On the Radiation Spectrum of Hot Quasi-Spherical Accretion Flow onto a Neutron Star

We study the observational properties of a hot, geometrically thick, optically thin accretion flow onto an unmagnetized (and weakly magnetized) spinning neutron star. We numerically compute the photon emission spectrum, assuming the flow is steady-state and the dominant emission mechanism is free-free emission. The key parameters are the flow (and the star) rotational velocity, $\Omega$, the radial infall velocity $v$, as well as the electron temperature and density, $T_e$ and $n_e$. The relativistic effects, important for rapidly rotating stars, are included. Various viewing angles with respect to the rotation axis are considered. We compare our results with available observational data.