Deep radio imaging of globular clusters and the cluster pulsar population

We have obtained deep multifrequency radio observations of seven globular c lusters using the Very Large Array and the Australia Telescope Compact Arra y. 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 stee p-spectrum radio sources in globular clusters. The sixth, the rich globular cluster Liller 1, had heretofore been undetected in the radio, and the sev enth, 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 66 24, reveal only point sources coincident with pulsars which have been disco vered subsequent to our first imaging survey; 21 and 18 cm images reveal se veral 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 ste ep 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 Te rzan 5 display strong steep-spectrum emission which cannot be associated wi th known pulsars. The 90 cm flux densities of Terzan 5, Liller 1, and NGC 6 544 are 35, 9, and 6 mJy, respectively, implying that a number of bright pu lsars in these clusters have been hidden from pulsed searches. The image of the rich cluster Terzan 5 displays numerous point sources with in 30 ", or 4 core radii, of the cluster center. The density of these objec ts 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 bright est individual sources, as well as the extended emission, possess the steep spectra expected of pulsars. Furthermore, the flux distribution of the sou rces agrees well with the standard pulsar luminosity function. The total lu minosity and number of objects observed suggest that Terzan 5 contains more pulsars than any other Galactic globular cluster.