  We report results from a study of XTE J1739-285, a transient neutron star low
mass X-ray binary observed with AstroSat and NuSTAR during its 2019-2020
outburst. We detected accretion-powered X-ray pulsations at 386 Hz during very
short intervals (0.5--1 s) of X-ray flares. These flares were observed during
the 2019 observation of XTE J1739-285. During this observation, we also
observed a correlation between intensity and hardness ratios, suggesting an
increase in hardness with the increase in intensity. Moreover, a thermonuclear
X-ray burst detected in our AstroSat observation during the 2020 outburst
revealed the presence of coherent burst oscillations at 383 Hz during its decay
phase. The frequency drift of 3 Hz during X-ray burst can be explained with r
modes. Thus, making XTE J1739-285 belong to a subset of NS-LMXBs which exhibit
both nuclear- and accretion-powered pulsations. The power density spectrum
created using the AstroSat-LAXPC observations in 2020 showed the presence of a
quasi-periodic oscillation at ~ 0.83 Hz. Our X-ray spectroscopy revealed
significant changes in the spectra during the 2019 and 2020 outburst. We found
a broad iron line emission feature in the X-ray spectrum during the 2020
observation, while this feature was relatively narrow and has a lower
equivalent width in 2019,~when the source was accreting at higher rates than
2020.
