## 2. Source grid of theoretical spectraIn this paper we analyse the set of model atmospheres of hot neutron stars, which was described and published in Madej (1991), hereafter Paper I. All the models were computed assuming plane-parallel geometry, hydrostatic and radiative equilibrium, and an equation of state for an ideal gas consisting of perfectly ionized hydrogen and helium of the solar number abundance, . No heavier elements were included in computations. A total of 20 model atmospheres were computed, and their parameters ( and surface ) are listed in Table 1 (cgs units).
Table 1 lists also logarithms of the critical gravities, at which radiation pressure gradient precisely compensates the gradient of gas pressure at some level in H/He atmosphere, thus limiting of hydrostatic models. Values of were obtained by extrapolation of non-grey hydrostatic models. The models in Paper I included very careful treatment of Compton
scattering, which is the dominant source of opacity in almost all
X-ray bursting neutron stars. The presence of very strong scattering
terms in the source functions generally causes deviations from the
Planckian shape and introduces non-local coupling in the atmosphere,
typical to NLTE models (Mihalas 1978). Both the equation of transfer
and the equation of radiative equilibrium included Compton scattering
frequency redistribution profiles (Pomraning 1973), which allow us to
trace in detail the transfer of photons in frequency space also in
cases that the photon energy is Figs. 1 and 2 display some of the X-ray spectra presented in Paper I, computed for extreme values of , together with the Planck function corresponding to that . It is evident, that all theoretical spectra are significantly harder, than the blackbody curve. Moreover, the theoretical X-ray spectra clearly exhibit a dependence on the surface gravity of a neutron star. In case of the coolest models at K (Fig. 1), a decrease of the surface gravity from 15.0 down to 12.5 (cgs units) causes distinct rise of the low-energy branch of X-ray spectra and a slight decrease around flux maximum (both effects were also discussed by Lewin et al. 1993). For the hottest models ( K) the most significant changes occur in a wide region around the peak in the flux distribution, with a dependence that is of opposite sign compared to the K models. The models in Paper I are quite accurate numerically, therefore it is worthwhile to seek for a simplified analytical representation of both gravity effects, for the subsequent fitting of observed spectra of X-ray burst sources.
© European Southern Observatory (ESO) 1997 Online publication: July 3, 1998 |