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

6. Nonzero gravitational redshift

Photons emitted from the surface of a neutron star lose part of their energy and change the rate at which they appear, while escaping to infinity. This is a general relativistic effect which is important in the extremely strong gravitational potential near neutron star. In case of nonrotating neutron star, all the corrections to observables are given by simple scalar factors (Thorne 1977).

The emerging spectrum of a star is squezeed along frequency axis, when measured at infinity (near the Earth). All photons decrease their initial frequency to , and

where is measured by a distant observer, and R is the actual radius of the photosphere measured locally.

Consider an arbitrary phase of a burst. If the redshift factor is guessed and the observed energy spectrum has been corrected for that factor, then the fitting procedure yields directly estimates of and in the photosphere at that phase. Both and yield mass to luminosity ratio, . In case if L is known (e.g. distance known) one may try to iterate and get momentary values of and () consistent.

Lewin et al. (1993) review also independent methods, which can be used for the estimates of .

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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