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

Abstract

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~6). The detector format was upgraded from a $4\times4$ to an $8\times8$ GAPD array with larger pixels ($100$ to $200~{μm}$), resulting in a 14".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 (SPs) emitted in each rotation. Moreover, we found an optical peak timing drift of $30\pm7.9~\mathrm{μs}$ over three days, with a significance of $3.9σ$. 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.

Figures (9)

• Figure 1: A schematic view of the IMONY readout system. Alt text: Block diagram of IMONY showing the signal flow from the $8\times8$ GAPD array to four FPGA boards via the front-end board.
• Figure 2: Observation results after the two-step quality cut on February 9, 2024. Approximately 80% of data was survived after the cut. The top-left panel shows the photon count rate over 64 pixels, and the bottom-left panel displays the optical 2D phaseogram. The time intervals in both plots are 60 seconds. The bottom-right panel illustrates the phaseogram with a bin width of $100~\mathrm{\mu s/bin}$, also summed over 64 pixels. Alt text: A two-dimensional histogram showing a relationship among the barycentric dynamical time (horizontal axis), rotational phase (vertical axis), and total photon counts accumulated over 60 second intervals. The projections onto the phase and time axes are displayed above and to the right of the histogram, respectively.
• Figure 3: Snapshot images corresponding to three rotational phase intervals of the Crab pulsar: MP (left), IP (middle), and off-pulse (right). Each frame is integrated over 10 minutes. The bright spot during the MP phase represents the pulsar. A nearby reference star (2MASS J05343217+2200560) is also visible to the northeast. North is up and East is to the left. Alt text: Snapshot images of the Crab pulsar region in the MP, IP, and the off-pulse phases.
• Figure 4: Comparison between the optical and radio pulse profiles. The red line shows the optical phaseogram divided into 1000 phase bins per rotation. The blue line shows the radio phaseogram recorded on February 7 and 8, 2024, in the S-band of the Usuda telescope. Phase 1.0 is aligned with the radio peak. The inset shows the full optical and radio profiles across two rotations. Alt text: Close-up optical and radio phaseograms of the Crab pulsar around the MP peak, from phase 0.970 to 1.005, with the full-range profiles shown in the inset panel.
• Figure 5: Comparison of the optical-radio time lags between our result and previous observations. Our result is consistent with Oosterbroek+06, Slowikowska+09, and Nakamori+25 within the $1\sigma$ error range. Alt text: Scatter plots with error bars, showing the comparison of optical-radio lags obtained in our observations and in previous studies. The horizontal axis spans 1999--2024 in MJD, and the vertical axis ranges from $-100~\mathrm{\mu s}$ to $500~\mathrm{\mu s}$.