Timing properties of the X-ray accreting pulsar 1A 0535+262 studied with Insight-HXMT

We report results on the timing analysis of the 2020 giant outburst of 1A 0535+262, using broadband data from Insight-HXMT. The analysis of the pulse profile evolution from the sub-critical luminosity to super-critical luminosity regime is presented for the first time. We found that the observed pulse profile exhibits a complex dependence on both energy and luminosity.A dip structure at the energy of the cyclotron resonant scattering features (CRSFs) is found for the first time in the pulse fraction-energy relation of 1A 0535+262, when the outburst evolves in a luminosity range from 4.8 $\times {10}^{37}$ erg s${}^{-1}$ to 1.0 $\times {10}^{38}$ erg s${}^{-1}$. The observed structure is luminosity dependent and appears around the source critical luminosity ($\sim$ 6.7 $\times {10}^{37}$ erg s${}^{-1}$).