3 Observations

By their very nature, relativistic binaries are compact and faint, so that observations of these systems are difficult unless they are in an interacting phase. Furthermore, observations of binaries that have segregated to the centers of clusters can be complicated by crowding issues. Nonetheless, observations across a broad spectrum using a variety of ground- and space-based telescopes have revealed populations of binaries and their tracers in many Milky Way and extra-galactic globular clusters.

In a CMD of a globular cluster, some stars can be found on the main sequence above and to the left of the turn-off (see Figure 6). These stars are known as blue stragglers. As their name suggests, if these stars are coeval with the stellar population of the cluster, they are too hot and too massive and should have evolved off the main sequence. They are thought to have been rejuvenated through mass transfer, merger, or direct collision of binaries [129, 97, 377, 268, 369]. As such, they can be interpreted as tracers of binary interactions within globular clusters. Recent observations of the blue straggler populations of 13 globular clusters indicate a correlation between the specific frequency of blue stragglers and the binary fraction in the globular cluster [441]. This supports observations which also seem to suggest that binary coalescences are the dominant formation mechanism for blue stragglers in globular clusters [291]. However, many of the populations that have been observed exhibit a bimodal radial distribution [127, 488, 123, 414, 478, 286, 87, 42, 86] for which the inner population can be interpreted as tracing dynamical interactions and the outer population as representative of the primordial binary population.

Figure 6: CMD of NGC 6205 from the Hubble Space Telescope Advanced Detector for Surveys. Different populations are identified as horizontal branch (HB), red giant branch (RGB), main sequence turn off (MSTO), and blue stragglers (BS). Image reproduced by permission from Leigh, Sills, & Knigge [292].

Blue stragglers are some of the most visible and populous evidence of the dynamical interactions that can also give rise to relativistic binaries. Current observations that have been revealing the blue straggler population and their variable counterparts (SX Phe stars) are the ACS Survey of Galactic Globular Clusters [418] and the Cluster AgeS Experiment (CASE) [76]. For a good description of surveys that use far-ultraviolet in detecting these objects, see Knigge [266]. For somewhat older but still valuable reviews on the implications of blue stragglers on the dynamics of globular clusters, see Hut [236] and Bailyn [25].

The globular cluster population of white dwarfs can be used to determine the ages of globular clusters [343], and so they have been the focus of targeted searches despite the fact that they are arguably the faintest electromagnetically detectable objects in globular clusters. These searches have yielded large numbers of globular cluster white dwarfs. For example, a recent search of  Centauri has revealed over 2000 white dwarfs [344], while Hansen et al. [191] have detected 222 white dwarfs in M4. Deep ACS observations of NGC 6397 [411] have identified a substantial population of approximately 150 white dwarfs [453]. Dieball et al. [107] have found approximately 30 white dwarfs and about 60 probably cataclysmic variables and white dwarf binaries in M80. In general, however, these searches uncover single white dwarfs. Optical detection of white dwarfs in binary systems tends to rely on properties of the accretion process related to the binary type. Therefore, searches for cataclysmic variables (CVs, see Section 3.1) generally focus on low-luminosity X-ray sources [251, 179, 470] and on ultraviolet-excess stars [104, 105, 177, 269, 319], but these systems are usually a white dwarf accreting from a low mass star. The class of “non-flickerers” which have been detected recently [82, 459] have been explained as He white dwarfs in binaries containing dark CO white dwarfs [116, 180, 190, 454].

