Photometric Follow-up of the Quasar 3C 138

ATel #17496; Ruixiang Hu (WHU), Shuaikang Yang (WHU), Xiao Fan (WHU), Han He (WHU), Saien Xu (WHU), Bei You (WHU), Zonghong Zhu (WHU), Xiaoyan Li (NIAOT), Kai Zhang (NIAOT), Zhengyang Li (NIAOT), Tong Zhou (NIAOT), Jianan Cong (NIAOT), Kaiwen Zheng (NIAOT), Yiqiao Yang (NIAOT), Chao Chen (NIAOT), Jiajia Wu (NIAOT), Yuan Liang (NIAOT)
on 17 Nov 2025; 06:38 UT
Credential Certification: Bei You (youbei@whu.edu.cn)

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Following the gamma-ray activity of the quazar 3C 138 (also known as 4FGL J0521.2+1637, Abdollahi et al. 2022, ApJS, 260, 53) detected by Fermi-LAT on 2025 October 29 (ATel #17461), we performed continued photometric observations of the source between October 30 and November 1, with the Wuhan University 1-meter Optical Telescope.

Direct imaging was conducted in the g filter on October 29, 2025, and on the second night, the wavelength coverage was extended to the u, r, and i filters. During the observations, the atmospheric transparency was relatively high, with a seeing of approximately 0.8''.

Aperture photometry was performed using the AUTOPHOT pipeline (https://github.com/Astro-Sean/autophot/). The spatial uniformity and accuracy of the calibration for 3C 138 were valiatied with the accompanying zero-point distribution and precision maps, which utilized a set of sources from Pan-STARRS and SDSS catalog. A source-check quality plot was created to identify unreliable detections based on parameters such as FWHM. Furthermore, a two-dimensional Point Spread Function (PSF) model was constructed to ensure accurate aperture corrections. The pipeline confirmed the optimal choice of the source aperture and local background annulus by producing a target aperture photometry plot for 3C 138. The photometric results are presented in the table below, and the reported uncertainties are thoroughly derived from the Poisson noise, the local sky background scatter, and the error in the photometric zero-point.

Our results indicate that the g-band magnitude for 3C 138 is approximately 16.5 mag. This is consistent with the median value of 16.48 calculated by ASAS-SN between MJD 60962 and 60982 . Additionally, the magnitudes in other wavelength bands shows slight variations, but they remain within a relatively bright range.

The Wuhan University 1-meter Optical Telescope, situated at the Lenghu Observatory, is owned and operated by Wuhan university. Designed as a Ritchey–Chrétien system with a 1-meter aperture, the telescope is equipped with a compact instrument composed of a multi-band photometric camera and a low-resolution spectrograph.