ssa: Strangeon star atmosphere

The strangeon star atmosphere (ssa) model describes the radiation from interstellar medium accreted plasma atmosphere on a strangeon star surface and its spectrum. The ssa could simply be regarded as the upper layer of a normal neutron star because the radiation from strangeon matter can be neglected (Wang et al. 2017, 1705.03763). The atmosphere is in radiative, thermal equilibrium and two-temperature. The ssa spectrum is based on bremsstrahlung from an extremely thin hydrogen plasma. More details of the ssa model are described in Wang et al. (2017, ApJ, 837, 81).

par1	Te, the electron temperature (keV)
par2	y, $= n_{i0}^2 T_i / R$, where $n_{i0}$ is the ion number density on the stellar surface (cm$^{-3}$), $T_i$ is the ion temperature (keV), and $R$ is the stellar radius (km). y is in units of $10^{42}$ keV/km/cm$^6$.
norm	$(R/d)^2$, where $d$ is the distance to the star in units of 10 kpc.