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 Hubble Space Telescope optical image and a radio continuum map for comparison. This remnant contains the recently discovered 16 ms X-ray pulsar PSR J0537-6910, the most rapidly rotating young pulsar known. Using phase-resolved Chandra imaging, we pinpoint the location of the pulsar at 5h37m47.ˢ36, -69°10'20.″4 (J2000), with an uncertainty of ≲1''. 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 that is strongly elongated (dimensions of ~0.6 × 1.7 pc), and a feature of large-scale (≳5 pc long) diffuse emission trailing from the pulsar. This latter comet-tail-shaped feature coexists with enhanced radio emission and is oriented nearly perpendicular to the major axis of the pulsar wind nebula. We propose the scenario below to explain these features. The bright, compact nebula is likely to be powered by a toroidal pulsar wind of relativistic particles that is partially confined by the ram pressure from the supersonic motion of the pulsar. The particles, after 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 confinement 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 luminosity ratio of ~0.2%, compared to similarly energetic pulsars. We estimate the pulsar wind Lorentz factor of N157B to be ~4 × 106 (with an uncertainty of a factor of ~2), consistent with that inferred from the modeling of the Crab Nebula.