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 theLarge Magellanic Cloud, which are presented together with an archival HSToptical image and a radio continuum map for comparison. This remnant containsthe recently discovered 16 ms X-ray pulsar PSR J0537-6910, the most rapidlyrotating young pulsar known. Using phase-resolved Chandra imaging, we pinpoint the location of the pulsarto within an uncertainty of less than 1 arcsec. PSR J0537-6910 is not detectedin any other wavelength band. The X-ray observations resolve three distinctfeatures: the pulsar itself, a surrounding compact wind nebula which isstrongly elongated and a feature of large-scale diffuse emission trailing fromthe pulsar. This latter comet tail-shaped feature coexists with enhanced radioemission and is oriented nearly perpendicular to the major axis of the pulsarwind nebula. We propose the following scenario to explain these features. Thebright, compact nebula is likely powered by a toroidal pulsar wind ofrelativistic particles which is partially confined by the ram-pressure from thesupersonic motion of the pulsar. The particles, after being forced out from thecompact nebula (the head of the ``comet''), are eventually dumped into a bubble(the tail), which is primarily responsible for the extended diffuse X-ray andradio emission. The ram-pressure confinement also allows a natural explanationfor the observed X-ray luminosity of the compact nebula and for the unusuallysmall X-ray to spin-down luminosity ratio, compared to similarly energeticpulsars. We estimate the pulsar wind Lorentz factor of N157B as about 4 times10^6 (with an uncertainty of a factor about 2, consistent with that inferredfrom the modeling of the Crab Nebula.