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 supernova remnant N157B (or 30 Dor B, SNR 0539-69.1) in the Large Magellanic 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-ray emission from N157B can be decomposed into a bright comet-shaped feature, superimposed on a diffuse emission region of a dimension ∼ 20 pc. The flat and nearly featureless spectrum of the remnant is distinctly different from those of young shell-like remnants, suggesting a predominantly Crab-like nature of N157B. Characterized by a power law with an energy slope ∼ 1.5, the spectrum of N157B above ∼ 2 keV is, however, considerably steeper than that of SNR 0540-69.3, which has a slope of ∼ 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 region. The hot gas has a characteristic thermal temperature of 0.4-0.7 keV. No significant periodic signal is detected from N157B in the period range of 3 × 10−3 − 2000 s. The pulsed fraction is ∼< 9% (99% confidence) in the 2− 7 keV range. We discuss the nature of the individual X-ray components. In particular, we suggest that the synchrotron radiation of relativistic particles from a fast-moving (∼ 103 km s−1) pulsar explains the size, morphology, spectrum, and energetics of the comet-shaped X-ray feature. We infer the age of the remnant as ∼ 5 × 103 yrs. The lack of radio polarization of the remnant may be due to Faraday dispersion by foreground H II gas. Subject headings: galaxies: individual (Large Magellanic Cloud) — supernova remnants — X-rays: ISM