Blue Straggler in the Making: An X-Ray Active Binary in Algol Configuration

We report an extremely rare X-ray active blue straggler star (BSS) in Collinder 261, undergoing ongoing Roche-lobe overflow in a semidetached binary in an Algol-type configuration with an orbital period of P _orb ∼ 2.112 days, providing direct observational evidence for the ongoing binary mass-transfer formation channel of BSS. Light-curve modeling with PHOEBE reveals a low mass ratio of q ∼ 0.19, corresponding to a mass of 1.67 M _⊙ for the BSS primary and a mass of 0.32 M _⊙ for the Roche-lobe-filling companion. Reflection effects and a hotspot near the inner Lagrangian point ( L _1 ) reproduce the observed light-curve asymmetries, while the observed X-ray luminosity is consistent with direct-impact accretion-driven emission, supporting ongoing mass transfer. Radial velocities from the Very Large Telescope/GIRAFFE are also consistent with the photometric solution and reveal enhanced primary rotation of $v\sin i\sim 69.55$ km s ^−1 , consistent with spin-up through direct-impact accretion. Evolutionary modeling with MESA reproduces the observed configuration for an initially detached system with M _d ∼ 1.12 M _⊙ , M _a ∼ 0.92 M _⊙ , and P _ini ∼ 0.83 day. Roche-lobe overflow and mass transfer begin at ∼5.46 Gyr with mass ratio reversal at ∼5.58 Gyr. The present configuration at ∼7 Gyr, consistent with the cluster age, corresponds to stable mass transfer of $\dot{M}\sim $ 2 × 10 ^−10 M _⊙ yr ^−1 . These results indicate that the system represents a very rare case of a BSS currently forming through binary mass transfer.