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The earliest detections of luminous X-ray sources (LX gtapprox 1036 erg s-1) in globular clusters were made with the Uhuru and OSO-7 Observatories (Giacconi et al. 1972 & 1974; Clark, Markert & Li, 1975; Canizares & Neighbours, 1975). About 10% of the luminous X-ray sources in our Galaxy are found in globular clusters. This implies that the probability (per unit mass) of finding a luminous X-ray source in a globular cluster is about two to three orders of magnitude higher than of finding one in the rest of our galaxy (Gursky 1973, Katz 1975). Clearly, the conditions in globular clusters are very special in that they must be very efficient breeding grounds for X-ray binaries. For reviews which reflect the ideas in the late seventies and early eighties, see Lewin (1980), Lewin & Joss (1983), Van den Heuvel (1983) and Verbunt & Hut (1987). At that time there was no evidence for a substantial population of binaries in globular clusters; e.g. Gunn & Griffin (1979) did not find a single binary in a spectroscopic search for radial velocity variations of 111 bright stars in M3.

Clark (1975) suggested that the luminous cluster sources are binaries formed by capture from the remnants of massive stars. Fabian, Pringle & Rees (1975) specified that they are formed via tidal capture of neutron stars in close encounters with main-sequence stars. Sutantyo (1975) suggested direct collisions between giants and neutron stars as a formation mechanism. Hills (1976) examined the formation of binary systems through star-exchange interactions between neutron stars and primordial binaries of low-mass stars. Hut & Verbunt (1983) compared the relative efficiencies of tidal capture and exchange encounters for neutron stars and for white dwarfs; and showed that the distribution of X-ray sources among globular clusters with different central densities and core sizes is compatible with the formation by close encounters (Verbunt & Hut 1987). The importance of mass segregation, which drives the neutron stars to the core, thereby enhancing the capture rate, was demonstrated by Verbunt & Meylan (1988).

As can be seen from the discovery references in Table 1, five luminous globular cluster X-ray sources were known by 1975, eight by 1980, ten by 1982, and thirteen to date. Twelve of these have shown type I X-ray bursts. Measurements of the black-body radii of the burst sources indicated that they are neutron stars (Swank et al. 1977; Hoffman, Lewin & Doty 1977a & 1977b; Van Paradijs 1978). Clearly, the luminous cluster sources are accreting neutron stars (Lewin, Van Paradijs & Taam, 1995). The absence of luminous accreting black holes in clusters of our galaxy is presumably a consequence of the small total number of sources, as discussed in Sect. 5.1.2.

Table 1. Some information on the luminous X-ray sources in globular clusters of our galaxy. Columns from left to right (1) cluster, (2) rough position (B1950, often used as source name) with reference to the currently most accurate position, (3) the satellite with which the source was discovered as a cluster source, (4) the satellite which detected the first burst of the source, (5) absolute magnitude with filter of optical counterpart, (6) orbital period, (7) indication (with a "T") whether the source is a transient. The last three columns indicate whether a normal (N) or ultrashort (U) orbital period is suggested by the comparison of optical with X-ray luminosity (8, under "O"), the X-ray spectrum (9, under "X") and the maximum flux reached during bursts (10, under 'B'). A "-" in columns 8-10 indicates that no information is available. Satellite names are abbreviated as O(SO-)7, O(SO-)8, UH(URU), H(EAO-)2, A(riel-)V, Ch(andra), Ha(kucho), RO(SAT), S(AS-)3, B(eppo)S(ax), EX(OSAT), Gi(nga). Note that the absolute magnitudes are subject to uncertainties in distance and reddening; also most sources are variable (see Deutsch et al. 2000).

cluster position discovery 1st burst Mlambda Pb TOXB  

NGC1851 0512-40 [104] O7 [25] UH [57] 5.6B [39]   UUU  
NGC6440 1745-20 [176] O7 [154] BS [112] 3.7B [231]   T-N-  
NGC6441 1746-37 [106] UH [66] EX [206] 2.4B [40] 5.7hr [191] -NN  
NGC6624 1820-30 [130] UH [66] ANS [75] 3.0B [2] 11.4m [202] UUU  
NGC6652 1836-33 [89] H2 [96] BS [111] 5.6B [89]   UUU  
NGC6712 1850-09 [76] AV [197] S3 [102] 4.5B [103] 20.6m [103] UUU  
NGC7078-1a 2127+12 [234] Ch [234]   0.7B [6] 17.1hr [110] --  
NGC7078-2a 2127+12 [234] Ch [234] Gi [45] 3.1U [234]   -U  
Terzan1 1732-30 [121] Ha [152] Ha [152]     T--  
Terzan2 1724-31 [76] O8b [205] O8 [205]     -NU  
Terzan5 1745-25 [90] Ha [152] Ha [152] 1.7J [90]   T-U-  
Terzan6 1751-31 [115] RO [179] BS [115]   12.36h [113] T-N-  
Liller1 1730-33 [105] S3c [145] S3 [101]     T--  

a A luminous X-ray source in NGC7078 was already found with Uhuru [66], the Chandra observations resolved this source into two sources
b X-ray source discovered before the globular cluster!
c X-ray source (the Rapid Burster) discovered before the globular cluster!

