Rotational broadening of atomic spectral features from neutron stars

The discovery of the first gravitationally redshifted spectral line from a neutron star (NS) by Cottam et al. has triggered theoretical studies of the physics of atomic line formation in NS atmospheres. Chang et al. showed that the hydrogenic Fe Ha line formed above the photosphere of a bursting NS is intrinsically broad. We now include rotational broadening within general relativity and compare the resulting profile to that observed during type I bursts from EXO 0748-676. We show that the fine-structure splitting of the line precludes a meaningful constraint on the radius. Our fitting of the data show that the line-forming Fe column is log (N-Fe,N-n=2/cm(-2)) = 17.9(-0.42)(+0.27) and gravitational redshift 1 + z = 1.345(-0.008+0.005) with 95% confidence. We calculate the detectability of this spectral feature for a large range of spins and inclinations, assuming that the emission comes from the entire surface. We find that at 300 and 600 Hz only 10% - 20% and 5% - 10% of NSs would have spectral features as deep as those seen in EXO 0748-676.