ROSAT HRI observations of the globular clusters M13 and M92

ABSTRACT We report on 40 kiloseconds of ROSAT HRI observations of the globular cluster M92(NGC 6341) and 20 kiloseconds of observations of the globular cluster M13 (NGC6205). We ﬁnd that the low-luminosity (10 32.5 erg sec −1 at 7.5 kpc) source previouslyobserved near the core of M92 with the ROSAT PSPC remains unresolved at HRIresolution; we can identify it with M92 with 99 per cent conﬁdence. In M13 we ﬁndthat the source seen with the ROSAT PSPC lies within the core and is possiblyassociated with the cluster (96 per cent conﬁdence). We ﬁnd probabilities of 99.8 percent (M92) and 98 per cent (M13) for the presence of additional unresolved emissionwithin these globular clusters. We interpret these results in light of current theoriesregarding the low-luminosity sources.Key words: globular clusters: individual: M92 – globular clusters: individual: M13– globular clusters: general – X-rays: general 1 INTRODUCTIONThe low-luminosity X-ray sources (L x < 10 34.5 erg sec −1 )in globular clusters were ﬁrst distinguished in the Einsteinglobular-cluster survey of Hertz & Grindlay (1983a,b; here-after HG83a and HG83b). Separated by 1.5 orders of mag-nitude in ﬂux from the high-luminosity sources that are con-vincingly explained as low-mass X-ray binaries (LMXBs; seeLewin, Van Paradijs & Taam 1993 for a review), they seemeda new population, and over a decade of observations has con-ﬁrmed this view.Recent optical and ultraviolet (UV) globular clusterobservations have yielded interesting objects in the clustercores, and these have been suggested as counterparts for thelow-luminosity X-ray sources. The ultraviolet variables V1and V2 detected in the core of 47 Tuc (Paresce, DeMarchi F Paresce & De Marchi 1994) are a case in point.But since the cores of clusters are crowded with interestingobjects, positional coincidence is not a convincing argumentfor association unless it is accompanied by correspondingtemporal variability or by a spatial pattern present in bothX-rays and the optical/UV. The latter is the case in the coreof NGC 6397, where three low-luminosity sources have beenidentiﬁed with three Hα emission objects that are probablycataclysmic variables (CVs; Cool et al. 1995a, Grindlay etal. 1995). These counterparts notwithstanding, the nature of thefull source population remains uncertain. Some of the ini-tial HG83b sources are almost certainly foreground or back-ground objects rather than low-luminosity sources (Margon& Bolte 1987; Verbunt et al. 1995; Cool et al. 1995b), whileproposals for the population itself include CVs (HG83a),LMXBs – either in quiescence (Verbunt, van Paradijs & El-son 1984; Van Paradijs et al. 1987) or viewed at high inclina-tion (White & Mason 1985) – millisecond pulsars (Danner,Kulkarni & Thorsett 1994), and RS CVn systems (Bailyn,Grindlay & Garcia 1990; Verbunt et al. 1993). With the addition of the data reported here, a total of38 galactic globular clusters have been subjected to deepimaging with ROSAT (HRI and PSPC). Low-luminositysources have been seen within or near the cores of 22 ofthese clusters; perhaps as many as 37 such sources have nowbeen detected (see Johnston & Verbunt1996 for afull discus-sion). The present observations aim to constrain this popula-tion further, taking advantage of the HRI’s high resolution(FWHM ∼ 7 arcsec) to evaluate the possible multiplicityof the sources seen in PSPC observations of M92 and M13(Hasinger, Johnston & Verbunt 1994; Margon et al. 1994a)and to search for extended emission from the cores of theseclusters.