Pulsars, although easily seen in radio, are difficult to detect when they occur in hard binaries, due to the Doppler shift of the pulse intervals. Thanks to an improved technique known as an “acceleration search” [328], which assumes a constant acceleration of the pulsar during the observation period, more short-orbital-period binary pulsars are being discovered [62, 64, 88, 90, 133, 140, 400]. For a good review and description of this technique, see Lorimer [296]. The progenitors of the ultracompact millisecond pulsars (MSPs) are thought to pass through a low-mass X-ray binary phase [100, 179, 246, 401, 405] (LMXBs, see Section 3.2). These systems are very bright and when they are in an active state, they can be seen anywhere in the Galactic globular cluster system. There are, however, several additional LMXBs that are currently quiescent [179, 207, 471, 185, 184] (qLMXBs). As these systems turn on, they can be added to the list of known LMXBs, which is currently at 15 [202]. Additional evidence of a binary spin-up phase for MSPs comes from measurements of their masses, which indicate a substantial mass-transfer phase during the spin-up. Several observed globular cluster MSPs in binary systems are seen to have masses above the canonical mass of  [135].

Although there are many theoretical predictions of the existence of black holes in globular clusters (see, e.g., [334, 388, 333, 91]), these have generally predicted that only a small number would be retained in the cluster. Prior to the recent discovery of two black hole binaries in M22 [451], there have been only a few observational hints of their existence. Measurements of the kinematics of the cores of M15 [145, 183], NGC 6752 [113], and Centauri [357] provide some suggestions of a large, compact mass. One proposed explanation would be a large black hole of in the cluster core. Black holes in this mass range are much larger than stellar mass black holes () but much smaller than the supermassive black holes () found in galactic centers. As such they are called intermediate mass black holes (IMBH) and may potentially form through mergers of stars or stellar-mass black holes in globular clusters. However, these observations can also be explained without requiring an IMBH [314, 367] and the existence of such objects is questionable. Furthermore, Centauri may be the stripped core of a dwarf galaxy. Therefore, any large black hole in its center may have arisen by other, non-dynamical means. VLA observations of M80, M62, and M15 do not indicate any significant evidence of radio emission, which can be used to place some limits on the likelihood of an accreting black hole in these clusters. While radio observations have provided the strongest limit on the mass of any IMBH in M15, M19, and M22 [452]. However, uncertainties in the expected gas density makes it difficult to place any upper limits on a black hole mass [32]. Recent observations of the kinematics of NGC 2808 [302] and NGC 6388 [303] have placed upper bounds of on any IMBH in their cores. The unusual millisecond pulsar in the outskirts of NGC 6752 has also been argued to be the result of a dynamical interaction with a possible binary intermediate mass black hole in the core [80]. If the velocity dispersion in globular clusters follows the same correlation to black hole mass as in galactic bulges, then there may be black holes with masses in the range in many globular clusters [493, 297]. Recent Hubble Space Telescope observations of the stellar dynamics in the core of 47 Tuc have placed an upper bound of for any intermediate mass black hole in this cluster [322]. Stellar mass black hole binaries may also be visible as low luminosity X-ray sources, but if they are formed in exchange interactions, they will have very low duty cycles and hence are unlikely to be seen [254]. For a good recent review on neutron stars and black holes in globular clusters, see Rasio et al. [406]. Heinke [202] has an excellent review of X-ray observations of Galactic globular clusters, including CVs, LMXBs and qLMXBs, MSPs, and other active binaries.

Recent observations and catalogs of known binaries are presented in the following sections.

3.1 Cataclysmic variables

Cataclysmic variables (CVs) are white dwarfs accreting matter from a companion that is usually a dwarf star or another white dwarf. They have been detected in globular clusters through identification of the white dwarf itself or through evidence of the accretion process. White dwarfs managed to avoid detection until observations with the Hubble Space Telescope revealed photometric sequences in several globular clusters [83, 82, 366, 408, 409, 410, 459, 191]. Spectral identification of white dwarfs in globular clusters has begun both from the ground with the VLT [342, 343] and in space with the Hubble Space Telescope [82, 116, 459, 344]. With spectral identification, it will be possible to identify those white dwarfs in hard binaries through Doppler shifts in the line. This approach has promise for detecting a large number of the expected double white dwarf binaries in globular clusters. Photometry has also begun to reveal orbital periods [349, 115, 255] of CVs in globular clusters.

