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 \sim 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 \sim 1.5 , the spectrum of N157B above \sim 2 keV is , however , considerably steeper than that of SNR 0540-69.3 , which has a slope of \sim 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 \times 10 ^ { -3 } -2000 s. The pulsed fraction is \lesssim 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 ( \sim 10 ^ { 3 } { ~ { } 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 \sim 5 \times 10 ^ { 3 } yrs .
The lack of radio polarization of the remnant may be due to Faraday dispersion by foreground \ion H2 gas .