Context. Gamma-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
discovered to emit at high energies, but their number is still scarce. 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 found for it.
Aims. The purpose of this work is to search for a gamma-ray source at GeV
energies to understand the origin of the TeV signal. For an unambiguous
identification of its binary nature, finding an orbital modulation is crucial.
Methods. We have 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 (XRT) and UltraViolet and
Optical Telescope (UVOT). We searched for periodicities in both X-ray and
gamma-ray bands using a Lomb-Scargle periodogram. Results. We find a periodic
modulation of \sim 86 days in the X-ray source XMMU J183245-0921539, together
with indications of gamma-ray modulation with a compatible period in 4FGL
J1832.9-0913. Neither an optical nor an UV counterpart is found at the X-ray
source location. The overall spectral energy distribution strongly resembles
the known gamma-ray binary HESS J0632+057. Conclusions. Both the spectrum and
the discovery of an orbital period allow the identification of the TeV source
HESS J1832-093 as a new member of the gamma-ray binary class.Comment: 9 pages, 9 figures. Accepted by A&
