An Extensive Census ofHubble Space TelescopeCounterparts toChandraX‐Ray Sources in the Globular Cluster 47 Tucanae. II. Time Series and Analysis

Abstract

We report time series and variability information for the optical identifications of X-ray sources in 47 Tucanae reported in Paper I (at least 22 cataclysmic variables [CVs] and 29 active binaries). The radial distribution of the CVs is indistinguishable from that of the millisecond pulsars (MSPs) detected by Freire et al. A study of the eight CVs with secure orbital periods (two obtained from the Chandra study of Grindlay et al.) shows that the 47 Tuc CVs have fainter accretion disks, in the V band, than field CVs with similar periods. These faint disks and the faint absolute magnitudes (M V ) of the 47 Tuc CVs suggests they have low accretion rates. One possible explanation is that the 47 Tuc objects may be a more representative sample of CVs, down to our detection threshold, than the CVs found in the field (where many low accretion rate systems are believed to be undiscovered), showing the advantages of deep globular cluster observations. The median F X =F opt value for the 47 Tuc CVs is higher than that of all known classes of field CV, partly because of the faint M V values and partly because of the relatively high X-ray luminosities (L X ). The latter are only seen in DQ Her systems in the field, but the 47 Tuc CVs are much fainter optically than most field DQ Her's. Previous work by Edmonds et al. has shown that the four brightest CVs in NGC 6397 have optical spectra and broadband colors that are consistent with DQ Her's having lower than average accretion rates. Some combination of magnetic behavior and low accretion rates may be able to explain our observations, but the results at present are ambiguous, since no class of field CV has distributions of both L X and M V that are consistent with those of the 47 Tuc CVs.The radial distribution of the X-ray detected active binaries is indistinguishable from that of the much larger sample of optical variables (eclipsing and contact binaries and BY Dra variables) detected in previous Wide Field Planetary Camera 2 (WFPC2) studies by Albrow et al. The X-ray properties of these objects (luminosity, hardness ratios, and variability) are consistent with those of active binaries found in field studies, and the F X =F opt distribution is significantly different from those of the CVs and the MSPs that are detected (or possibly detected) in the optical. Despite these results, we examine the possibility that a few of the active binaries are MSPs with main-sequence companions resulting from double exchanges in the crowded core of 47 Tuc. No solid evidence is found for a significant population of such objects, and therefore, using the methods of Grindlay et al., we estimate that the number of MSPs in 47 Tuc with luminosities above 10 30 ergs s À1 is $30-40, near the previous lower limit. We present the results of a new, deeper search for faint low-mass X-ray binaries (LMXBs) in quiescence. One reasonable and one marginal candidate for optical identification of a quiescent LMXB was found (one is already known). Finally, it is shown that the periods of the blue va...

A strong upper limit on the pulsed radio luminosity of the compact object 1RXS J141256.0+792204

“…Heinke et al (2005) used a deep Chandra observation of the cluster to perform a population analysis which concluded that there are likely ∼25 radio MSPs (<60 at 95% confidence) residing in the cluster, independent of beaming. Comparing this with optical and radio studies of the MSP population by Edmonds et al (2003) and McConnell et al (2004) respectively, who both conclude that the total MSP population in 47 Tuc is ∼30, Heinke et al (2005) conclude that the beaming fraction is > ∼ 37%. While these various studies suggest that the beaming fraction of MSPs is significantly larger than for normal pulsars, and could be quite high, we cannot strongly rule out beaming as the cause of our non-detection of 1RXS J141256.0+792204.…”

New cataclysmic variables and other exotic binaries in the globular cluster 47 Tucanae*

“…Examples in 47 Tuc are the cases of the X-ray sources W42 (an accreting black hole candidate, Miller-Jones et al 2015) and W82 (a millisecond pulsar -MSP-, Bogdanov et al 2006) which were initially classified as CVs by Paresce et al (1992) and Edmonds et al (2003a, hereafter E03a), respectively. Although, we note that Edmonds et al 2003b (from now on E03b) also suggested that W82 could be an MSP. To mitigate the risk of misclassification, the combination of photometric information at different wavelengths is important.…”

Chandra studies of the globular cluster 47 Tucanae: a deeper X-ray source catalogue, five new X-ray counterparts to millisecond radio pulsars and new constraints to r-mode instability window

“…We identified W96 as its X-ray counterpart. The possibility of W96 being an MSP was previously suggested by Edmonds et al (2003b), based on the lack of an optical Note-The BBODYRAD and NSATMOS models used CFLUX and the FLUX command respectively, to determine the unabsorbed luminosity. MSPs ab & S were fit with the χ 2 statistic (and the null hypothesis probability is quoted rather than the C-stat "goodness" statistic), the rest were fit with the C-statistic.…”