ROSAT OBSERVATIONS OF THE LMC PULSAR PSR 0540-69

The LMC pulsar PSR 0540-69, a young, rapidly rotating neutron star of dynamic age approximately 1000 yr, was observed with the Position Sens itive Proportional Counter (PSPC) onboard ROSAT for approximately 29,0 00 s in 1990 June, 1990 July, and 1991 February. The timing data are c onsistent with contemporaneous optical ephemerides suggesting that the optical and X-ray pulses are in phase. The characteristic broad sinus oidal pulse profile of PSR 0540-69 is observed at a pulsed fraction of approximately 15% measured against the pulsar plus the unresolved neb ula. The spectral distribution of the pulsar + nebula counts is well d escribed by a simple power law with a photon index of alpha = (2.4-1.8 5) and a neutral hydrogen column density of N(H) = (3.6-4.6) x 10(21) cm-2 at the 90% confidence level. The implied model luminosity in the 0.1-2.4 keV.band is 1.6 x 10(37) ergs s-1 which is approximately 11 % of the available rotational energy loss rate and similar to the value observed in the Crab nebula + pulsar. A positive correlation between t he X-ray hardness and the pulse phase intensity implies an underlying spectrum for the pulsar which is harder than the nebula. A phase-resol ved spectral fit to the pulsed component supports a harder spectrum fo r the pulsar and gives a power-law photon index of alpha(pulsed) = 1,3 , The derived pulsed luminosity of 1.5 x 10(36) ergs s-1 is approximat ely 9% of the total X-ray luminosity.