Continued broadband radio and optical brightening and record-high 22 GHz and R-band flux density of 3C 138

ATel #17539; Yu. V. Sotnikova, T. V. Mufakharov, V. V. Vlasyuk (SAO RAS), Tao An (ShAO CAS), O. I. Spiridonova, A. K. Erkenov, R. Yu. Udovitskij, T. A. Semenova, (SAO RAS), Yu. A. Kovalev (ASC Lebedev), Yu. Y. Kovalev (MPIfR), A. V. Popkov (MIPT, ASC Lebedev), M. A. Kharinov, I. A. Rahimov, T. S. Andreeva (IAA RAS)
on 11 Dec 2025; 07:25 UT
Credential Certification: Timur Mufakharov (timur.mufakharov@gmail.com)

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We report continued long-term broadband radio and optical brightening of the compact steep-spectrum (CSS) quasar 3C 138, with the variability amplitude increasing toward higher frequencies. In addition to earlier reports of enhanced radio emission (ATel #17077 and #17104), new RATAN-600 (SAO RAS) observations show a record-high flux density at 22.3 GHz of 3.4 ± 0.2 Jy in 2025 November 15, exceeding the previous maximum of 2.7 ± 0.3 Jy reported in 2024 August. The 11.2-22.3 GHz spectral index (S ~ nu^alpha) has flattened over the past three years, indicating progressive spectral hardening at cm wavelengths. In October 2025, the spectral index had become rising and reached the value of +0.1-0.2 on 15-16 November.

We estimated the linear long-term slopes of the light curves across all monitored frequencies. Positive trends are found at 22.3, 11.2, 8.2, and 4.7 GHz: +0.14, +0.05, +0.04, and +0.04 Jy/yr, respectively. In contrast, the 2.3 and 1.2 GHz measurements exhibit only low-amplitude fluctuations with a marginally negative slope (–0.04 and –0.03 Jy/yr). This behavior suggests that the low-frequency emission arises predominantly from more extended, slowly varying components, while the high-frequencies trace the ongoing outburst in the compact core.

According to the Zwicky Transient Facility (ZTF; X. Chen et al. 2020) archive data, the optical brightness of 3C 138 varied slowly between 0.2 and 0.3 mJy from 2018 until late 2023. In 2024, the source underwent 2 optical flares, each reaching approximately 0.6 mJy in March and October. No earlier optical flux-density measurements are available in the ZTF archive.

Optical monitoring of 3C 138 in the R-band has been performed since July 2025 with the SAO RAS 1-m Zeiss-1000 and 0.5-m AS-500/2 reflectors. After the quasar emerged from solar conjunction, its behavior changed dramatically - starting at the same brightness level as observed in October 2024, the source brightened rapidly to 2 mJy by 31 October2025, consistent with the estimates reported in ATel #17496. Following the peak, the source began to fade with a slope -0.03 mJy/day, a decline that continues at present (see the accompanying figure at the link below).

The following data are available at the link below: RATAN-600 and RT-32 light curves (2011-2025) at 2.3, 4.7, 8.2, 11.2, and 22.3 GHz; spectral index evolution plot and optical R-band light curves (2018-2025).

The RATAN-600 and RT-32 light curves of 3C 138 at 2.3-22.3 GHz