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

Abstract

The gamma-ray binary LS I +61${}^{\circ }$303 is a well established source from centimeter radio up to very high energy (VHE; E$>$100 GeV). Its broadband emission shows a periodicity of $\sim$26.5 days, coincident with the orbital period. A longer (super-orbital) period of 1667 $\pm$ 8 days was discovered in radio and confirmed in optical and high energy (HE; E>100 MeV) gamma-ray observations. We present a four-year campaign performed by MAGIC together with archival data concentrating on a search for a long timescale signature in the VHE emission. We focus on the search for super-orbital modulation of the VHE peak and on the search for correlations between TeV emission and optical determination of the extension of the circumstellar disk. A four-year campaign has been carried out by MAGIC. The source was observed during the orbital phases when the periodic VHE outbursts have occurred ($\varphi$=0.55-0.75). Additionally, we included archival MAGIC observations and data published by the VERITAS collaboration in these studies. For the correlation studies, LS I +61${}^{\circ }$303 has also been observed during the orbital phases where sporadic VHE emission had been detected in the past ($\varphi$=0.75-1.0). These MAGIC observations were simultaneous with optical spectroscopy from the LIVERPOOL telescope. The TeV flux of the periodical outburst in orbital phases $\varphi$=0.5--0.75 was found to show yearly variability consistent with the $\sim$4.5 years long-term modulation found in the radio band. This modulation of the TeV flux can be well described by a sine function with the best fit period of $1610\pm 58$ days. The complete dataset span two super-orbital periods. There is no evidence for a correlation between the TeV emission and the mass-loss rate of the Be star but this may be affected by the strong, short timescale (as short as intra-day) variation displayed by the H$\alpha$ fluxes.

arXiv comment7 pages, 4 figures, to be published in A&A

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• The GeV emission of PSR B1259-63 during its last three periastron passages observed by Fermi-LAT
• Galactic Observatory Science with the ASTRI Mini-Array at the Observatorio del Teide
• Superorbital modulation at GeV energies in the gamma-ray binary LS I +61 303
• Radio QPO in the $\gamma$-ray-loud X-ray binary LS I +61${}^{\circ }$303
• Modelling the correlated keV/TeV light curves of Be/gamma-ray binaries
• Understanding the periodicities in radio and GeV emission from LS I+61303
• A Precessing Jet Scenario for the Multi-Wavelength Long-Term Modulation of LS I +61${}^{\circ }$303
• Observing the Energetic Universe at Very High Energies with the VERITAS Gamma Ray Observatory
• A precessing stellar disk model for superorbital modulations of the gamma-ray binary LS I+61${}^{\circ }$ 303
• The likely Fermi detection of the Be X-ray binary GRO J1008-57: hint for gamma-ray production in neutron-star X-ray binaries
• A Very-High-Energy Gamma-Ray View of the Transient Sky
• The Very Energetic Radiation Imaging Telescope Array System (VERITAS)
• VHE observations of binary systems performed with the MAGIC telescopes
• The Major Gamma-ray Imaging Cherenkov Telescopes (MAGIC)