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

B. Marcote (1) , M. Ribó (1) , J. M. Paredes (1) , C. H. Ishwara-Chandra (2) , J. D. Swinbank (3) , J. W. Broderick (4) , S. Markoff (5) , R. Fender (4) , R. A. M. J. Wijers (5) , G. G. Pooley (6) , A. J. Stewart (4) , M. E. Bell (7) , R. P. Breton (8) , D. Carbone (5) , Stéphane Corbel (9, 10) , J. Eislöffel (11) , H. Falcke (12) , Jean-Mathias Griessmeier (10, 13) , M. Kuniyoshi (14) , M. Pietka (4) , A. Rowlinson (5, 15) , M. Serylak (4) , A. J. van Der Horst (5) , J. van Leeuwen (5, 15) , M. W. Wise (5) , P. Zarka (16, 10)

1 DAM - Departament d'Astronomia i Meteorologia [Barcelona]
2 NCRA - National Centre for Radio Astrophysics [Pune]
3 Department of Astrophysical Sciences [Princeton]
4 Oxford Astrophysics
5 AI PANNEKOEK - Astronomical Institute Anton Pannekoek
6 MRAO - Mullard Radio Astronomy Observatory
7 University of Southampton
8 Jodrell Bank Centre for Astrophysics
9 AIM (UMR7158 / UMR_E_9005 / UM_112) - Astrophysique Interprétation Modélisation
10 USN - Unité Scientifique de la Station de Nançay
11 TLS - Thüringer Landessternwarte Tautenburg
12 Radboud University [Nijmegen]
13 LPC2E - Laboratoire de Physique et Chimie de l'Environnement et de l'Espace
14 MPIFR - Max-Planck-Institut für Radioastronomie
15 ASTRON - Netherlands Institute for Radio Astronomy
16 LESIA - Laboratoire d'études spatiales et d'instrumentation en astrophysique

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.

Keywords: radiation mechanisms: non-thermal – binaries: close – stars: individual: LS I + 61 303 – gamma-rays: stars – radio continuum: stars.

Domains: Cosmology and Extra-Galactic Astrophysics [astro-ph.CO]

B. Marcote, M. Ribó, J. M. Paredes, C. H. Ishwara-Chandra, J. D. Swinbank, et al.. Orbital and superorbital variability of LS I +61 303 at low radio frequencies with GMRT and LOFAR. Monthly Notices of the Royal Astronomical Society, 2016, 456 (2), pp.1791-1802. ⟨10.1093/mnras/stv2771⟩. ⟨insu-01338384⟩