Anomalous orbital expansion of the low-mass X-ray binary 2A 1822-371: the existence of a circumbinary disk?

Abstract

Source 2A 1822-371 is an eclipsing low-mass X-ray binary (LMXB) consisting of a neutron star (NS) and a ∼0.5 M⊙donor star in an orbit of 5.57 h. Based on the timing of the eclipse arrival times, this source was found to experience a rapid orbital expansion with an orbital period derivative asṖorb= (1.51 ± 0.05) × 10−10s s−1, implying that the mass-transfer rate is probably higher than at least three times the Eddington accretion rate. The standard magnetic braking (MB) model cannot produce a mass-transfer rate this high. The modified MB model can produce a high mass-transfer rate, resulting in a highṖorb. We propose an alternative model to account for the anomalously high mass-transfer rate andṖorbof 2A 1822-371. During the mass transfer, a tiny fraction of the transferred material is thought to form a circumbinary (CB) disk around the LMXB, which can efficiently extract orbital angular momentum from the system by the interaction between the CB disk and the binary. We used the MESA code to model the formation and evolution of 2A 1822-371 for different CB-disk masses. When the CB-disk mass is 2.3 × 10−8 M⊙, the simulation can reproduce the observed donor-star mass, orbital period, and orbital period derivative. This CB disk can accelerate the evolution of the binary and produce a high mass-transfer rate of 1.9 × 10−7M⊙yr−1, driving the binary to evolve toward a wide-orbit system. Therefore, we propose that CB disks may cause the rapid orbital changes observed in some LMXBs.