2 Observations and data

2.1 RXTE PCA

The PCA (Jahoda 1994) consists of five Proportional Counter Units (PCU) sensitive in the $\sim$2-60 keV range with an energy resolution less than 18% at 6 keV and a large field of view of 1 $\hbox{$^\circ$ }$ FWHM. Each PCU has three xenon layers which provide the basic scientific data. When running in Good Xenon configuration each photon captured in a xenon layer is registered with a timing resolution of 1 $\mu$s. The individual triggers do not contain any spatial information.

The PCA observed the LMC many times with PSR B0540-69 in its large field of view. We analysed archival data from observations performed in the Good Xenon configuration with offset angles less than 30 $\hbox {$^\prime $ }$ and durations longer than $\sim$5 ks (see Table 1), resulting in a total dataset containing 684 ks of exposure, after screening for Earth occultations, South Atlantic Anomaly (SAA) passages and enhanced background due to high-energy particles. We used this exceptionally long exposure of PSR B0540-69 for our timing analysis. To boost the signal-to-noise ratio for this weak source we selected the photons registered in the first xenon layer of the PCUs.

 

 

Table 1: List of all RXTE observations used in this study with PSR B0540-69 within 30 $\hbox {$^\prime $ }$ of the pointing direction. The exposures mentioned are effective values screened for Earth occultations, SAA passages and contaminating particles.

Obs. ID	Target	Offset	Exposure
		( $\hbox {$^\prime $ }$)	(ks)
10206	PSR B0540-69	0.04	39.1
10218	SN1987A	25.30	74.1
10250	LMC X-1	24.80	33.4
20188	LMC X-1	24.80	234.8
30087	LMC X-1	24.77	73.5
40139	PSR J0537-69	15.96	109.0
50103	PSR J0537-69	15.94	120.0

2.2 RXTE HEXTE

The HEXTE instrument (Rothschild et al. 1998) aboard RXTE consists of two independent detector clusters (A and B) each containing 4 Na(Tl)/CsI(Na) phoswich scintillation detectors passively collimated to a 1 $\hbox{$^\circ$ }$ FWHM field of view and co-aligned with the PCA. The instrument is sensitive to photons with energies in the energy range 15-250 keV with an energy resolution of about 15.4% at 60 keV. The net open area of the 8 detectors is 1600 cm². The events can be tagged with a maximum time resolution of $7.6~\mu$s. In its default operation mode the field of view of each cluster is switched on and off source to provide instantaneous background measurements reducing the effective source-on exposure by roughly a half. Due to high-energy particle events the overall performance of the instrument is degraded resulting in a dead-time fraction of about 40%. When the source of interest is observed off-axis the response is further reduced by the collimator/detector assembly. For a source observed  $30\hbox{$^\prime$ }$ off-axis the sensitive area is reduced to about 0.5 of its on-axis value.

In the spectral analysis of HEXTE data all these effects which reduce the effective sensitive area are taken into account. Due to the co-alignment of HEXTE with the PCA the same observations have been used as for the PCA. For the spectral analysis HEXTE data from all observations listed in Table 1 have been used, not limited to those observations falling within the stable gain period used in the PCA spectral analysis.

The total dead-time and off-axis corrected Cluster-A and B on-source (=PSR B0540-69) exposures are 248.4 ks and 260.7 ks, respectively.

   
2.3 ROSAT PSPC

To extend the spectral coverage to the soft X-ray regime we also included ROSAT PSPC data in our study. The PSPC is a gas-filled imaging proportional counter which operates in photon counting mode (Pfeffermann & Briel 1986). It is sensitive over the 0.01-2.5 keV energy band. The energy resolution is $\Delta E=0.45 E^{0.5}$. The events are tagged on board with a precision of $130~\mu$s, but the erratic spacecraft clock makes absolute timing difficult.

We could identify in the ROSAT archive 3 PSPC observation sequences with PSR B0540-69 nearly on-axis. The observation identifiers are RP400052N00 (PSPC-B), RP150044N00 (PSPC-C) and RP400133N00 (PSPC-B), with effective exposure times of 8.5 ks, 5.1 ks and 1.7 ks, respectively.