Super-orbital variability of LS I +61 ° 303 at TeV energies

Astronomy and astrophysics 591, A76(2016). doi:10.1051/0004-6361/201527964
The gamma-ray binary LS I +61°303 is a well-established source from centimeter radio up to very high energy (VHE;E > 100 GeV).The broadband emission shows a periodicity of $\sim$ 26.5 days, coincident with the orbital period. A longer (super-orbital) period of 1667 $\pm$ 8 dayswas proposed from radio variability and confirmed using optical and high-energy (HE; E > 100 MeV) gamma-ray observations. In this paper, wereport on a four-year campaign performed by MAGIC together with archival data concentrating on a search for a long-timescale signature in the VHE emission from LS I +61°303 Aims.We focus on the search for super-orbital modulation of the VHE emission, similar to that observed at other energies, and on the search forcorrelations between TeV emission and an optical determination of the extension of the circumstellar disk.Methods. A four-year campaign has been carried out using the MAGIC telescopes. The source was observed during the orbital phases when theperiodic VHE outbursts have occurred ($\phi$ = 0:55–0.75, one orbit=26.496 days). Additionally, we included archival MAGIC observations anddata published by the VERITAS collaboration in these studies. For the correlation studies, LS I +61°303 has also been observed during the orbitalphases where sporadic VHE emission had been detected in the past ($\phi$ = 0:75–1.0). These MAGIC observations were simultaneous with opticalspectroscopy from the LIVERPOOL telescope.Results.The TeV flux of the periodical outburst in orbital phases $\phi$ = 0:5–0.75 was found to show yearly variability consistent with the long-term modulation of$\sim$ 4.5 years found in the radio band. This modulation of the TeV flux can be well described by a sine function with a best-fit period of 1610 $\pm$ 58 days. The complete data, including archival observations, span two super-orbital periods. There is no evidence for a correlation betweenthe TeV emission and the mass-loss rate of the Be star, but this may be a ected by the strong, short-timescale (as short as intra-day) variation displayed by the Hfluxes.
Published by EDP Sciences, Les Ulis