Long-term monitoring of LS I+61303 with INTEGRAL

TL;DR

This study presents a comprehensive analysis of LS I+61303's hard X-ray emission using INTEGRAL data, revealing its spectral properties and orbital periodicity over an extended observation period.

Contribution

It provides the first long-term systematic analysis of LS I+61303's hard X-ray properties, doubling previous data and confirming its orbital period in X-rays.

Findings

Detection of the source in 18-60 keV band with high significance.

Hard X-ray spectrum well described by a power law with photon index ~1.7.

Confirmation of a 27-day orbital period in hard X-ray emission.

Abstract

LS I+61303 is one of the few high-mass X-ray binaries that have been recently observed at TeV and GeV energies. Here we investigate the hard X-ray spectral and timing properties of this source using the IBIS/ISGRI instrument on-board the INTEGRAL satellite. We carry out a systematic analysis based on all available INTEGRAL data since December 28, 2002 up to April 30, 2008. The total exposure time analyzed amounts to 2.1 Ms, hence more than doubling the previous reported sample. The source is best detected in the 18-60 keV band, with a significance level of 12.0 sigma. The hard X-ray data are best fit with a simple power law with a photon index of ~ 1.7+-0.2. We detect a periodical signal at 27 +- 4 days, matching the orbital period of 26.496 days previously reported at other wavelengths. The hard X-rays orbital lightcurve is obtained and compared with those derived at other frequencies.