Detection of an X-ray pulsar wind nebula and tail in SNR N157B

We report Chandra X-ray observations of the supernova remnant N157B in the Large Magellanic Cloud, which are presented together with an archival Hubbl e Space Telescope optical image and a radio continuum map for comparison. T his remnant contains the recently discovered 16 ms X-ray pulsar PSR J0537-6 910, the most rapidly rotating young pulsar known. Using phase-resolved Cha ndra imaging, we pinpoint the location of the pulsar at 5(h)37(m)47(s).36, -69 degrees 10'20."4 (J2000), with an uncertainty of PSR J0537-6910 is not detected in any other wavelength band. The X-ray observations resolve three distinct features: the pulsar itself, a surrounding compact wind nebula th at is strongly elongated (dimensions of similar to0.6 x 1.7 pc), and a feat ure of large-scale (greater than or similar to5 pc long) diffuse emission t railing from the pulsar. This latter comet-tail-shaped feature coexists wit h enhanced radio emission and is oriented nearly perpendicular to the major axis of the pulsar wind nebula. We propose the scenario below to explain t hese features. The bright, compact nebula is likely to be powered by a toro idal pulsar wind of relativistic particles that is partially confined by th e ram pressure from the supersonic motion of the pulsar. The particles, aft er being forced out from the compact nebula (the head of the "comet"), are eventually dumped into a bubble (the tail), which is primarily responsible for the extended diffuse X-ray and radio emission. The ram-pressure confine ment also allows a natural explanation for the observed X-ray luminosity of the compact nebula and for the unusually small X-ray to spin-down luminosi ty ratio of similar to0.2%, compared to similarly energetic pulsars. We est imate the pulsar wind Lorentz factor of N157B to be similar to4 x 10(6) (wi th an uncertainty of a factor of similar to2), consistent with that inferre d from the modeling of the Crab Nebula.