1 Introduction

PSR B0540-69 is a young Crab-like pulsar located in the Large Magellanic Cloud (LMC) and was discovered at X-rays by Seward et al. (1984) who used the Einstein X-ray Observatory. Like the Crab it is embedded in a bright synchrotron wind nebula which was confirmed in the optical waveband (Chanan et al. 1984) shortly after the discovery of the pulsar. The rotational characteristics of PSR B0540-69 resemble those of the Crab pulsar as well, therefore their spin-down ages are of the same order of magnitude, $\sim$10³ yr.

The pulse profile of PSR B0540-69, however, is considerably different from that of the Crab. The Crab profile shows two sharp peaks from optical to high-energy gamma-rays (for the most recent high-energy picture from soft X-rays up to high-energy gamma-rays see Kuiper et al. 2001), while the pulse profile of PSR B0540-69 consists mainly of a broad asymmetric pulse with indications for fine structure (Middleditch & Pennypacker 1985; Boyd et al. 1995; Seward et al. 1984). Most recently, Mineo et al. (1999) described the pulse shape as nearly sinusoidal with a minor structure on the left side of the maximum on the basis of higher-statistic BeppoSAX data. Hirayama et al. (2002) analysed all available ASCA data on this source. They confirmed the broad profile and discussed a hump on the leading wing of the profile as a possible interpulse, analogous to the case of the Crab.

Measurements of the total pulsed spectrum of PSR B0540-69 between 0.2 and 10 keV by the Chandra X-ray Observatory suggest a hard spectrum with a photon spectral index of  $1.83\pm 0.13$ (Kaaret et al. 2001), consistent with the BeppoSAX (Mineo et al. 1999) and ASCA (Hirayama et al. 2002) spectral findings. At soft X-rays (ROSAT 0.1-2.4 keV) Finley et al. (1993) reported an index $1.3 \pm 0.5$ (90% errors), statistically consistent with the indices measured above 2 keV. If the pulsed spectrum would extend from medium energy X-rays with the same power-law index into the hard X-ray or soft gamma-ray regime, then pulsed emission should have been detected by the BATSE (20-600 keV) and OSSE (50 keV-10 MeV) instruments aboard the Compton Gamma Ray Observatory (CGRO). However, no detection of pulsed soft gamma-ray emission from PSR B0540-69 has been reported sofar (Hertz et al. 1995; Wilson et al. 1993).

The Rossi X-ray Timing Explorer (RXTE) archive contains a huge amount of hard X-ray data with PSR B0540-69 in the field of view. A part of these data had been used by Zhang et al. (2001) to provide an accurate measurement of the spin frequency of the pulsar and its first and second derivatives by phase-linking on a limited time interval. In this paper we present the hard X-ray characteristics of PSR B0540-69 using all these available RXTE Proportional Counter Array (PCA, 2-60 keV) and RXTE High Energy X-ray Timing Experiment (HEXTE, 15-250 keV) data. In order to augment the spectral coverage towards lower energies we also used soft X-ray data ($\sim$0.01-2.5 keV) from the ROSAT PSPC in a consistent analysis.