A study of the luminosity variability of distant quasars by near-infrared imaging with Nayuta/NIC

The origin of supermassive blackholes in quasars, distant Active Galactic Nuclei, is a great mystery. One possible way to address this problem is to observe the luminosity variability of quasars. In this paper, we present time variability analyses for four quasars, PSO 183+05 (redshift z = 6.44), PSO 338+29 (redshift z = 6.66), ULAS 1120+0641 (redshift z = 7.09) and ULAS 1342+0928 (redshift z = 7.54) based on near-infrared imaging observations with NIC (Nishiharima Infrared Camera) on the Nayuta Telescope over one week to two years. We have examined whether these targets have time variability in luminosity in our data and in literature. J band magnitude of PSO 338+29 and ULAS J1342+0928 show ∼ 2σ level possible signs of the variability. We need to continue to observe these quasars to confirm whether the variability is real. For the variability research, it is important to obtain magnitude accurately. In this paper, we have carefully examined the magnitude uncertainty which is considered to be composed of Poisson errors, zero-point errors, and systematic errors compared with stars around the targets. Furthermore, we carried out 7-point dithering observations during the 2021 run, instead of the default setting of 10-point dithering, in order to reduce the uncertainties. However, we could not see the reduction of the Poisson error by the change of dithering points. The effect on other errors is a subject for future study.