X-rays from the eclipsing millisecond pulsar PSR J1740-5340 in the globular cluster NGC 6397

Abstract

<i>Aims. <i/>The millisecond pulsar PSR J1740-5340 in the globular cluster NGC 6397 shows radio eclipses over ~40% of its binary orbit. A first Chandra observation revealed indications for the X-ray flux being orbit dependent as well. In this work, we analysed five datasets of archival Chandra data taken between 2000 and 2007 to investigate the emission across the pulsar's binary orbit.<i>Methods. <i/>Utilizing archival Chandra observations of PSR J1740-5340, we performed a systematic timing and spectral analysis of this binary system.<i>Results. <i/>Using a -test, the significance for intra-binary orbital modulation was found to be between 88.5% and 99.6%, depending on the number of phase bins used to construct the light curve. Applying the unbiased statistical Kolmogorov-Smirnov (KS) test did not indicate any significant intra-binary orbital modulation. However, comparing the counting rates observed at different epochs, a flux variability on times scales of days to years is indicated. The possible origin of the X-ray emission is discussed in a number of different scenarios.