Deep Chandra observations of TeV binaries - I. LS I +61°303

We report on a 95 ks Chandra observation of the TeV emitting high-mass X-ray binary LS I +61 degrees 303, using the ACIS-S camera in continuous clocking mode to search for a possible X-ray pulsar in this system. The observation was performed while the compact object was passing from phase 0.94 to 0.98 in its orbit around the Be companion star (hence close to the apastron passage). We did not find any periodic or quasi-periodic signal (at this orbital phase) in a frequency range of 0.005-175 Hz. We derived an average pulsed fraction (PF) 3 Sigma upper limit for the presence of a periodic signal of less than or similar to 10 per cent (although this limit is strongly dependent on the frequency and the energy band), the deepest limit ever reached for this object. Furthermore, the source appears highly variable in flux and spectrum even in this very small orbital phase range, in particular we detect two flares, lasting thousands of seconds, with a very hard X-ray spectrum with respect to the average source spectral distribution. The X-ray PF limits we derived are lower than the PF of any isolated rotational-powered pulsar, in particular having a TeV counterpart. In this scenario, most of the X-ray emission of LS I +61 degrees 303 should necessarily come from the interwind or innerpulsar wind zone shock rather than from the magnetosphere of the putative pulsar. On the other hand, very low X-ray PFs are not unseen in accreting neutron star systems, although we cannot at all exclude the black hole nature of the hosted compact object, a pulsar with a beam pointing away from our line of sight or spinning faster than similar to 5.6 ms, nor that pulsations might have a transient appearance in only a small fraction of the orbit. Furthermore, we did not find evidence for the previously suggested extended X-ray emission.