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

Abstract 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>v</mml:mi> <mml:mi>sin</mml:mi> <mml:mi>i</mml:mi> <mml:mo>∼</mml:mo> <mml:mn>69.55</mml:mn> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> <mml:mo>∼</mml:mo> </mml:math> 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.