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Astron. Astrophys. 329, L37-L40 (1998)

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1. Introduction

The details of the formation mechanism of low-mass X-ray binaries, in the galactic disk and in globular clusters, are not well understood. It is thought that low-mass X-ray binaries evolve directly from binaries in the galactic disk, but originate from close stellar encounters in globular clusters in which a neutron star captures a low-mass star via tidal energy dissipation - a tidal capture - or in which a neutron star takes the place of a binary member in a three-body encounter - an exchange collision (see reviews by Verbunt 1993, Hut et al. 1992). It has been suggested that close stellar encounters may also produce low-mass X-ray binaries in the galactic disk, in open clusters (Mardling 1996).

A key observation that any formation theory must answer is the relative frequency of low-mass X-ray binaries with a black hole and with a neutron star as accreting object. In the galactic disk, an increasing number of low-mass X-ray binaries with an accreting black hole has been found in the last years. Interestingly, all of these are transient systems, in which the X-ray and optical luminosities are high only during outbursts, and rather low in the quiescent intervals between the outbursts. Since we do not know the distribution of the durations of the inter-outburst intervals (the observations obviously being biased to short intervals) the total number of low-mass X-ray binaries with a black hole cannot be estimated accurately; but it is quite possible that their number is of the same order of magnitude as that of low-mass X-ray binaries with a neutron star (for reviews see Tanaka & Shibazaki 1996, or Chen, Shrader & Livio 1997).

In contrast, none of the X-ray sources in globular clusters has so far been found to contain a black hole (Hut et al. 1992; Verbunt et al. 1995). Twelve X-ray sources in globular clusters have shown X-ray luminosities [FORMULA], of which seven are permanently bright, and five transients. Type I X-ray bursts, thought to be caused by thermonuclear flashes on or near a neutron star identifying the accreting object unambiguously as a neutron star, have been detected in all permanently bright sources in globular clusters and in the transients in Terzan 5 and Liller 1. The nature of the accreting object in the remaining three transients, in NGC 6440, NGC 6652 and Terzan 6 is still unknown. If the same statistics would apply in globular clusters as in the galactic disk, these transients have a relatively high probability of containing a black hole compared to that of the permanently bright sources.

In this article we discuss the first time discovery of two type I X-ray bursts in the transient X-ray source in NGC 6652. This X-ray source was detected with HEAO-1 (Hertz & Wood 1985) and shown to be associated with the globular cluster on the basis of the data from the ROSAT All-Sky Survey (Predehl et al. 1991, Verbunt et al. 1995). The X-ray spectrum between 0.1-2.5 keV can be described with a powerlaw [FORMULA] (where [FORMULA] is the energy flux), absorbed by a column of [FORMULA] (Johnston et al. 1996). The X-ray luminosity was about the same during the ROSAT Survey in September 1990 and during the ROSAT pointing in April 1992, at [FORMULA] (for a distance of 14.3 kpc; Djorgovski 1993). This is about ten times more luminous than observed with HEAO-1 in August 1977 to April 1978, which is why Verbunt et al. (1995) classify this source as a transient.

The observations and data analysis are described in Sect. 2, and a brief discussion follows in Sect. 3.

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© European Southern Observatory (ESO) 1998

Online publication: December 8, 1997