Discovery of an Ultrafast X-Ray Pulsar in the Supernova Remnant N157B

We present the serendipitous discovery of 16 ms pulsed X-ray emission fromthe Crab-like supernova remnant N157B in the Large Magellanic Cloud. This isthe fastest spinning pulsar associated with a supernova remnant (SNR).Observations with the Rossi X-ray Timing Explorer (RXTE), centered on the fieldcontaining SN1987A, reveal an X-ray pulsar with a narrow pulse profile.Archival ASCA X-ray data confirm this detection and locate the pulsar within 1'of the supernova remnant N157B, 14' from SN1987A. The pulsar manifests evidencefor glitch(es) between the RXTE and ASCA observations which span 3.5 years; themean linear spin-down rate is Pdot = 5.126 X 10E-14 s/s. The backgroundsubtracted pulsed emission is similar to other Crab-like pulsars with a powerlaw of photon index of ~ 1.6. The characteristic spin-down age (~ 5000 years)is consistent with the previous age estimate of the SNR. The inferred B-fieldfor a rotationally powered pulsar is ~ 1 X 10E12 Gauss. Our result confirms theCrab-like nature of N157B; the pulsar is likely associated with a compact X-raysource revealed by ROSAT HRI observations.Comment: 9 pages with 3 eps figure, LaTex (aas2pp2, psfig). Submitted to the Astrophysical Journal Letter