Evolving LMXBs: CARB Magnetic Braking

Abstract

Abstract The formation of low-mass X-ray binaries (LMXBs) is an ongoing challenge in stellar evolution. An important subset of LMXBs is the binary systems with a neutron star (NS) accretor. In NS LMXBs with nondegenerate donors, the mass transfer (MT) is mainly driven by magnetic braking (MB). The discrepancies between the observed MT rates and the theoretical models were known for a while. Theory predictions of the MT rates are too weak and differ by an order of magnitude or more. Recently, we showed that with the standard MB, it is not possible to find progenitor binary systems such that they could reproduce—at any time of their evolution—most of the observed persistent NS LMXBs. In this Letter we present a modified MB prescription, Convection And Rotation Boosted (CARB). CARB MB combines two recent improvements in understanding stellar magnetic fields and magnetized winds—the dependence of the magnetic field strength on the outer convective zone and the dependence of the Alfvèn radius on the donor’s rotation. Using this new MB prescription, we can reproduce the observed MT rates at the detected mass ratio and orbital period for all well-observed to-the-date Galactic persistent NS LMXBs. For the systems where the effective temperature of the donor stars is known, theory agrees with observations as well.