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

¹ Università degli Studi di Pavia, Dipartimento di Fisica Nucleare e Teorica and INFN-Pavia, via Bassi 6, 27100 Pavia, Italy e-mail: paoloesp@iasf-milano.inaf.it
² INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, via Bassini 15, 20133 Milano, Italy
³ NASA/Goddard Space Flight Center, Greenbelt, Maryland 20771, USA

Received: 23 July 2007
Accepted: 16 August 2007

Abstract

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.Twenty-five snapshot observations of LS I +61° 303 were 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 cm^-2 and photon index . No accretion disk signatures, such as an iron line, are found in the spectrum.