X-ray and γ-ray orbital variability from the γ-ray binary HESS J1832-093

Abstract

Context.γ-ray binaries are systems composed of a massive star and a compact object whose interaction leads to particle acceleration up to relativistic energies. In the last fifteen years, a few binaries have been found to emit at high energies, but their number is still low. The TeV source HESS J1832-093 has been proposed as a binary candidate, although its nature is unclear. Neither a GeV counterpart nor a period was detected.
Aims: The purpose of this work is to search for a GeV counterpart to understand the origin of the TeV signal detected by H.E.S.S. For an unambiguous identification of its binary nature, finding an orbital modulation is crucial.
Methods: We analysed data spanning more than 10 years from the Fermi Large Area Telescope (Fermi-LAT), together with Swift archival observations taken between 2015 and 2018, using both the X-Ray Telescope and UV/Optical Telescope. We searched for periodicities in both X-ray and GeV bands.
Results: We find a periodic modulation of ∼ 86 days in the X-ray source candidate counterpart XMMU J183245-0921539, together with indications of γ-ray modulation with a compatible period in the GeV candidate counterpart 4FGL J1832.9-0913. Neither an optical nor a UV counterpart is found at the X-ray source location. The overall spectral energy distribution strongly resembles the known γ-ray binary HESS J0632+057.
Conclusions: Both the spectral energy distribution and the discovery of an orbital period allow the identification of the TeV source HESS J1832-093 as a new member of the γ-ray binary class.