To Be or Not To Be: The Role of Rotation in Modeling Galactic Be X-Ray Binaries
Kyle A. Rocha, Vicky Kalogera, Z. Doctor, Jeff J. Andrews, Meng Sun, Seth Gossage, Simone S. Bavera, Tassos Fragos, Konstantinos Kovlakas, Matthias U. Kruckow, Devina Misra, Philipp M. Srivastava, Zepei Xing, Emmanouil Zapartas
Abstract
Abstract Be X-ray binaries (Be-XRBs) are one of the largest subclasses of high-mass X-ray binaries, comprised of a rapidly rotating Be star and neutron star companion in an eccentric orbit, intermittently accreting material from a decretion disk around the donor. Originating from binary stellar evolution, Be-XRBs are of significant interest to binary population synthesis (BPS) studies, encapsulating the physics of supernovae, common envelope, and mass transfer (MT). Using the state-of-the-art BPS code, POSYDON , which relies on precomputed grids of detailed binary stellar evolution models, we investigate the Galactic Be-XRB population. POSYDON incorporates stellar rotation self-consistently during MT phases, enabling detailed examination of the rotational distribution of Be stars in multiple phases of evolution. Our fiducial BPS and Be-XRB model aligns well with the orbital properties of Galactic Be-XRBs, emphasizing the role of rotational constraints. Our modeling reveals a rapidly rotating population ( ω / ω crit ≳ 0.3) of Be-XRB-like systems with a strong peak at intermediate rotation rates ( ω / ω crit ≃ 0.6) in close alignment with observations. All Be-XRBs undergo an MT phase before the first compact object forms, with over half experiencing a second MT phase from a stripped helium companion (Case BB). Computing rotationally limited MT efficiencies and applying them to our population, we derive a physically motivated MT efficiency distribution, finding that most Be-XRBs have undergone highly nonconservative MT ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mover accent="true"> <mml:mrow> <mml:mi>β</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>¯</mml:mo> </mml:mrow> </mml:mover> </mml:mrow> <mml:mrow> <mml:mi>rot</mml:mi> </mml:mrow> </mml:msub> <mml:mo>≃</mml:mo> <mml:mn>0.15</mml:mn> </mml:math> ). Our study underscores the importance of detailed angular momentum modeling during MT in interpreting Be-XRB populations, emphasizing this population as a key probe for the stability and efficiency of MT in interacting binaries.