2024PASP..136g4202N

We present optical follow-up of IGR J16194-2810, a hard X-ray source discovered by the INTEGRAL mission. The optical counterpart is a ∼500 L_⊙ red giant at a distance of 2.1 kpc. We measured 17 radial velocities (RVs) of the giant over a period of 271 days. Fitting these RVs with a Keplerian model, we find an orbital period of P_orb = 192.73 ± 0.01 days and a companion mass function f(M₂) = 0.365 ± 0.003 M_⊙. We detect ellipsoidal variability with the same period in optical light curves from the ASAS-SN survey. Joint fitting of the RVs, light curves, and the broadband spectral energy distribution allows us to robustly constrain the masses of both components. We find a giant mass of ${M}_{\star }={0.99}_{-0.03}^{+0.02}\,{M}_{\odot }$ and a companion mass of ${M}_{2}={1.23}_{-0.03}^{+0.05}\,{M}_{\odot }$, implying that the companion is a neutron star (NS). We recover a 4.06 hr period in the system's TESS light curve, which we tentatively associate with the NS spin period. The giant does not yet fill its Roche lobe, suggesting that current mass transfer is primarily via winds. Modules for Experiments in Stellar Astrophysics evolutionary models predict that the giant will overflow its Roche lobe in 5-10 Myr, eventually forming a recycled pulsar + white dwarf binary with a ∼900 days period. IGR J16194-2810 provides a window on the future evolution of wide NS + main sequence binaries recently discovered via Gaia astrometry. As with those systems, the binary's formation history is uncertain. Before the formation of the NS, it likely survived a common envelope episode with a donor-to-accretor mass ratio ≳10 and emerged in a wide orbit. The NS likely formed with a weak kick (v_kick ≲ 50 km s^–1), as stronger kicks would have disrupted the orbit.