Hydrogen/helium model atmospheres of X-ray burst sources predict either the rise or decrease (at K) of a low-energy branch of the spectrum, in case when surface gravity decreases down to the critical gravity (cf. Paper I). Simultaneously, there appears a slight flattening of the spectrum around the peak. Therefore such deviations of X-ray spectrum from a blackbody shape are signatures of gravity, , in the photosphere.
Tabulated X-ray spectra are approximated by a new formula, Eq. (2), which involves two parameters ( and µ), which depend on the effective temperature , and . Both can be used for the fitting of the observed counts, and the temperature and gravity estimation. Numerical results are valid in case, when the atmosphere of a bursting neutron star consists of hydrogen and helium, and no heavy metals are present.
The referee pointed out, that the largest deviations of theoretical spectra from a blackbody shape occur at low energies, where the effects of interstellar absorption are most important. This is a very troublesome fact at the lowest temperatures of bursts (see Fig. 1, where all deviations from blackbody occur below 1 keV). However, with rising of a burst, predicted differences quickly shift above 1 keV, where interstellar extinction decreases (see Fig. 2, case of the extreme ). Therefore at higher determination of gets more reliable.
© European Southern Observatory (ESO) 1997
Online publication: July 3, 1998