  We have discovered a triple-peaked X-ray burst from the low-mass X-ray binary
(LMXB) 4U 1636-53 with the Rossi X-ray Timing Explorer (RXTE). This is the
first triple-peaked burst reported from any LMXB using RXTE, and it is only the
second burst of this kind observed from any source. (The previous one was also
from 4U 1636-53, and was observed with EXOSAT.) From fits to time-resolved
spectra, we find that this is not a radius-expansion burst, and that the same
triple-peaked pattern seen in the X-ray light curve is also present in the
bolometric light curve of the burst. Similar to what was previously observed in
double-peaked bursts from this source, the radius of the emitting area
increases steadily during the burst, with short periods in between during which
the radius remains more or less constant. The temperature first increases
steeply, and then decreases across the burst also showing three peaks. The
first and last peak in the temperature profile occur, respectively,
significantly before and significantly after the first and last peaks in the
X-ray and bolometric light curves. We found no significant oscillations during
this burst. This triple-peaked burst, as well as the one observed with EXOSAT
and the double-peak bursts in this source, all took place when 4U 1636-53
occupied a relatively narrow region in the colour-colour diagram, corresponding
to a relatively high (inferred) mass-accretion rate. No model presently
available is able to explain the multiple-peaked bursts.
