HOT ATMOSPHERES AROUND ACCRETING NEUTRON-STARS - A POSSIBLE SOURCE FOR HARD X-RAY-EMISSION

The structure of static atmospheres around unmagnetized neutron stars undergoing steady spherical accretion is discussed. We focus on the '' hot'' configurations presented earlier by Turolla et al, and calculate the radiation spectrum using a characteristics method. In particular, it is found that e(+)-e(-) pair production may significantly affect t he external atmospheric layers, where positron and proton number densi ties become of the same order. The consequent increase of the scatteri ng opacity lowers the Eddington limit, and this, in turn, may drive a dynamical instability if the accretion luminosity is large enough, ult imately producing a rapid expulsion of the envelope. If ''hot'' states are indeed accessible, this mechanism could give rise to transient ph enomena in hard X-rays of great potential astrophysical interest.