2015ApJ...814...88S

We present multi-wavelength observations of the unassociated γ-ray source 3FGL J2039.6-5618 detected by the Fermi Large Area Telescope. The source γ-ray properties suggest that it is a pulsar, most likely a millisecond pulsar, for which neither radio nor γ-ray pulsations have been detected. We observed 3FGL J2039.6-5618 with XMM-Newton and discovered several candidate X-ray counterparts within/close to the γ-ray error box. The brightest of these X-ray sources is variable with a period of 0.2245±0.0081 days. Its X-ray spectrum can be described by a power law with photon index Γ_X= 1.36±0.09, and hydrogen column density N_H< 4x10²⁰/cm2, which gives an unabsorbed 0.3-10 keV X-ray flux of 1.02x10^–13 erg/cm2/s. Observations with the Gamma-Ray Burst Optical/Near-Infrared Detector discovered an optical counterpart to this X-ray source, with a time-averaged magnitude g' ∼ 19.5. The counterpart features a flux modulation with a period of 0.22748±0.00043 days that coincides, within the errors, with that of the X-ray source, confirming the association based on the positional coincidence. We interpret the observed X-ray/optical periodicity as the orbital period of a close binary system where one of the two members is a neutron star. The light curve profile of the companion star, which has two asymmetric peaks, suggests that the optical emission comes from two regions with different temperatures on its tidally distorted surface. Based upon its X-ray and optical properties, we consider this source as the most likely X-ray counterpart to 3FGL J2039.6-5618, which we propose to be a new redback system.