A small subset of thermonuclear X-ray bursts on neutron stars exhibit such a strong photospheric expansion that for a few seconds the photosphere is located at a radius r _ { ph } \ga 10 ^ { 3 } \textrm { km } .
Such ‘ superexpansions ’ imply a large and rapid energy release , a feature characteristic of pure He burst models .
Previous calculations have shown that during a pure He burst , the freshly synthesized heavy-element ashes of burning can be ejected in a strong radiative wind and produce significant spectral absorption features .
We search the burst data catalogs and literature and find 32 superexpansion bursts , 24 of which were detected with BeppoSAX and three with RXTE at high time resolution .
We find that these bursts exhibit the following interesting features : ( 1 ) At least 31 are from ( candidate ) ultracompact X-ray binaries in which the neutron star accretes hydrogen-deficient fuel , suggesting that these bursts indeed ignite in a helium-rich layer .
( 2 ) In two of the RXTE bursts we detect strong absorption edges during the expansion phase .
The edge energies and depths are consistent with the H-like or He-like edge of iron-peak elements with abundances \ga 100 times solar , suggesting that we are seeing the exposed ashes of nuclear burning .
( 3 ) The superexpansion phase is always followed by a moderate expansion phase during which r _ { ph } \sim 30 \textrm { km } and the luminosity is near the Eddington limit .
( 4 ) The decay time of the bursts , \tau _ { decay } , ranges from short ( \approx 10 \textrm { s } ) to intermediate ( \ga 10 ^ { 3 } \textrm { s } ) .
However , despite the large range of \tau _ { decay } , the duration of the superexpansion is always a few seconds , independent of \tau _ { decay } .
By contrast , the duration of the moderate expansion is always of order \tau _ { decay } .
( 5 ) The photospheric radii r _ { ph } during the moderate expansion phase are much smaller than steady state wind models predict .
We show that this may be further indication that the wind contains highly non-solar abundances of heavy elements .