Evolving LMXBs: CARB Magnetic Braking

TL;DR

This paper introduces the CARB magnetic braking model, which improves the theoretical understanding of mass transfer in neutron star low-mass X-ray binaries, aligning predictions with observed data.

Contribution

The paper presents a new magnetic braking prescription, CARB, that incorporates recent insights into stellar magnetic fields and rotation, resolving previous discrepancies in LMXB evolution models.

Findings

CARB magnetic braking reproduces observed mass transfer rates.

The model aligns theory with observed orbital periods and mass ratios.

It agrees with donor star temperature measurements.

Abstract

The formation of low-mass X-ray binaries (LMXBs) is an ongoing challenge in stellar evolution. The important subset of LMXBs are the binary systems with a neutron star (NS) accretor. In NS LMXBs with non-degenerate donors, the mass transfer is mainly driven by magnetic braking. The discrepancies between the observed mass transfer (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 magnetic braking, 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 ${\it Letter}$ we present a modified magnetic braking prescription, CARB (Convection And Rotation Boosted). CARB magnetic braking 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 Alfv\`en radius on the donor's rotation. Using this new magnetic braking prescription, we can reproduce the observed mass transfer 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.