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We present the X-ray timing and spectral analysis of the new Galactic X-raytransient Swift J1658.2-4242 observed with LAXPC and SXT instruments onboard{\it Astrosat}. We detect prominent C-type quasi-periodic oscillations (QPOs)of frequencies varying from $\sim 1.5$ Hz to $\sim 6.6$ Hz along with distinct2nd harmonics and sub-harmonics. The QPO detected at $\sim 1.56$ Hz drifts to ahigher centroid frequency of $\sim 1.74$ in the course of the observation,while the QPO detected at $\sim 6.6$\,Hz disappeared during hard flarings. Thefractional rms at the QPO and the sub-harmonic frequencies increases withphoton energy, while at the 2nd harmonic frequencies the rms seems to beconstant. In addition, we have observed soft time lag at QPO and sub-harmonicfrequencies up to a time scale of $\sim 35$ ms, however, at the 2nd harmonicfrequencies there is weak/zero time lag. We attempt spectral modeling of thebroadband data in the 0.7--25 keV band using the doubly absorbed disk plusthermal Comptonization model. Based on the spectral and timing properties, weidentified the source to be in the hard intermediate state of black hole X-raybinaries. To quantitatively fit the energy and frequency-dependent fractionalrms and time lag, we use a single zone fluctuation propagation model anddiscuss our results in the context of that model.

Spectral and Timing Properties of the Galactic X-ray transient Swift~J1658.2--4242 using Astrosat Observations