ROSAT AND ASCA OBSERVATIONS OF THE CRAB-LIKE SUPERNOVA REMNANT N157B IN THE LARGE-MAGELLANIC-CLOUD

We report the results of ROSAT and ASCA X-ray observations of the supe rnova remnant N157B (or 30 Dor B, SNR 0539-69.1) in the Large Magellan ic Cloud. For comparison, we also briefly describe the results on SNR 0540-69.3, the only confirmed Crab-like remnant in the Cloud. The X-ra y emission from N157B can be decomposed into a bright comet-shaped fea ture, superposed on a diffuse emission region with a dimension of simi lar to 20 pc. The flat and nearly featureless spectrum of the remnant is distinctly different from those of young shell-like remnants, sugge sting a predominantly Crab-like nature of N157B. Characterized by a po wer law with an energy slope of similar to 1.5, the spectrum of N157B above similar to 2 keV is, however, considerably steeper than that of SNR 0540-69.3, which has a slope of similar to 1.0. At lower energies, the spectrum of N157B presents marginal evidence for emission lines, which if real most likely arise in hot gas of the diffuse emission reg ion. The hot gas has a characteristic thermal temperature of 0.4-0.7 k eV. No significant periodic signal is detected from N157B in the perio d range of 3 x 10(-3)-2000 s. The pulsed fraction is less than or simi lar to 9% (99% confidence) in the 2-7 keV range. We discuss the nature of the individual X-ray components. In particular, we suggest that th e synchrotron radiation of relativistic particles from a fast-moving ( similar to 10(3) km s(-1)) pulsar explains the size, morphology, spect rum, and energetics of the comet-shaped X-ray feature. We infer the ag e of the remnant as similar to 5 x 10(3) yr. The lack of radio polariz ation of the remnant may be due to Faraday dispersion by foreground H II gas.