Because of the observed correlation between the occurrence of a luminous X-ray source in a globular cluster and its high central density, it was expected already early on that these luminous sources would be located close to the cluster centers. These expectations were confirmed by measurements, carried out with the SAS-3 X-ray Observatory, which showed that the positional error circles with radii of 20-30 arcsec (90% confidence) included the optical centers of the clusters (Jernigan & Clark, 1979). Later work with the Einstein observatory greatly refined the positional measurements (Grindlay et al. 1984). Bahcall & Wolf (1976) have shown that under certain assumptions, the average mass of the X-ray sources can be derived from their positions with respect to the cluster center. Even if one accepts the assumptions made, the average mass derived this way for the luminous X-ray sources in globular clusters was not sufficiently accurate to classify these sources, but the result was consistent with the earlier conclusions (see e.g., Lewin 1980; Lewin & Joss 1983) that these are accreting neutron stars (Grindlay et al. 1984).

Sources with Lx ltapprox 1035erg s-1were first found in globular clusters with Einstein (Hertz & Grindlay 1983). More were found with ROSAT, by a variety of authors (Table 2); a final, homogeneous analysis of the complete ROSAT data was made by Verbunt (2001). On the basis of these Einstein and ROSAT results, it has gradually become clear that these sources are a mix of various types. Hertz & Grindlay (1983) suggested that they were mainly cataclysmic variables, and noted that the low-luminosity source in NGC6440 could be the quiescent counterpart of the luminous transient source in that cluster. Verbunt et al. (1984) argued that the more luminous of the low-luminosity sources are all quiescent low-mass X-ray binaries. The first pulsar detected as a low-luminosity source in a globular cluster is the pulsar in NGC6626 (M28, Saito et al. 1997). Finally, Bailyn et al. (1990) pointed out that magnetically active binaries also reach X-ray luminosities in the range of the less luminous sources detected with ROSAT. The various classes of X-ray sources in globular clusters are illustrated in Figure 1.

Figure 1

Figure 1. Various types of X-ray sources in globular clusters; sg, ms, wd, and ns stand for subgiant, main-sequence star, white dwarf, and neutron star, respectively. From top to bottom: luminous low-mass X-ray binary, low-luminosity low-mass X-ray binary, recycled radio pulsar (here with a white dwarf companion), cataclysmic variable, and magnetically active binary. Lsd stands for spin-down luminosity. Approximate maximum luminosities (in the 0.5-4.5 keV range) are indicated on the right. The low-mass X-ray binaries in the figure are shown harboring a neutron star, we refer to these as LMXBNS; when they harbor a back hole, we refer to them as LMXBBH, and we refer to both groups together as LMXB.

It was also realized that some of the sources could be unresolved multiple sources; and unresolved emission was found e.g. by Fox et al. (1996) in NGC6341 and NGC6205. However, it is fair to say that the actual plethora of sources shown by the Chandra observations in virtually every cluster that it observed (Tables 2, 4) was unpredicted. These observations confirmed that quiescent low-mass X-ray binaries, cataclysmic variables, pulsars, and magnetically active binaries are all X-ray sources in globular clusters, as is discussed in Sect. 4. Whereas some of the Einstein and ROSAT sources are confirmed with Chandra as single sources, others have been resolved into multiple sources; details are given in Table 2.

The positions obtained with ROSAT were sufficiently accurate to find plausible optical counterparts in HST observations in a number of cases. This work was pioneered in NGC6397 with a search for Halpha emitting objects by Cool et al. (1993, 1995), and spectroscopic followup confirming the classification as cataclysmic variables by Grindlay et al. (1995), Cool et al. (1998) and Edmonds et al. (1999). Plausible candidate counterparts were also found for two X-ray sources in the core of omegaCen (Carson et al. 2000). All of these suggested counterparts were confirmed with the more accurate positions obtained with Chandra. In 47Tuc, of the candidate counterparts suggested by a variety of authors, Verbunt & Hasinger (1998) only retain three, on the basis of more accurate positions of the X-ray sources: these also were confirmed with Chandra. Ferraro et al. (1997) suggested ultraviolet stars as counterparts for two sources found by Fox et al. (1996) in NGC6205 (see also Verbunt 2001). An ultraviolet counterpart suggested by Ferraro et al. (2000) for a source in NGC6341 is, in fact, incompatible with the position of that source (Geffert 1998, Verbunt 2001). Another approach is to look for X-rays of an already known object. Thus a dwarf nova known since 1941 well outside the central region of NGC5904 (Oosterhoff 1941) was detected with ROSAT (Hakala et al. 1997), and a pulsar in M28 (Lyne et al. 1987) was detected with ASCA (Saito et al. 1997). Before Chandra, no magnetically active binary was suggested as optical counterpart for a specific X-ray source.

