  X-ray burst spectra have long been used to estimate neutron star masses and
radii. These estimates assumed that burst spectra are accurately described by
the model atmosphere spectra developed over the last three decades. We compared
RXTE data from a superburst with these spectra and found that the spectra
predicted by previously published model atmospheres are strongly inconsistent
with these high-precision measurements. In contrast, a simple Bose-Einstein
spectrum is fully consistent with the data, as are recently published model
atmosphere spectra. We discuss the implications of our results for
determinations of neutron star masses and radii via constraints on their
surface gravity and redshift, as originally suggested by Majczyna and Madej.
