10 MICRON DETECTION OF THE HARD X-RAY TRANSIENT GRO J0422- FREE-FREE EMISSION FROM AN X-RAY DRIVEN ACCRETION DISK WIND(32 )

We report the detection of 10 mum emission from the transient low-mass X-ray binary (LMXB) and optical nova GRO J0422+32 near the maximum of its outburst. We discuss this result in terms of (1) a ''standard'' m odel according to which low-energy radiation of LMXB is caused by repr ocessing of X-rays in an accretion disk; (2) emission from a cool seco ndary star; (3) emission from dust grains heated by the transient X-ra ys, and (4) free-free emission from an X-ray-driven wind from the accr etion disk. Only the fourth alternative provides a viable explanation for the observed 10 mum emission, with a mass-loss rate in the disk wi nd that may be substantially higher than the rate of accretion onto th e compact star. The presence of such a wind may have a profound effect on the evolution of the binary, and contribute to the solution of the ''birthrate problem'' of millisecond radio pulsars.