We present sensitive images of five globular clusters made with the VLA at 1.4 GHz. In M4, M15, and M28, we detect point sources within the core which can be identified with already known pulsars. In M15 we also detect a point radio source coincident with the 8 hr X-ray binary, AC 211. A weak (180 μJy) source is detected close to the center of M3. So far, no pulsations have been detected toward this cluster, suggesting that the source is either an unrelated background object or a pulsar in a very compact binary system. In M92 we do not detect any source at the level of 100 μJy (4 σ), a limit that is challenging even for the giant Arecibo telescope. In the cores of M4 and M28, we find that additional pulsars, if present, must be fainter by a factor of ≳10 relative to the two known pulsars. Assuming a cluster-weighting function favored by Phinney and Kulkarni, we find our data are consistent with the standard luminosity function, dN/dL ∝ L-2. Our principal conclusion is that there is not a large population of pulsars in compact binaries which have escaped detection in single-dish pulse searches. Such binaries have been postulated to result from tidal captures of single main-sequence stars. We conclude by suggesting that most tidal captures involving neutron stars ultimately result in the formation of a spun-up single pulsar and the complete disruption of the main-sequence star.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science