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

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Research Note

Analytical fits to theoretical spectra of X-ray burst sources: a hint on the surface gravity

J. Madej

Astronomical Observatory, Warsaw University, Al. Ujazdowskie 4, PL-478 Warszawa, Poland

Received 23 January 1996 / Accepted 25 September 1996


Simple analytic formulae have been developed to fit the existing grid of X-ray spectra of very hot neutron star atmospheres. Model atmospheres analysed here were computed in both the radiative and hydrostatic equilibrium, and included precisely the effects of relativistic Compton scattering, The grid of fitted models covers the range of [FORMULA] from [FORMULA] K up to [FORMULA] K, and [FORMULA] down to the Eddington limit, but was restricted to fully ionized hydrogen/helium gas with the He number abundance [FORMULA]. Both [FORMULA] and [FORMULA] in the above models are defined on the surface of a neutron star.

Fitting parameters given in the paper define the relation [FORMULA] between the observed color temperature of a general X-ray burst and the effective temperature, and depend also on the surface gravity [FORMULA] and the general relativistic redshift. Results of this paper can be used for the estimates of both [FORMULA] and [FORMULA] directly from the observed spectra of X-ray bursts, as long as the composition of accreted matter is restricted to H and He alone, and the redshift correction is negligible or at least can be guessed.

Key words: X-rays: general – stars: neutron – X-rays: bursts

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998