  The low mass X-ray binary 4U 1728-24 was observed with AstroSat/LAXPC on 8th
March 2016. Data from a randomly chosen one orbit of over 3 ks was analyzed for
detection of rapid intensity variations. We found that the source intensity was
nearly steady but towards the end of the observation a typical Type-1 burst was
detected. Dynamical power spectrum of the data in the 3 to 20 keV band, reveals
presence of a kHz Quasi-Periodic Oscillation (QPO) whose frequency drifted from
around 815 Hz at the beginning of the observation to about 850 Hz just before
the burst. The QPO is also detected in the 10 to 20 keV band, which was not
obtainable by earlier RXTE observations of this source. Even for such a short
observation with a drifting QPO frequency, the time-lag between the 5 to 10 and
10 to 20 keV bands can be constrained to be less than 100 microseconds. The
Type-1 burst that lasted for about 20 secs had a typical profile. During the
first four seconds dynamic power spectra reveal a burst oscillation whose
frequency increased from 361.5 to 363.5 Hz. This is consistent with the earlier
results obtained with RXTE/PCA, showing the same spin frequency of the neutron
star. The present results demonstrate the capability of LAXPC instrument for
detecting millisecond variability even from short observations. After RXTE
ceased operation, LAXPC on AstroSat is the only instrument at present with
capability of detecting kHz QPOs and other kind of rapid variations from 3 keV
to 20 keV and possibly at higher energies also.