Accretion onto the white dwarf builds up a layer of hydrogen-rich material that may eventually lead to a dwarf nova outburst as the layer undergoes a thermonuclear runaway. These dwarf novae cause an increase in luminosity of a factor of a few to a few hundred. Identifications of globular cluster CVs have been made through such outbursts in the cores of M5 [319], 47 Tuc [365], NGC 6624 [431], M15 [428], and M22 [16, 58]. With the exception of V101 in M5 [319], original searches for dwarf novae performed with ground based telescopes proved unsuccessful. This is primarily due to the fact that crowding obscured potential dwarf novae up to several core radii outside the center of the cluster [425, 427]. Since binaries tend to settle into the core, it is not surprising that none were found significantly outside of the core. Subsequent searches using the improved resolution of the Hubble Space Telescope eventually revealed a few dwarf novae close to the cores of selected globular clusters [424, 426, 431, 428, 16]. To date, there have been 14 found [374, 423], using the Hubble Space Telescope and Las Campanas Observatory (CASE).

A more productive approach has been to look for direct evidence of the accretion around the white dwarf. This can be in the form of excess UV emission and strong emission [124, 178, 269, 270, 104] from the accretion disk. This technique has resulted in the discovery of candidate CVs in 47 Tuc [124, 269], M92 [126], NGC 2808 [104], NGC 6397 [82, 116, 459], and NGC 6712 [125]. The accretion disk can also be discerned by very soft X-ray emissions. These low luminosity X-ray binaries are characterized by a luminosity , which distinguishes them from the low-mass X-ray binaries with . Initial explanations of these objects focused on accreting white dwarfs [24], and a significant fraction of them are probably CVs [471, 480]. There have been 10 identified candidate CVs in NGC 6752 [382], 15 in NGC 6397 [78], 19 in NGC 6440 [381], 2 in  Cen [147], 5 in Terzan 5 [204], 22 in 47 Tuc [114], 5 in M80 [206], 7 in M54 [399], 2 – 5 in NGC 288 [273], 4 in M30 [301], 4 in NGC 2808 [422], 1 in M71 [227], and 1 in M4 [33]. However, some of the more energetic sources may be LMXBs in quiescence [471], or even candidate QSO sources [33].

The state of the field at this time is one of rapid change as Chandra results come in and optical counterparts are found for the new X-ray sources. A living catalog of CVs was created by Downes et al. [108], but it has been allowed to lapse and is now archival [109, 110]. It may still be the best source for confirmed CVs in globular clusters up to 2006.

3.2 Low-mass X-ray binaries

The X-ray luminosities of low-mass X-ray binaries are in the range . The upper limit is close to the Eddington limit for accretion onto a neutron star, so these systems must contain an accreting neutron star or black hole. All of the LMXBs in globular clusters contain an accreting neutron star as they also exhibit X-ray bursts, indicating thermonuclear flashes on the surface of the neutron star [251]. Compared with  100 such systems in the galaxy, there are 15 LMXBs known in globular clusters. The globular cluster system contains roughly 0.1% of the mass of the galaxy and roughly 10% of the LMXBs. Thus, LMXBs are substantially over-represented in globular clusters. Within the Galactic globular cluster system, high metallicity clusters are more likely to host an LMXB, as noted by Grindlay in 1993 [176] and Bellazzini in 1995 [50].

Because these systems are so bright in X-rays, the globular cluster population is completely known – we expect new LMXBs to be discovered in the globular cluster system only as quiescent (qLMXBs) and transient systems become active. The 15 sources are in 12 separate clusters. Five have orbital periods greater than a few hours, six ultracompact systems have measured orbital periods less than one hour, and four have undetermined orbital periods. A member of the ultracompact group, 4U 1820–30 (X1820–303) in the globular cluster NGC 6624, has an orbital period of 11 minutes [447]. This is one of the shortest known orbital periods of any binary and most certainly indicates a degenerate companion. The orbital period, X-ray luminosity, and host globular clusters for these systems are given in Table 1.

