ASCA observations of the Crab-like pulsar/nebula system PSR B0540-69

The ASCA satellite has observed the 50-ms pulsar PSR B0540-69 and its associated supernova remnant SNR 0540-693 in the Large Magellanic Cloud 15 times in total, at least once a year, between 1993 June and 1999 November, including six observations with the pulsar as the main target. In all the observations, a point-like X-ray source was clearly detected at the position of PSR B0540-69 and the X-ray pulsations were detected at large significance. Folded light curves are identical throughout the observations, showing one broad pulse with a small notch at the top. Pulse frequency history during the 6.4 yr of the ASCA observations is well characterized by a second-order polynomial with a breaking index of 2.10+/-0.05, assuming no glitches occurred during the period. The frequency history smoothly follows the Ginga and ROSAT results and connects to the RXTE results without a measurable discontinuity. The X-ray spectrum of the whole source is well represented by a single power-law model of photon index, alpha =2.00+/-0.02, and a small photoelectric absorption, N (H) =(4.3+/-0.2)x10(21) cm(-2) . The X-ray spectrum can also be represented by a pulsed power-law spectrum with photon index of 1.80+/-0.05 with 24 per cent in amplitude at 1 keV, embedded in a constant power-law component with photon index of 2.17+/-0.02 ascribed to a nebular component. Spectral properties of the X-ray emission from the pulsar and the nebula remain unchanged over the six years and are quite similar to those of the Crab pulsar and nebula.