Orbital evolution of low-mass X-ray binaries due to radiation driven mass transfer

Abstract

A LOW-MASS X-ray binary (LMXB) consists of a compact star, probably a neutron star, accreting mass from a low-mass (≤ M⊙) companion via an accretion disk. Of ∼100 known LMXBs in the Galaxy, only four have stable enough X-ray modulations to have allowed the reliable determination of orbital period changes. For these four LMXBs, all of which have P_orb≤5.6h, the measured values^1–4 of P⊙_orb/P_orb disagree markedly with what would be expected for orbital evolution driven by angular momentum loss due to gravitational radiation, possibly supplemented by magnetic braking; the empirically derived timescale for orbital evolution is ∼100 times less than expected. On the assumption that the observed period changes are secular, and not due to some longer-term periodic change, I argue here that the observed behaviour of LMXBs can be explained as the result of mass loss from the companion star caused by irradiation of the secondary star and accretion disk by the primary⁵. The typical lifetime of a radiation-driven LMXB is expected to be ∼10⁶–10⁷yr. This reduced evolutionary timescale can resolve the statistical discrepancy between the number of binary millisecond pulsars and of their presumed LMXB progenitors if about half of all the LMXBs are radiation-driven^5–7.