A&A 474, 575-578 (2007)
DOI: 10.1051/0004-6361:20078334

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

P. Esposito^{1, 2}, P. A. Caraveo², A. Pellizzoni², A. De Luca², N. Gehrels³, and M. A. Marelli<sup>2

1</sup>  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.

Key words: X-rays: individual: LS I +61° 303 -- X-rays: binaries

© ESO 2007

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.

• If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
• You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
• You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.