Swift/XRT monitoring of five orbital cycles of LSI +61 303

Abstract

LSI +61 303 is one of the most interesting high-mass X-ray binaries owing to its spatially resolved radio emission and its TeV emission, generally attributed to non-thermal particles in an accretion-powered relativistic jet or in the termination shock of the relativistic wind of a young pulsar. Also, the nature of the compact object is still debated. Only LS 5039 and PSR B1259-63 (which hosts a non-accreting millisecond pulsar) have similar characteristics. We study the X-ray emission from LSI +61 303 covering both short-term and orbital variability. We also investigate the source spectral properties in the soft X-ray (0.3-10 keV) energy range. 25 snapshot observations of LSI +61 303 have been collected in 2006 with the XRT instrument on-board the Swift satellite over a period of four months, corresponding to about five orbital cycles. Since individual data sets have too few counts for a meaningful spectral analysis, we extracted a cumulative spectrum. The count rate folded at the orbital phase shows a clear modulation pattern at the 26.5 days period and suggests that the X-ray peak occurs around phase 0.65. Moreover, the X-ray emission appears to be variable on a timescale of ~1 ks. The cumulative spectrum is well described by an absorbed power-law model, with hydrogen column density Nh=(5.7+/-0.3)E+21 cm^-2 and photon index 1.78+/-0.05. No accretion disk signatures, such as an iron line, are found in the spectrum.