Significant Radio Brightening of 3C 138 Revealed by KVN Multi-Frequency Observations at 22-129 GHz

ATel #17077; Shan Li (Korea Astronomy and Space Science Institute, Korea University of Science and Technology), Sang-Sung Lee (Korea Astronomy and Space Science Institute, Korea University of Science and Technology), Whee Yeon Cheong (Korea Astronomy and Space Science Institute)
on 12 Mar 2025; 03:30 UT
Credential Certification: Sang-Sung Lee (sslee@kasi.re.kr)

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Motivated by recent reports of enhanced gamma-ray activity in the compact steep-spectrum (CSS) source 3C 138 (ATel #16845), we conducted simultaneous multi-frequency VLBI observations using the Korean VLBI Network (KVN). KVN consists of four 21-m radio telescopes located in Seoul, Ulsan, PyeongChang, and Jeju Island in Korea, capable of simultaneous observations at 22, 43, 86, and 129 GHz.

Our observations, conducted on 2024 Dec 12 (MJD 60656), resulted in the following CLEAN flux densities (i.e., total flux density on milli-arcsecond (mas) scales) of 3C 138:

22 GHz: 1.65 +/- 0.16 Jy
43 GHz: 1.64 +/- 0.16 Jy
86 GHz: 1.14 +/- 0.23 Jy
129 GHz: 1.11 +/- 0.33 Jy

At 22 GHz, a 2-D Gaussian model fitting resolves two prominent components (emission regions) separated by ~5 mas, with the corresponding flux densities of two components measured as 1.21 +/- 0.14 Jy and 0.41 +/- 0.06 Jy, respectively. This corresponds to approximately two times the flux density measured in the Very Long Baseline Array (VLBA) observations at 23 GHz on 2024-06-18. The VLBA raw data, obtained from the NRAO archive (project code: UC003), were processed through data reduction and imaging, yielding flux densities of 0.67 +/- 0.02 Jy and 0.18 +/- 0.02 Jy for two distinct components (separated by ~5 mas) . These results indicate that the inner jet region of 3C 138 is currently experiencing an increase in radio flux density.

Further observations and analysis of VLBI imaging and polarization measurements are ongoing to investigate the nature of the emission variability. For inquiries, please contact shan_li@kasi.re.kr.

We are grateful to the staff of the KVN who helped to operate the array and to correlate the data. The KVN is a facility operated by KASI (Korea Astronomy and Space Science Institute). The KVN observations and correlations are supported through the high-speed network connections among the KVN sites provided by KREONET (Korea Research Environment Open NETwork), which is managed and operated by KISTI (Korea Institute of Science and Technology Information). We also acknowledge the use of the VLBA under the US Naval Observatory's time allocation. This work supports USNO's ongoing research into the celestial reference frame and geodesy. The National Radio Astronomy Observatory (NRAO) is a facility of the National Science Foundation (NSF), operated under a cooperative agreement by Associated Universities, Inc.