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Astron. Astrophys. 320, 181-184 (1997)

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7. Conclusions

Hydrogen/helium model atmospheres of X-ray burst sources predict either the rise or decrease (at [FORMULA] K) of a low-energy branch of the spectrum, in case when surface gravity [FORMULA] decreases down to the critical gravity [FORMULA] (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, [FORMULA], in the photosphere.

Tabulated X-ray spectra are approximated by a new formula, Eq. (2), which involves two parameters ([FORMULA] and µ), which depend on the effective temperature [FORMULA], and [FORMULA]. 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 [FORMULA] of a burst, predicted differences quickly shift above 1 keV, where interstellar extinction decreases (see Fig. 2, case of the extreme [FORMULA]). Therefore at higher [FORMULA] determination of [FORMULA] gets more reliable.

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998