Hard X - ray timing and spectral properties of PSR B 0540 - 69

We report the hard X-ray properties of the young Crab-like LMC pulsar PSR B0540-69, using archival RXTE PCA (2 – 60 keV) and RXTE HEXTE (15 – 250 keV) data. Making use of the very long effective exposure of 684 ks, we derived a very detailed master pulse profile for energies 2 – 20 keV. We confirm the broad single-pulse shape with a dip in the middle and with a significant fine structure to the left of the dip. For the first time pulse profiles in the 10 – 50 keV energy interval are shown. Remarkably, the coarse pulse shape is stable from the optical up to X-ray energies analogous to the case of the Crab pulsar (PSR B0531+21). The profiles can be described with two Gaussians with a phase separation of ∼ 0.2; the distribution of the ratios between the two components from the optical to the X-ray range is consistent with being flat. Therefore we cannot conclude that the profile consists of two distinct components. We also derived a new total pulsed spectrum in the ∼ 0.01 – 50 keV range in a consistent analysis including also archival ROSAT PSPC (0.01 – 2.5 keV) data. This spectrum cannot be described by a single power-law, but requires an additional energy dependent term. The bending of the spectrum around 10 keV resembles that of the Crab pulsar spectrum. Although model calculations using Outer Gap scenarios could probably explain the high-energy characteristics of PSR B0540-69 as they successfully do for the Crab, our measurements do not entirely agree with the latest calculations by Zhang & Cheng (2000). The small discrepancies are likely to be caused by uncertainties in the pulsar's geometry.