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A new outburst of GX 339$-$4 in 2021 was monitored by the Hard X-rayModulation Telescope (\textit{Insight}-HXMT). By using the data of\textit{Insight}-HXMT from February to March 2021, we make the X-ray timinganalysis of this new outburst. Based on the results of count rates,hardness-intensity diagram (HID) and power density spectrum (PDS), we confirmthat the source exhibits spectral transitions from the low-hard state (LHS) tothe hard-intermediate state (HIMS). During the transition from the LHS to theHIMS, Low-frequency Quasi-periodic oscillations (LFQPOs) are detected in thePDS. We found that these QPOs are all type-C QPOs with centroid frequenciesevolving from $0.1 -0.6$ Hz in the LHS and in the $1 -3$ Hz frequency range inHIMS. The QPO features above 50 keV are reported for the first time in thisblack hole by \textit{Insight}-HXMT. The QPO rms stays stable with time butdecreases with energy at higher energy above $\sim 10$ keV. We also find thatthe phase lag of the type-C QPO is close to zero in the early outburst stage,but becomes positive as the outburst evolves, with a hard lag of $\sim$ 0.6-1.2rad in $50 -100 $ keV. The implications of the phase lag in high energy bandsand possible physical mechanisms to explain those observations are alsodiscussed.

Quasi-periodic oscillations in GX 339-4 during the 2021 Outburst observed with Insight-HXMT