XMM-Newton observation of a spectral state transition in the peculiar radio/X-ray/γ-ray source LS I+61 303

We report the results of XMM-Newton and BeppoSAX observations of the radio and X-ray emitting star LS I + 61 303, likely associated with the gamma-ray source 2CG 135+01 and recently detected also at TeV energies. The data include a long XMM-Newton pointing carried out in January 2005, which provides the deepest look ever obtained for this object in the 0.3-12 keV range. During this observation the source flux decreased from a high level of similar to 13 x 10(-12) erg cm(-2) s(-1) to 4 x 10(-12) erg cm(-2) s(-1) within 2-3 h. This flux range is the same seen in shorter and less sensitive observations carried out in the past, but the new data show for the first time that transitions between the two levels can occur on short time scales. The flux decrease was accompanied by a significant softening of the spectrum, which is well described by a power law with photon index changing from 1.62 +/- 0.01 to 1.83 +/- 0.01. A correlation between hardness and intensity is also found when comparing different short observations spanning almost 10 years and covering various orbital phases. LS I + 61 303 was detected in the 15-70 keV range with the PDS instrument in one of the BeppoSAX observations, providing evidence for variability also in the hard X-ray range. The X-ray spectra, discussed in the context of multiwavelength observations, place some interesting constraints on the properties and location of the high-energy emitting region.