Orbital X‐Ray Variability of the Microquasar LS 5039

The properties of the orbit and the donor star in the high mass X-ray binarymicroquasar LS 5039 indicate that accretion processes should mainly occur via aradiatively driven wind. In such a scenario, significant X-ray variabilitywould be expected due to the eccentricity of the orbit. The source has beenobserved at X-rays by several missions, although with a poor coverage thatprevents to reach any conclusion about orbital variability. Therefore, weconducted RossiXTE observations of the microquasar system LS 5039 covering afull orbital period of 4 days. Individual observations are well fitted with anabsorbed power-law plus a Gaussian at 6.7 keV, to account for iron lineemission that is probably a diffuse background feature. In addition, we havetaken into account that the continuum is also affected by significant diffusebackground contamination. Our results show moderate power-law flux variationson timescales of days, as well as the presence of miniflares on shortertimescales. The new orbital ephemeris of the system recently obtained byCasares et al. have allowed us to show, for the first time, that an increase ofemission is seen close to the periastron passage, as expected in an accretionscenario. Moreover, the detected orbital variability is a factor of ~4 smallerthan the one expected by using a simple wind accretion model, and we suggestthat an accretion disk around the compact object could be responsible for thisdiscrepancy. On the other hand, significant changes in the photon index arealso observed clearly anti-correlated with the flux variations. We interpretthe overall X-ray spectral characteristics of LS 5039 in the context of X-rayradiation produced by inverse Compton and/or synchrotron processes in the jetof this microquasar.Comment: published in Astrophysical Journal, submission format (real number of pages: 7, 4 figures