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

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5. Fitting of observed spectra

Let us temporarily ignore the existence of gravitational reddening. In such a case one can directly compare theoretical spectra discussed above to the observed spectra of X-ray burst sources.

Fitting coefficients [FORMULA], computed here, are useless for the estimate of [FORMULA], the radius to distance ratio. This important parameter can be estimated according to the usual blackbody fitting procedure (Lewin et al. 1993). Consequently, values of fitted [FORMULA] are not essential in the following procedure, which does not include [FORMULA] determination.

An observed X-ray spectrum (expressed in erg/cm2 sec Hz) should be fitted by the Eq. (2) with the parameter [FORMULA] set to 2.710. Then, the observed [FORMULA] can be either interpolated between values from Table 2 to find some estimate of [FORMULA], or simply taken as [FORMULA]. Both best fitted [FORMULA] and µ can be used for the surface gravity determination, by two dimensional interpolation between values given in Table 2.

The value of gravity, [FORMULA], is measured at the photosphere of the X-ray burster, which does not necessarily coincide with the surface of the neutron star itself. In principle the above fitting offers a chance for tracing [FORMULA] variations during a burst, and tracing of radius expansion or contraction.

Such a procedure can fail if matter emitting X-rays contains some amounts of heavy elements, like iron. In such case theoretical spectra of X-ray bursts must exhibit some discrete features of highly ionized ions, like Fe [FORMULA] and Fe [FORMULA]. Fitting parameters presented in this paper get irrelevant in such a situation. Also in case, when [FORMULA] just approaches [FORMULA], values of [FORMULA] (or rather factors t) in Table 2 are too low, since t approaches 2 when [FORMULA] (Babul & Paczyski 1987).

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998