The improved resolution of Chandra allows for the possibility of identifying optical counterparts to LMXBs. If an optical counterpart can be found, a number of additional properties and constraints for these objects can be determined through observations in other wavelengths. In particular, the orbital parameters and the nature of the secondary can be determined. So far, optical counterparts have been found for X0512–401 in NGC 1851 [224], X1745–203 in NGC 6440 [473], X1746–370 in NGC 6441 [99], X1830–303 in NGC 6624 [265], X1832–330 in NGC 6652 [100, 203], X1850–087 in NGC 6712 [84, 26, 354], X1745–248 in Terzan 5 [204], and both LMXBs in NGC 7078 [20, 485]. Continued X-ray observations will also further elucidate the nature of these systems [347].

The 15 bright LMXBs are thought to be active members of a larger population of lower luminosity quiescent low mass X-ray binaries (qLMXBs) [486]. Early searches performed with ROSAT data (which had a detection limit of ) revealed roughly 30 sources in 19 globular clusters [251]. A more recent census of the ROSAT low luminosity X-ray sources, published by Verbunt [470], lists 26 such sources that are probably related to globular clusters. Recent observations with the improved angular resolution of Chandra have begun to uncover numerous low luminosity X-ray candidates for CVs [179, 180, 203, 225, 204, 206, 114, 115, 147, 382, 381]. For a reasonably complete discussion of recent observations of qLMXBs in globular clusters, see Verbunt and Lewin [471] or Webb and Barret [480] and references therein. Table 2 lists the 27 qLMXBs known to date.

3.3 Millisecond pulsars

The population of known millisecond pulsars (MSPs) is one of the fastest growing populations of relativistic binaries in globular clusters. Several ongoing searches continue to reveal millisecond pulsars in a number of globular clusters. Previous searches have used deep multi-frequency imaging to estimate the population of pulsars in globular clusters [140]. In this approach, the expected number of pulsars beaming toward the earth, , is determined by the total radio luminosity observed when the radio beam width is comparable in diameter to the core of the cluster. If the minimum pulsar luminosity is and the total luminosity observed is , then, with simple assumptions on the neutron star luminosity function,

In their observations of 7 globular clusters, Fruchter and Goss have recovered previously known pulsars in NGC 6440, NGC 6539, NGC 6624, and 47 Tuc [140]. Their estimates based on Eq. (19) give evidence of a population of between 60 and 200 previously unknown pulsars in Terzan 5, and about 15 each in Liller 1 and NGC 6544 [140]. Additional Fermi LAT work uses -ray emission to estimate total numbers of  2600 – 4700 MSPs in the Galactic globular cluster system, which corresponds to estimates from encounter rates [8].

Current searches include the following: Arecibo, which is searching over 22 globular clusters [214]; Green Bank Telescope (GBT), which is working both alone and in conjunction with Arecibo [249, 214]; the Giant Metrewave Radio Telescope (GMRT), which is searching over about 10 globular clusters [134]; and Parkes, which is searching over 60 globular clusters [89]. Although these searches have been quite successful, they are still subject to certain selection effects such as distance, dispersion measure, and acceleration in compact binaries [63]. For an excellent review of the properties of all pulsars in globular clusters, see the review by Camilo and Rasio [63] and references therein. The properties of known pulsars in binary systems with orbital period less than one day are listed in Table 3, which has been extracted from the online catalog maintained by Freire [397].

With the ongoing searches, it can be reasonably expected that the number of millisecond pulsars in binary systems in globular clusters will continue to grow in the coming years.

3.4 Black holes

Radio observations of M22 have revealed two stellar mass black holes with likely low-mass stellar companions [451]. These objects have flat radio spectra, but no observed X-ray emission. As such, they are likely to be undergoing very low level accretion from their companions. Mass estimates put these black holes at . There have been no confirmed observations of X-ray black hole binaries in Galactic globular clusters. All of the Galactic globular cluster high luminosity LMXBs exhibit the X-ray variability that is indicative of nuclear burning on the surface of a neutron star. It is possible that some of the recently discovered low luminosity LMXBs may house black holes instead of neutron stars [471], it is more likely that they are simply unusual neutron star LMXBs in quiescence [486]. Finally, there is very circumstantial evidence for the possible existence of an intermediate mass black hole (IMBH) binary in NGC 6752 based upon an analysis of the MSP binary PSR A [79, 80]. Further evidence for black holes in globular clusters will probably have to wait for the next generation of gravitational wave detectors.

