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From: atel@astronomerstelegram.org
Subject: ATel  17508
Message-Id: <20251129070603.130C428807AD@server.astronomerstelegram.org>
Date: Sat, 29 Nov 2025 02:06:03 -0500 (EST)

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Posted: Within the last 24 hours
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ATEL #17508							     ATEL #17508

Title:	Historical Optical Flaring of the BL Lac Object S2 0109+22
		
Author:	C. Lorey, E. Bommert, S. Duerr, M. Gräf, T. Kaplan, F. Lach,
		B. Lehmann, L. Lehmann, F. Pfeuffer, G. Rinke, M. Feige, M. Kopp,
		D. Reinhart, R. Steineke, N. Zottmann(all Hans-Haffner-Sternwarte,
		Friedrich-Koenig-Gymnasium, Wuerzburg, Germany), J. Hessdoerfer, M.
		Kadler, L. Haury, K. Mannheim, F. Roesch W. Schulga (all University
		of Wurzburg, Germany) P. Benke (GFZ Potsdam), L. Debbrecht (MPIfR),
		F. Eppel (JIVE), A. Gokus (WUSTL), S. Hämmerich (FAU), S. Kim (KASI),
		D. Elsaesser (TU Dortmund, Germany)
Queries:	dominik.elsaesser@tu-dortmund.de
Posted:	28 Nov 2025; 10:14 UT
Subjects:Radio, Optical, AGN, Blazar

We report exceptional activity of the BL Lac S2 0109+22 (ICRS J2000 coords.
R.A.: 01 12 05.825 Dec.: +22 44 38.786 (Gaia Collaboration 2020)) and z=0.265-0.49
(Koljonen et al. 2024, MNRAS, 531, 5084).

The source showed an increase in R-band brightness from 14.564 mag in early
October to 13.364 mag early on November 22nd 2025.

In February 2025 and August 2024, S2 0109+22 already showed very high optical
brightness (ATel #17025 and ATel #16783). Another flare in June 2023 was
accompanied by a historical radio flare (ATel #16093).

After the flare in February 2025, the optical brightness fluctuated around
13.75 mag until March 2025, before decreasing to between 14 mag and 14.75
mag.

From the beginning of October 2025, the brightness started increasing again,
with a high state in the night from November 21st to 22nd 2025, (R-band:
13.364 mag).

At high radio frequencies (19 - 45 GHz) the source has shown a continuous
rise over the last half a year, doubling its flux density from (1.42 &#177;
0.12) Jy at 14mm in April to (2.601 &#177; 0.060) Jy at 14mm and (2.62
&#177; 0.11) Jy at 7mm in November 2025, as revealed through regular monitoring
observations within the TELAMON program (Eppel et al. 2024, A&A, 684, A11).

This behavior in rise time and amplitude is similar to flares previously
seen in this source, however, the most recent flux densities exceed previous
flares with the exception of the historical radio flare in June 2023 where
the source reached flux densities ~4.5 Jy (ATel # 16093).

For the optical range, we present the following preliminary magnitudes:

JD 2460951,55915: 14.564 &#177; 0.004

JD 2460964.35121: 14.104 &#177; 0.003

JD 2460967.34281: 14.311 &#177; 0.006

JD 2460979.40792: 13.794 &#177; 0.005

JD 2460991.30064: 14.236 &#177; 0.004

JD 2460993.41008: 14.311 &#177; 0.013

JD 2460997.43133: 14.005 &#177; 0.002

JD 2460998.26291: 13.928 &#177; 0.003

JD 2461000.38077: 13.708 &#177; 0.002

JD 2461001.52561: 13.364 &#177; 0.002

JD 2461002.25071: 13.430 &#177; 0.002

JD 2461002.45829: 13.501 &#177; 0.002

JD 2460967.34727: 14.566 &#177; 0.010

JD 2460991.30484: 14.653 &#177; 0.005

JD 2460993.41430: 14.683 &#177; 0.032

JD 2460997.43553: 14.403 &#177; 0.004

JD 2460998.26720: 14.320 &#177; 0.004

JD 2461000.38497: 14.092 &#177; 0.004

JD 2461001.52981: 13.726 &#177; 0.004

JD 2461002.25501: 13.726 &#177; 0.004

JD 2461002.46368: 13.726 &#177; 0.003

JD 2460967.35162: 15.292 &#177; 0.013

JD 2460991.30905: 15.261 &#177; 0.007

JD 2460993.41851: 15.309 &#177; 0.009

JD 2460997.43973: 15.025 &#177; 0.005

JD 2460998.27252: 14.926 &#177; 0.005

JD 2461000.38917: 14.707 &#177; 0.004

JD 2461001.53401: 14.314 &#177; 0.005

JD 2461002.25931: 14.398 &#177; 0.005

JD 2461002.46799: 14.497 &#177; 0.004

The full light curve measured within the scope of TELAMON can be inspected
at https://telamon.astro.uni-wuerzburg.de/sources/0112-2244 .

TELAMON uses the Effelsberg 100-m telescope to monitor the radio spectra
of very-high-energy emitting active galactic nuclei (AGN), namely TeV blazars
and candidate neutrino-associated AGN, with high-cadence high-frequency
observations every ~3 weeks and at multiple frequencies up to 44 GHz.

Optical comparison stars and their magnitudes were taken from Ciprini et
al. 2024, A&A, 400, 487-498.

These measurements are carried out as part of the AGN monitoring program
of the Naturwissenschaftliches Labor fuer Schueler am Friedrich-Koenig-Gymnasium
(FKG), Universitaet Wuerzburg, and TU Dortmund. Data were acquired through
Bessel filters (Chroma) with the 0.5m CDK-astrograph (PlaneWave) and a
Moravian C4-16000EC camera at the school / university observatory Hans-Haffner-Sternwarte
in Hettstadt.

Hans-Haffner-Sternwarte: https://schuelerlabor-wuerzburg.de/en/observatory/
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Password Certification: Dominik Elsaesser (dominik.elsaesser@tu-dortmund.de)
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