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 orbital cycle of 26.5 d and a superorbital modulation with a period of 4.6 yr. We have performed a detailed study of the low-frequency radio emission of LS I +61 303 by analyzing all the archival GMRT data at 150, 235 and 610 MHz and conducting regular LOFAR observations within the Radio Sky Monitor at 150 MHz. We have detected the source for the first time at 150 MHz, which is also the first detection of a gamma-ray binary at such low frequency. We have obtained the light-curves of the source at 150, 235 and 610 MHz, showing orbital and superorbital modulation. A comparison with contemporaneous RT and OVRO 15-GHz observations shows remarkable differences. At 15 GHz we see clear outbursts, whereas at low frequencies we observe variability with wider maxima. The light-curve at 235 MHz seems to be uncorrelated with the one at 610 MHz. We model the shifts between the maxima at different frequencies as due to changes in the physical parameters of the emitting region assuming either free-free absorption or synchrotron self-absorption, obtaining subrelativistic expansion velocities which are close to the stellar wind velocity for some of the cases.