewin 1976; nearly concurrent SAS-3 observations are available), the Ha flare may have been due to a ^ 10 min X-ray flare of the source such as occasionally observed (e.g., Cañizares and Neighbours 1975). Such flares may be increasingly expected at accretion rates nearly high enough for the ^10 s bursts (Grindlay 19776). Clearly, more extensive optical and X-ray observations are needed.…”

A supersoft variable low-luminosity X-ray source in the globular cluster M3

“…One of the truly new and unexpected classes of X-ray sources discovered with the Einstein Observatory was the class of low luminosity globular cluster X-ray sources (LLGCXS). Seven high luminosity (L X > 10 36 ergs s ?1 ) globular cluster X-ray sources were known prior to the Einstein observations (Grindlay 1977). A survey of more than 80 Galactic globular clusters with Einstein failed to detect any additional high luminosity sources; however, 14 unresolved LLGCXS were detected in 8 globular clusters with X-ray luminosities L X < 10 34:5 ergs s ?1 (Hertz & Grindlay 1983b).…”

Neutron Star and Degenerate Dwarf Models of X‐ray Bursts*

“…The nature of the burst sources has been addressed earlier in reviews by Grindlay, 63 Clark," and Lewin7 and is also addressed elsewhere in this volume in the reviews by Gursky,' Lewin,' and Ostriker." It may be possible, as suggested by Gursky, to decide the nature of the underlying compact object independent of model details.…”