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M. Hirayama (University of California, Santa Cruz), F. Nagase, T. Endo (The Institute of Space and Astronautical Science), N. Kawai (The Institute of Physical and Chemical Research), M. Itoh (Kobe University)
The ASCA satellite observed the 50 ms pulsar PSR B0540-69 and its associated supernova remnant SNR 0540-693 in the Large Magellanic Cloud fifteen times in total, at least once a year, since June 1993 through November 1999, including six observations with the pulsar as a 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 dip on the top.
The X-ray spectrum of the best quality was obtained by the observation on 23 September 1993 during the payload verification phase. 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, NH = (4.3 ±0.2) x 1021 cm-2, resulting in an X-ray luminosity of the source (1--10 keV) of LX = ( 1.16 ±0.03 ) x 1037 erg s-1 with the distance of 55 kpc assumed.
Pulse-phase resolved spectra indicate variation of photon index over pulse phase ranging from 2.0 to 2.2, while a photoelectric absorption remain constant. Pulsed X-ray flux is estimated LX = (7.9 ±0.8) x 10-12 erg s-1 cm-2 (1--10 keV). The total X-ray spectrum can also be represented by a pulsed power-law spectrum with photon index of 1.80 ±0.08 with 20% in amplitude at 1 keV, embedded in constant power-law component with photon index of 2.17+0.10-0.03 ascribed to a nebular component.
Long-term trend of pulse period change was also investigated. Spin periods of the pulsar at the centers of the observations were determined within accuracy of a few \muHz for all the observations. Pulsar's spin-down history during the 6.4-year span of the observations is well characterized by the second order polynomial with a breaking index of 2.09 ±0.05.