Observations

3 Observations

Observational evidence for relativistic binaries in globular clusters has undergone an explosion in recent years, thanks to concentrated pulsar searches, improved X-ray source positions from Chandra, and optical follow-ups with HST and ground-based telescopes. There are challenges to detecting most binaries since they have generally segregated to the cores of the clusters where crowding can be a problem. Nonetheless, numerous observations of both binaries and their tracer populations have been made in several globular clusters.

Figure 5:

CMD of M3 from the Hubble Space Telescope WFPC2. Note the stars above and to the left of the turn-off. These are the blue stragglers. Figure taken from Zhao and Bailyn [246 ].

One tracer population of the dynamical processes that may lead to the formation of relativistic binaries is the population of blue stragglers. These are stars that appear on the main sequence above and to the left of the turn-off in the CMD of a globular cluster (see Figure 5

). These stars are hot and massive enough that they should have already evolved off the main sequence. Consequently, these objects are thought to arise from stellar coalescences either through the gradual merger of the components of binaries or through direct collisions [62

, 180 ]. Blue stragglers are some of the most visible and populous evidence of the dynamical interactions that can also give rise to relativistic binaries. For a good description of the use of far-ultraviolet surveys in detecting these objects, see Knigge [132 ]. For somewhat older but still valuable reviews on the implications of blue stragglers on the dynamics of globular clusters, see Hut [115

] and Bailyn [10

Recent observations of the blue straggler populations of 13 globular clusters indicates a correlation between the specific frequency of blue stragglers and the binary fraction in the globular cluster [221 ]. This supports observations which also seem to suggest that binary coalescences are the dominant formation mechanism for blue stragglers in globular clusters [143 ].Update

The globular cluster population of white dwarfs can be used to determine the ages of globular clusters [163

], and so they have been the focus of targeted searches despite the fact that they are arguably the faintest objects in a globular cluster. These searches have yielded large numbers of globular cluster white dwarfs. For example, a recent search of $ω$ Centauri has revealed over 2000 white dwarfs [164

], while Hansen et al. [93

] have detected 222 white dwarfs in M4. Deep ACS observations of NGC 6397 [200 ] have identified a substantial population of approximately 150 white dwarfs [226 ].Update

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 generally focus on low-luminosity X-ray sources [124

, 88

, 233

] and on ultraviolet-excess stars [50

, 51 , 86 , 133

, 152

], but these systems are usually a white dwarf accreting from a low mass star.Update

The class of “non-flickerers” which have been detected recently [36

, 228

] have been explained as He white dwarfs in binaries containing dark CO white dwarfs [56

, 89

, 92 ].

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” [158 ], which assumes a constant acceleration of the pulsar during the observation period, more short orbital period binary pulsars are being discovered [26 , 27 , 39 , 41 , 65 , 71

, 194 ]. For a good review and description of this technique, see Lorimer [144

]. The progenitors of the ultracompact millisecond pulsars (MSPs) are thought to pass through a LMXB phase [48

, 88

, 121 , 195

, 198

]. These systems are very bright and all of them in the globular cluster system are known. There are, however, several additional LMXBs that are currently quiescent [88

, 234

]. 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 $1.4M ⊙$ [68 ].Update

Although there are many theoretical predictions of the existence of black holes in globular clusters (see, e.g., [160

, 189

, 159

, 42 ]), there are very few observational hints of them. Measurements of the kinematics of the cores of M15 [74 , 90 ], NGC 6752 [53 ], and $ω$ Centauri [170 ] provide some suggestions of a large, compact mass.Update

However, these observations can also be explained without requiring an intermediate mass black hole [148 , 175 ]. 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. However, uncertainties in the expected gas density makes it difficult to place any upper limits on a black hole mass [13 ].Update

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 [35

]. 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 $1 − 103M ⊙$ in many globular clusters [247 ]. Recent Hubble Space Telescope observations of the stellar dynamics in the core of 47 Tuc have placed an upper bound of $1500 M ⊙$ for any intermediate mass black hole in this cluster [153 ].Update

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 [127 ].

Recent observations and catalogs of known binaries are presented in the following Sections 3.1, 3.2, 3.3, and 3.4.