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To: stefano@bicar.it
From: atel@astronomerstelegram.org
Subject: ATel  17539, 17540
Message-Id: <20251212071133.20A5128808C9@server.astronomerstelegram.org>
Date: Fri, 12 Dec 2025 02:11:33 -0500 (EST)

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Posted: Within the last 24 hours
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ATEL #17539							     ATEL #17539

Title:	Continued broadband radio and optical brightening and record-high
		22 GHz and R-band flux density of 3C 138 
Author:	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)
Queries:	timur.mufakharov@gmail.com
Posted:	11 Dec 2025; 07:25 UT
Subjects:Radio, Optical, AGN, Quasar

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 &plusmn
0.2 Jy in 2025 November 15, exceeding the previous maximum of 2.7 &plusmn
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: https://rat.sao.ru/scientific-results/continued-broadband-radio-and-optical-brightening-and-record-high-22-ghz-and-r-band-flux-density-of-/
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Password Certification: Timur Mufakharov (timur.mufakharov@gmail.com)
https://www.astronomerstelegram.org/?read=17539
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ATEL #17540							     ATEL #17540

Title:	The CSS 3C 138: secondary R band maximum after 40 days
Author:	V. V. Vlasyuk, O. I. Spiridonova (Special Astrophysical Observatory
		of the Rus. Ac. Sci.)
Queries:	mosk@sao.ru
Posted:	11 Dec 2025; 21:32 UT
Subjects:Optical, Quasar

The compact steep-spectrum (CSS) quasar 3C 138 demonstrate enhanced activity
in optical 
range (R band) since 2025 September. As we reported in ATel #17539 its
brightness achieved 2 
mJy level (R~15.45 +/- 0.01 mag) (October 31) and faded after with a slope
-0.03 mJy/day. But 
after stabilization of flux between November 24 and November 29 the quasar
started new phase 
of re-brightening and reached 2 mJy level again (R~15.49 +/- 0.02 mag)
at December 11/12 night. 
The slope of light curve was 2 times steeper in comparison with fading
phase --- about 0.06 
mJy/day, which is similar to behavior of 3C 138 in 2025 October.

All estimates for the 3C 138 re-brightening phase were provided with 0.5-meter
SAO RAS 
telescope. They were calibrated using our own secondary standards in the
3C 138 field.

More accurate reduction is necessary. We strongly encourage further multi-wavelength

observations.

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Password Certification: Alexander Moskvitin (mosk@sao.ru)
https://www.astronomerstelegram.org/?read=17540
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