Orbital and superorbital variability of LS I +61 303 at low radio frequencies with GMRT and LOFAR

Abstract : LS I +61 303 is a gamma-ray binary that exhibits an outburst at GHz frequencies each orbitalcycle of 26:5 d and a superorbital modulation with a period of 4.6 yr. We have performeda detailed study of the low-frequency radio emission of LS I +61 303 by analysing all thearchival GMRT data at 150, 235 and 610 MHz, and conducting regular LOFAR observationswithin the Radio Sky Monitor (RSM) at 150 MHz. We have detected the source for the firsttime at 150 MHz, which is also the first detection of a gamma-ray binary at such a low frequency.We have obtained the light-curves of the source at 150, 235 and 610 MHz, all of themshowing orbital modulation. The light-curves at 235 and 610 MHz also show the existenceof superorbital variability. A comparison with contemporaneous 15-GHz data shows remarkabledierences with these light-curves. At 15 GHz we see clear outbursts, whereas at lowfrequencies we see variability with wide maxima. The light-curve at 235 MHz seems to beanticorrelated with the one at 610 MHz, implying a shift of 0.5 orbital phases in the maxima.We model the shifts between the maxima at dierent frequencies as due to changes in thephysical parameters of the emitting region assuming either free-free absorption or synchrotronself-absorption, obtaining expansion velocities for this region close to the stellar wind velocitywith both mechanisms.