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 , computed here, are useless for the estimate of , 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 are not essential in the following procedure, which does not include determination.
An observed X-ray spectrum (expressed in erg/cm2 sec Hz) should be fitted by the Eq. (2) with the parameter set to 2.710. Then, the observed can be either interpolated between values from Table 2 to find some estimate of , or simply taken as . Both best fitted and µ can be used for the surface gravity determination, by two dimensional interpolation between values given in Table 2.
The value of gravity, , 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 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 and Fe . Fitting parameters presented in this paper get irrelevant in such a situation. Also in case, when just approaches , values of (or rather factors t) in Table 2 are too low, since t approaches 2 when (Babul & Paczyski 1987).
© European Southern Observatory (ESO) 1997
Online publication: July 3, 1998