Exceptionally high brightness of PG 1553+113 in the optical and radio range

ATel #15915; D. Kuberek, L. Bertsch, M. Hensel, M. Kardasch, E. von Raesfeld, R. Steineke, D. Reinhart, N. Zottmann, M. Feige, C. Lorey (all Friedrich-Koenig-Gymnasium / Hans-Haffner-Sternwarte, Wuerzburg, Germany), D. Kirchner, F. Roesch, J. Hessdoerfer, P. Weber, K. Mannheim, M. Kadler, F. Eppel (all University of Wurzburg, Germany), D. Elsaesser (TU Dortmund, Germany), P. Benke, G. F. Paraschos (all MPIfR Bonn), J. Sinapius (DESY Zeuthen), S. Haemmerich (Dr. Remeis Observatory Bamberg & ECAP), A. Gokus (Washington University, St. Louis, USA), J. A. Acosta Pulido (Instituto de Astrofisica de Canarias and Universidad de La Laguna, Tenerife, Spain), N. Marchili, S. Righini, M. Giroletti (INAF, Istituto di Radioastronomia, Bologna, Italy), S. G. Jorstad, C. Pauley (Institute for Astrophysical Research, Boston University, Boston, MA, USA), C. M. Raiteri, M. Villata, and M. I. Carnerero (INAF, Osservatorio Astrofisico di Torino, Italy).
on 23 Feb 2023; 11:24 UT
Credential Certification: Dominik Elsaesser (dominik.elsaesser@tu-dortmund.de)

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The high-synchrotron-peaked BL Lac type object PG 1553+113 (RA: 15 55 43.0440 Dec: +11 11 24.3656; ICRS J2000; z approx. 0,4; cf. Danforth et al. 2010, ApJ 720, 976) reached a high state of brightness in the optical band in the last nights.
A quasi-periodic brightness oscillation with a period of about 2.2 years has been reported for this object. This oscillation is seen both in the gamma-ray range with a period of 2.18 +/- 0.08 years (cf. M. Ackermann et al. 2015, ApJL 813, L41) and in the optical range with a median period of 2.21 +/- 0.04 years (cf. A. Agarwal et al. 2022 J. Astrophys. Astr.).
We have observed a steady increase in the optical and radio bands over the last few months. In the night from Feb 21-22, 2023, the source reached the highest emission state both in the optical (R band) and radio (7mm) bands.
This matches the expectations from the claimed quasi-periodicity quite well, with the last two maxima reported in April 2021 (ATel # 14520 -gamma ray-) and April 2019 (ATel # 12695, ATel # 12649 -optical- ATel # 12645 -NIR- # 12641 -gamma ray-).


Optical: We announce the following brightness values (R band):

JD 2459931.7000: 13.783 ± 0.008 (HHO)
JD 2459945.7435: 13.560 ± 0.004 (HHO)

JD 2459984.0117: 13.233 ± 0.005 (PT)
JD 2459984.0176: 13.239 ± 0.010 (PT)

JD 2459984.6514: 13.243 ± 0.004 (HHO)
JD 2459984.9600: 13.248 ± 0.008 (PT)
JD 2459984.9756: 13.255 ± 0.010 (PT)

JD 2459985.9893: 13.279 ± 0.004 (PT)
JD 2459985.9932: 13.270 ± 0.003 (PT)
JD 2459985.9971: 13.274 ± 0.008 (PT)

JD 2459989.7188: 13.237 ± 0.005 (IAC80)
JD 2459989.7195: 13.246 ± 0.005 (IAC80)

JD 2459996.7263: 13.168 ± 0.029 (HHO)
JD 2459996.7295: 13.200 ± 0.018 (HHO)

JD 2459997.4884: 13.177 ± 0.005 (HHO)
JD 2459997.5291: 13.161 ± 0.004 (HHO)
JD 2459997.5773: 13.168 ± 0.004 (HHO)

Stations: Hans-Haffner Observatory (HHO); IAC80 Telescope (IAC80); Perkins telescope (PT).
The optical observations were conducted with a 0.82 m telescope at the Instituto de Astrofísica de Canarias (https://www.iac.es/en/observatorios-de-canarias/telescopes-and-experiments/iac80-telescope), with a 1,82 m telescope at Perkins Observatory (https://www.bu.edu/pto/) and with a 0.5 m telescope at the Hans-Haffner Observatory (https://schuelerlabor-wuerzburg.de/en/observatory/).
All data were acquired with a Rc standard filter and using the photometric sequence for this source made available by the WEBT Collaboration (https://www.oato.inaf.it/blazars/webt/1553113-pg-1553113/)

Radio: In the radio band, the 32 m radio telescope of Medicina (Italy) measured a flux density of 0.42 ± 0.02 Jy at 36mm on JD=2459997.518037.

Furthermore, the source was observed with the Effelsberg 100 m telescope as part of the TELAMON program on Feb 21, 2023. Thereby, flux densities of (405 ± 6) mJy at 20mm, (398 ± 4) mJy at 14mm and (415 ± 22) mJy at 7mm were found.
The 7mm value is the highest observed so far. All values are averaged over several sub-frequencies within the respective band. We note that the source is currently close to its historical maximum in all observed radio bands and has been rising in radio flux density since approx. mid 2021, where we observed a flux density of (274 ± 9) mJy at 14mm and (233 ± 36) mJy at 7mm on May 21, 2021.
An averaged light curve of the source is available under: telamon.astro.uni-wuerzburg.de/sources/1555-1111