A search for the iron absorption edge in the tail of an X-ray burst from X1636–53

Model atmosphere calculations of the spectrum of a neutron star cooling after an X-ray burst show that the photoelectric edge of iron should be prominent. We find no clear evidence for such a redshifted feature in the spectrum of a burst from X1636–53 and conclude that the iron abundance there must be less than 0.3 solar. Unless the iron abundance of the surface matter on the neutron star is highly time-dependent, our result argues against the 4.1-keV absorption line seen in some bursts from X1636-53 by Waki et al. being due to iron. We note that the iron edge will be a powerful diagnostic of the surface redshift of the neutron star in burst sources where the iron abundance is more nearly solar.