Deep Radio Imaging of Globular Clusters and the Cluster Pulsar Population

Abstract

We have obtained deep multifrequency radio observations of seven globular clusters using the Very Large Array and the Australia Telescope Compact Array. Five of these, NGC 6440, NGC 6539, NGC 6544, NGC 6624, and Terzan 5, had previously been detected by Fruchter & Goss in a shallower survey for steep-spectrum radio sources in globular clusters. The sixth, the rich globular cluster Liller 1, had heretofore been undetected in the radio, and the seventh, 47 Tucanae, was not included in our original survey. High-resolution 6 and 20 cm images of three of the clusters, NGC 6440, NGC 6539, and NGC 6624, reveal only point sources coincident with pulsars which have been discovered subsequent to our first imaging survey; 21 and 18 cm images reveal several point sources within a few core radii of the center of 47 Tuc. Two of these are identified pulsars, and a third, which is variable and has a steep spectrum, is also most likely a pulsar previously identified by a pulsed survey. However, the 6, 20, and 90 cm images of NGC 6544, Liller 1, and Terzan 5 display strong steep-spectrum emission which cannot be associated with known pulsars. The 90 cm flux densities of Terzan 5, Liller 1, and NGC 6544 are 35, 9, and 6 mJy, respectively, implying that a number of bright pulsars in these clusters have been hidden from pulsed searches. The image of the rich cluster Terzan 5 displays numerous point sources within 30", or 4 core radii, of the cluster center. The density of these objects rises rapidly toward the core, where an elongated region of emission is found. This feature may be due to the presence of a higher density of point sources than can be resolved by the 1" resolution of our data. The brightest individual sources, as well as the extended emission, possess the steep spectra expected of pulsars. Furthermore, the flux distribution of the sources agrees well with the standard pulsar luminosity function. The total luminosity and number of objects observed suggest that Terzan 5 contains more pulsars than any other Galactic globular cluster.