Determination of the night sky background around the Crab pulsar using its optical pulsation

Abstract The poor angular resolution of imaging γ-ray telescopes is offset by the large reflector areas of next generation telescopes such as MAGIC (17 m diameter), which makes the study of optical emission associated with some γ-ray sources feasible. Furthermore, the extremely fast time response of photomultipliers (PMs) makes them ideal detectors for fast (subsecond) optical transients and periodic sources like pulsars. The optical pulse of the Crab pulsar was detected with the HEGRA CT1 central pixel using a modified PM, similar to the future MAGIC camera PMs. The HEGRA CT1 telescope is a 2.1 m radius Cerenkov telescope, which has seen the Crab optical pulsations. The corresponding observation time required for a detection is 200 s, but MAGIC will only require ∼30 s if the same strategy with standard PMs is used. The constancy of the pulsed fraction (with a relatively small error) during the 2 h CT1 measurements shows that the pointing did not change during the observations. The purpose of these periodic observations was to determine the light of the night sky (LONS) for the galactic anticenter Crab region, and to compare it with the LONS for a nearby bright star ( ζ Tauri). These obtained LONS values vary between 4.6 and 4.3 × 10 12 ph m −2 s −1 sr −1 (with a coarse estimated error of 10%), being the first value from the timing signature of the Crab pulsar, and the second one was derived from the technique using the ζ Tauri standard star. Our results are between 2.5 and 3 times larger than the previously measured LONS (outside the galactic plane) of (1.7 ± 0.4) × 10 12 ph m −2 s −1 sr −1 , as expected since the Crab pulsar is in the galactic plane, which implies a slightly higher energy threshold for Crab observations, if the higher value of CT1 measured LONS rate for galactic sources is used.