X-rays from the Eclipsing Millisecond Pulsar PSR J1740-5340 in the Globular Cluster NGC 6397

TL;DR

This study analyzes multiple Chandra X-ray observations of the eclipsing millisecond pulsar PSR J1740-5340 in NGC 6397, revealing potential orbital flux modulation and variability over days to years, contributing to understanding pulsar emission mechanisms.

Contribution

It provides a systematic timing and spectral analysis of archival X-ray data, exploring orbital dependence and flux variability of the pulsar for the first time.

Findings

Orbital modulation significance ranges from 88.5% to 99.6%.

No significant intra-binary orbital modulation detected by KS test.

Flux variability observed over days to years.

Abstract

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 data sets of archival Chandra data taken between 2000 and 2007 in order to investigate the emission across the pulsar's binary orbit. Utilizing archival Chandra observations of PSR J1740-5340, we have performed a systematic timing and spectral analysis of this binary system. Using a chi-square-test the significance for intra-binary orbital modulation is 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, though. 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.