Hints of γ-ray orbital variability from γ2 Velorum

Abstract

Context. Colliding wind binaries are massive systems featuring strong, interacting stellar winds which may act as particle accelerators. Therefore, such binaries are good candidates for detection at high energies. However, only the massive binary η Carinae has been firmly associated with a γ-ray signal. A second system, γ2 Velorum, is positionally coincident with a γ-ray source, but we lack unambiguous identification. Aims. Observing orbital modulation of the flux would establish an unambiguous identification of the binary γ2 Velorum as the γ-ray source detected by the Fermi Large Area Telescope (Fermi-LAT). Methods. We used more than ten years of observations with Fermi-LAT. Events are phase-folded with the orbital period of the binary to search for variability. We studied systematic errors that might arise from the strong emission of the nearby Vela pulsar with a more conservative pulse-gated analysis. Results. We find hints of orbital variability, indicating maximum flux from the binary during apastron passage. Conclusions. Our analysis strengthens the possibility that γ-rays are produced in γ2 Velorum, most likely as a result of particle acceleration in the wind collision region. The observed orbital variability is consistent with predictions from recent magnetohydrodynamic simulations, but contrasts with the orbital variability from η Carinae, where the peak of the light curve is found at periastron.