The persistent shift in spin-down rate following the largest Crab pulsar glitch rules out external torque variations due to starquakes

It was previously believed that, the long-term persistent increase in the spin-down rate of the Crab pulsar following a glitch is direct evidence of a starquake-induced glitch or at least related to a starquake. Using radio data covering 1710 days following the 2017 glitch, we obtain an extreme persistent increase of the spin-down rate, which allows to test two prevailing models related to starquake through an interrelation analysis between glitch size (the amplitude of the frequency step at a glitch) and persistent increase in the spin-down rate of the star. Our results do not support the hypothesis that glitches induce the external torque variation of the Crab pulsar, which may indicate no occurrence of starquake during the Crab pulsar glitch. This can explain why no changes in the radio and X-ray flux, pulse profile and spectrum of the Crab pulsar have been observed. We also suggest an internal mechanism due to superfluidity as an explanation for the long-term persistent shift in spin-down rate of the Crab pulsar following the relatively large glitches.