Long-term X-ray monitoring of the TeV binary LS i +61 303 with the Rossi X-ray Timing Explorer

We report on the results of a long-term X-ray monitoring campaign of the galactic binary LS I +61 303 performed by the Rossi X-ray Timing Explorer. This data set consists of 1 ks pointings taken every other day between 2007 August 28 and 2008 February 2. The observations covered six full cycles of the 26.496 day binary period and constitute the largest continuous X-ray monitoring data set on LS I +61 303 to date with this sensitivity. There is no statistically strong detection of modulation of the flux or the photon index with orbital phase; however, we do find a strong correlation between the flux and photon index, with the spectrum becoming harder at higher fluxes. The data set contains three large flaring episodes, the largest of these reaching a flux level of 7.2 ^+0.1 _-0.2 × 10^-11 erg cm^-2 s^-1 in the 3-10 keV band, which is a factor 3 times larger than the flux levels typically seen in the system. Analysis of these flares shows the X-ray emission from LS I +61 303 changing by up to a factor of 6 over timescales of several hundred seconds as well as doubling times as fast as 2 s. This is the fastest variability ever observed from LS I +61 303 at this wavelength and places constraints on the size of the X-ray emitting region.