Discovery of state transition behaviors in PSR J1124--5916

With the twelve-year long observations by {\sl Fermi}-LAT, we discover two pairs of spin-down state transitions of PSR J1124--5916, making it the second young pulsar detected to have such behaviors. PSR J1124--5916 shows mainly two states according to its spin-down rate evolution, the normal spin-down state and the low spin-down state. In about 80\% of the observation time, the pulsar is in the normal spin-down state, in which the spin-down rate decreases linearly and gives a braking index of $1.98\pm0.04$. The two transitions to the low spin-down state are in MJD 55183--55803 and MJD 56114--56398 respectively, with fractional amplitudes both $\sim0.4\%$. No significant difference between the $\gamma$-ray profiles of the two spin-down states is detected, which is similar to PSR B0540-69, the other young pulsar with state transition detected.