  We analysed thermonuclear (type-I) X-ray bursts observed from the low-mass
X-ray binary 4U1728-34 by RXTE, Chandra and INTEGRAL. We compared the variation
in burst energy and recurrence times as a function of accretion rate with the
predictions of a numerical ignition model including a treatment of the heating
and cooling in the crust. We found that the measured burst ignition column
depths are significantly below the theoretically predicted values, regardless
of the assumed thermal structure of the neutron star interior. While it is
possible that the accretion rate measured by Chandra is underestimated, due to
additional persistent spectral components outside the sensitivity band, the
required correction factor is typically 3.6 and as high as 6, which is
implausible. Furthermore, such underestimation is even more unlikely for RXTE
and INTEGRAL, which have much broader bandpasses. Possible explanations for the
observed discrepancy include shear-triggered mixing of the accreted helium to
larger column depths, resulting in earlier ignition, or the fractional covering
of the accreted fuel on the neutron star surface.
