Discovery of an ultrafast X-ray pulsar in the supernova remnant N157B

We present the serendipitous discovery of 16 ms pulsed X-ray emission from the Crab-like supernova remnant N157B in the Large Magellanic Cloud. This is the fastest spinning pulsar associated with a supernova remnant (SNR). Observations with the Rossi X-Ray Timing Explorer (RXTE), centered on the field containing SN 1987A, reveal an X-ray pulsar with a narrow pulse profile. The archival Advanced Satellite for Cosmology and Astrophysics (ASCA) X-ray data confirm this detection and locate the pulsar within 1' of the supernova remnant N157B, 14' from SN 1987A. The pulsar shows evidence of glitch(es)between the 3.5 yr separating the RXTE and ASCA observations; the mean linear spin-down rate is P = 5.126 x 10(-14) s s(-1). The background-subtracted, pulsed emission is similar to other Crab-like pulsars with a power law of photon index similar to 1.6. The characteristic spin-down age (similar to 5000 yr) is consistent with;he previous age estimate of the SNR. The inferred B-field for a rotationally powered pulsar is similar to 1 x 10(12) G. Our result confirms the Crab-like nature of N157B; the pulsar is likely associated with a compact X-ray source revealed by ROSAT HRI observations.