Swift / XRT monitoring of five orbital cycles of LS I + 61 ◦ 303

Context. LS I+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. Aims. We study the X-ray emission from LS I+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. Methods. 25 snapshot observations of LS I+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. Results. 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)× 1021 cm−2 and photon index Γ = 1.78 ± 0.05. No accretion disk signatures, such as an iron line, are found in the spectrum.