Late evolution of very low mass X-ray binaries sustained by radiation from their primaries

Under suitable conditions, it is possible for part of the radiation originating in the vicinity of weakly magnetized neutron stars in low-mass X-ray binaries (LMXBs) to illuminate the low-mass secondary and power a wind from it. This evaporative wind can produce a Roche lobe overflow of the secondary with a mass loss close to the neutron star's Eddington limit. This can occur even when gravitational radiation or other causes of loss of angular momentum from the LMXB are weak. A bootstrapping mechanism can keep the LMXB X-ray luminosity near the Eddington limit until the secondary's mass falls below a few hundredths of a solar mass. Thereafter it would be expected that the LMXB X-ray luminosity would either drop abruptly by several orders of magnitude or cease completely, if the accretion spun-up neutron star period is less than several milliseconds. As the LMXB secondary mass traverses the range 10^-1 to a few times 10^-2 M_⊙, the primary radiation can substantially shorten the lifetime of an LMXB compared with that predicted by standard gravitational radiation scenarios. However, it will not greatly affect those earlier stages of evolution when the companion is heavier. Because the LMXB evolution speeds up dramatically through this very low mass secondary range, few LMXBs in this phase would be expected to be observed. Several features of LMXB observations can be plausibly explained by this model.