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, superposed on a diffuse emission region with a dimension of ~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 of ~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 yr. The lack of radio polarization of the remnant may be due to Faraday dispersion by foreground H II gas.