One-week optical observations of pulsed emission from the Crab pulsar with IMONY on the 3.8 m Seimei telescope

We report our optical observations of the Crab pulsar using the Imager of MPPC-based Optical photoN counter from Yamagata (IMONY), a high-time-resolution photon-counting imager with 100 ns timing resolution, mounted on the 3.8 m Seimei telescope in Japan (f/D$\sim$6). The detector format was upgraded from a $4\times 4$ to an $8\times 8$ GAPD array with larger pixels (100–$200~\mathrm{\mu m}$), resulting in a $14{_{.}^{\prime\prime}} 5$ field of view on the Seimei telescope. We conducted nightly optical observations for one week, including two nights of simultaneous optical and radio observations with the 64 m Usuda radio telescope. Thanks to the large diameter of the Seimei telescope and the high time resolution of IMONY, we successfully detected optical single pulses emitted in each rotation. Moreover, we found an optical peak timing drift of $30\pm 7.9~\mathrm{\mu s}$ over three days, with a significance of $3.9\sigma$. The corresponding emission region size is 9.1 km, which is equivalent to 0.006 times the light cylinder radius of the Crab pulsar. We ruled out the possibility of a pulsar glitch and suggested that the optical pulsed emission region of the Crab pulsar may fluctuate due to the spatial drift and variations in the magnetospheric caustics.