Multiwavelength Observations of a New Redback Millisecond Pulsar Candidate: 3FGL J0954.8-3948

We present a multiwavelength study of the unassociated Fermi Large Area Telescope (LAT) source, 3FGL J0954.8-3948, which is likely the γ-ray counterpart of a 9.3 hr binary in the field. With more than 9 years of Pass 8 LAT data, we updated the γ-ray spectral properties and the LAT localization of the γ-ray source. While the binary lies outside the cataloged 95% error ellipse, the optimized LAT ellipse is 0.°1 closer and encloses the binary. The system is likely spectrally hard in X-rays (photon index Γ_x = 1.4^+1.2 _-1.0) with orbital modulations detected in optical, ultraviolet, and possibly X-rays. A steep spectrum radio counterpart (spectral index α ≈ -1.6) is also found in the TIFR GMRT Sky Survey, implying that it is a pulsar system. We obtained a series of SOAR and Gemini spectroscopic observations in 2017/2018, which show a low-mass secondary orbiting in a close circular orbit with K ₂ = 272 ±4 km s^-1 under strong irradiation by the primary compact object. All the observations as well as the modeling of the X/γ-ray high-energy emission suggest that 3FGL J0954.8-3948 is a redback millisecond pulsar in a rotation-powered state.