Table 2. Observations of low-luminosity sources in globular clusters, giving numbers of sources found with Einstein (under E), ROSAT (R) and Chandra or XMM (C/X). Numbers following the + sign indicate sources outside the cluster core. Note that the detection limits are very different between clusters. References are Hertz & Grindlay (1983) for Einstein sources, Verbunt (2001) and references therein for ROSAT sources. References for Chandra and XMM-Newton are listed in Table 4 for Chandra and XMM sources. Under comments we provide additional references for ROSAT, and indicate the relation between sources observed by subsequent satellites. = identical; > resolved into multiple sources; A neq B source A not detected by satellite B, due to significant variability. Source numbers under comments are those in the references given.

cluster E R C/X comments

NGC104/47Tuc 1 5+4 39+66 E=R9=C42 (CV) R5=C58 R7=C46
        R6=C56 R10=C27 R11=C25 R13=C2
        R19=C30 R4 outside C-frame
NGC288   1   [194]
NGC362   2    
Pal 2   0+1   [184]
NGC1904/M79 1 0+1   E=R
NGC5139 / omegaCen 1+4 3+3 3+97 [224] core: EC>(R9a=C6/R9b=C4)
        both CVs; out-of-core: EB=R7=C3 qLMXB;
        EA=R3, ED=R4, EE=R5 foreground stars
NGC5272/M3 1 1   [46] E=R, CV/SSS? also [97]
NGC5824 1 0   R limit just below E detection level
NGC5904/M5   0+1 10a [85]
NGC6093/M80   1 9+10 R>(C1/C2/C4/C7..)
NGC6121/M4   1 12+19 R=C1
NGC6139   1    
NGC6205/M13   2+1 2+1 core: RGa=X3 qLMXB; RGb neq X
        X2 neq R out-of-core: RF=X6
NGC6266/M62   1 45a  
NGC6341/M92   1   [120], [59]
NGC6352   0+1   [122]
NGC6366   1 1a [122]
NGC6388   0+1    
NGC6397   5+1 9+11 R4a/b/c/d/e=C19/17/23/22/18 R13=C24
NGC6440 1 2 24 E>(R1=C2/C4/C5..,R2=C1/C3...)
NGC6541 1 1    
NGC6626/M28   3+1 12+34 core: (R2a+2b)=C26 R2c=C19
        out-of-core: R7=C17
NGC6656/M22 1+3 1 3+24 core: E=R=X16, opt id.[1] X18 neq R
        out-of-core: E prob. not related to cluster
NGC6752   4+2 9+8 core: R7a>C4/7/9 R21>C11/12/18 R7b=C1
        R22=C6 out-of-core: R6=C3 R14=C2
NGC6809   1   [122]
NGC7099   0+1 5a [120]
total: 8+7 37+18    

a number within half-mass radius from [177], detailed analysis not yet published

The luminous X-ray sources in globular clusters are binary systems, and most (if not all) of the low-luminosity X-ray sources are also binary systems or have evolved from them. The presence of binaries is a very important factor in the evolution of a globular cluster (Hut et al. 1992). Theoretical considerations and numerical calculations show that a cluster of single stars is unstable against collapse of its core (Hénon 1961). If binaries are present, however, close binary-single star encounters can increase the velocity of the single stars by shrinking the binary orbits. Binaries can therefore become a substantial source of energy for the cluster, sufficient even to reverse the core collapse. Even a handful of very close binaries can significantly modify the evolution of a globular cluster (Goodman & Hut, 1989). With a million stars in the cluster as a whole, the number of stars in the core of a collapsed cluster may be only a few thousand. A close binary system, such as an X-ray binary, will have a binding energy that can easily be a few hundred times larger than the kinetic energy of a single star. A dozen such systems, as they were formed, released an amount of energy that is comparable to the kinetic energy of the core as a whole. Encounters between such binaries and other single stars or binaries have the potential to change the state of the core dramatically by increasing or decreasing the core size, and by kicking stars and binaries into the cluster halo or even out of the cluster altogether. The study of the binaries, and X-ray binaries in particular, is therefore of great importance as they play a key role in the cluster's dynamical evolution.

It has been suggested that globular clusters are responsible for the formation of all or some of the low-mass X-ray binaries in our Galaxy, also those outside clusters now (e.g. Grindlay & Hertz 1985). Specifically, such an origin was suggested by Mirabel et al. (2001) for the black-hole X-ray binary XTEJ1118+480, and by Mirabel & Rodrigues (2003) for Sco X-1, on the basis of their orbits in the galaxy. The discovery of very large populations of cluster X-ray sources in other galaxies has rekindled the question of cluster origin for non-cluster sources, as we will discuss in Sect. 3.5. We will argue in that Section that most X-ray binaries in the disk of our Galaxy were formed there; and do not originate in globular clusters.

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