1994A&A...291..842V

We have studied the effect of X-rays from the accretion disc or corona on the evolution of a Low Mass X-ray Binary (LMXB), via heating and blowing up the secondary. Hameury et al. (1993) concluded that the effect of the X-ray heating is much smaller than earlier supposed to be: the heating blanket is so thin that only the outer radiative zone of the facing hemisphere is heated. Their result was that the star does not expand and the evolution of a LMXB should not deviate much from that of a CV. We included the form of a typical observed LMXB X-ray spectrum into the computations. The absorption of the incident flux was computed properly at all envelope layers, from the surface down to the convective zone. For a large range of stellar masses and incident X-ray fluxes, about 10 per cent of the energy flux is absorbed in the deeper convective part of the envelope. If the X-ray flux is occasionally very hard, almost all of the heating penetrates into the adiabatic part of the envelope. The luminosity released in the convective layer is distributed in a short thermal time scale of the zone around the star. This time is short when compared to the evolutionary time scale. For this reason, the use of the efficiency 10 per cent is well justified (η=0.1) with the assumption of a spherical heating. We computed two evolutionary sequences using η=0.1 and 0.001. The sequence with η=0.1 passed very close to the observed values of the eclipsing system X1822-371 (P=5.57hr ) explaining e.g. the value of its period increase. The sequence with η=0.001 penetrated into the 2-3h period gap (while accreting) and might explain the many AM Her's found inside the gap (compensating the lower X-ray luminosity by a higher efficiency and the harder spectrum).