3.5 Extragalactic globular clusters

Recently, the X-ray observatories XMM-Newton, Chandra, and Swift have been combined with HST observations to search for extragalactic globular cluster binaries. These have yielded a number of systems found in M31, M104, NGC 1399, NGC 3379, NGC 4278, NGC 4472, and NGC 4697.

Three super soft X-ray sources have been found in M31 globular clusters [212, 375, 213]. Super soft sources have little or no radiation at energies above 1 keV and have a blackbody spectrum corresponding to temperatures between to . These are assumed to be from classical novae, and two have been associated with optical novae [212, 213]. Stiele et al. [448] have found 36 LMXBs and 17 candidate LMXBs that are co-located with M31 globular clusters, while Peacock et al. [368] claim 41 LMXBs in 11% of the M31 globular clusters. Barnard et al. [31] have found 4 black hole X-ray binaries associated with M31 globulars. Finally, there is the suspected intermediate mass black hole in G1 [17, 102].

The X-ray luminosity function (XLF) of the population of LMXB’s in individual galaxies has been found to obey a power law with a slope in the range of 1.8 to 2.2 [164, 258]. When the XLF’s for many galaxies are combined, a broken power law provides a better fit [258]. At the break luminosity of , the XLF steepens to a slope consistent with . The break luminosity is near the Eddington luminosity for a accretor. The XLFs for several individual galaxies are shown in Figure 7 and the combined XLFs from Kim and Fabbiano [258] are shown in Figure 8. When the globular cluster XLF is separated from the field XLF, we see a dramatic drop in the number of low luminosity LMXBs for the globular cluster population, indicative of a different formation mechanism [477, 492].

Figure 7: X-ray luminosity functions for several individual galaxies. Each can be fit will with a single power law. Image reproduced by permission from Kim & Fabbiano [258], copyright by IOP.

Figure 8: X-ray luminosity function for several galaxies combined together. On the left is the best fit to a single power law, while a broken power law fit is shown in the right. Image reproduced by permission from Kim & Fabbiano [258], copyright by IOP.

Searches for bright X-ray sources in other nearby galaxies have yielded LMXB’s thought to contain black holes based on their luminosity. There are 2 in NGC 1399 [432, 243, 313], and Paolillo et al. [364] estimate that 65% of globular clusters in NGC 1399 host LMXBs. Brassington et al. [60] have found 3 LMXBs in globular clusters associated with NGC 3379, of which 1 is a presumed black hole. Fabbiano et al. [120] have found 4 LMXBs in globular clusters associated with NGC 4278, all of which are bright enough to be black holes. Maccarone and collaborators [308, 310] have found 2 LMXBs containing black holes in NGC 4472 globular clusters. Kim et al. [259] claim 75 globular clusters LMXBs in NGC 3379, NGC 4278, and NGC 4697. Finally, Li et al. [293] have found 41 X-ray sources that are presumably LMXBs in globular clusters in M104. The extragalactic LMXBs have been found to follow the same trend toward over-representation in red (or metal rich) globular clusters [440, 282, 307, 261, 283], as can be seen in Figure 9.

Figure 9: GC magnitudes (Mz) vs. GC colors (g – z), with integrated histograms of the properties above and to the right. GCs unmatched to LMXBs are indicated by small black dots and open black histograms (scaled down by a factor of 10). Blue GCs with LMXBs are indicated by filled blue squares and histograms. Red GCs with LMXBs are indicated by filled red circles and histograms. The histograms of the GCs with LMXBs are stacked on each other. GCs that are redder and brighter are more likely to contain LMXBs. Image reproduced by permission from Sivakoff et al. [440], copyright by IOP.