Historical Optical Flaring of the BL Lac Object S2 0109+22
ATel #17508; 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)
on 28 Nov 2025; 10:14 UT
Credential Certification: Dominik Elsaesser (dominik.elsaesser@tu-dortmund.de)
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 ± 0.12) Jy at 14mm in April to (2.601 ± 0.060) Jy at 14mm and (2.62 ± 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:
R:
JD 2460951,55915: 14.564 ± 0.004
JD 2460964.35121: 14.104 ± 0.003
JD 2460967.34281: 14.311 ± 0.006
JD 2460979.40792: 13.794 ± 0.005
JD 2460991.30064: 14.236 ± 0.004
JD 2460993.41008: 14.311 ± 0.013
JD 2460997.43133: 14.005 ± 0.002
JD 2460998.26291: 13.928 ± 0.003
JD 2461000.38077: 13.708 ± 0.002
JD 2461001.52561: 13.364 ± 0.002
JD 2461002.25071: 13.430 ± 0.002
JD 2461002.45829: 13.501 ± 0.002
V:
JD 2460967.34727: 14.566 ± 0.010
JD 2460991.30484: 14.653 ± 0.005
JD 2460993.41430: 14.683 ± 0.032
JD 2460997.43553: 14.403 ± 0.004
JD 2460998.26720: 14.320 ± 0.004
JD 2461000.38497: 14.092 ± 0.004
JD 2461001.52981: 13.726 ± 0.004
JD 2461002.25501: 13.726 ± 0.004
JD 2461002.46368: 13.726 ± 0.003
B:
JD 2460967.35162: 15.292 ± 0.013
JD 2460991.30905: 15.261 ± 0.007
JD 2460993.41851: 15.309 ± 0.009
JD 2460997.43973: 15.025 ± 0.005
JD 2460998.27252: 14.926 ± 0.005
JD 2461000.38917: 14.707 ± 0.004
JD 2461001.53401: 14.314 ± 0.005
JD 2461002.25931: 14.398 ± 0.005
JD 2461002.46799: 14.497 ± 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
