Radio flare in CSS quasar 3C 138

In 2024 the enhanced gamma-ray activity from 3C 138, also known as 4FGL J0521.2+1637, was observed with the Fermi/LAT (Large Area Telescope) (ATel #16845). VLBI observations using the Korean VLBI Network (KVN) on 2024 December 12 found that the flux densities of two components in 3C138 became about two times brighter than the flux density measured in the Very Long Baseline Array (VLBA) observations at 23 GHz on 2024 June 18 (ATel #17077). 

We detected a significant increase of the flux densities of this quasar at 11.2 and 22.3 GHz measured with the RATAN-600 radio telescope. This increase continues since August 2022 (see the light curves at 2.3, 4.7, 8.2, 11.2, 22.3 GHz at the link below). The flux densities have reached the following high values: 2.69 (&plusmn 0.3) Jy at 22.3 GHz on 2024 August 31 and 2.94 (&plusmn 0.3) Jy at 11.2 GHz on 2025 March 1. The values at 22.3 GHz are noticeably higher than 1.65 Jy measured by Shan Li at al. (ATel #17077). Calibrations of the RATAN-600 measurements were made using the gain curves, constructed based on observations of the standard non-variable sources: PKS 1151-34, 3C 48, 3C 147, 3C 161, 3C 286, 3C 295, 3C 309.1, 3C 249.1 and NGC 7027 with their flux densities adopted from the Baars et al. (1977), Ott et al. (1994), Tabara & Inoue (1980), and Perley & Butler (2013). We will continue monitoring of 3C 138 to follow up on the development of the flare at 2-22 GHz.