A strong optical flare from the Blazar Ton 599

ATel #16488; E. Ankara, F. Kaplan, N. Boutter, N. Bader, J. Seufert, N. Mildner, G. Rinke, S. Duerr, O. Koppitz, M. Heidemann, D. Kuberek, G. Manhalter, O. Domann, R. Steineke, S. Huesam, M. Feige, D. Reinhart, C. Lorey (Hans-Hafner-Sternwarte – Friedrich-Koenig-Gymnasium, Wuerzburg, Germany) K. Mannheim, A. Scherbantin, M. Kadler, F. Eppel, F. Roesch, J. Hessdoerfer (all University of Wurzburg, Germany) D. Elsaesser (TU Dortmund, Germany) A. C. Gupta (Aryabhatta Research Institute of Observational Science (ARIES), Manora Peak, Nainital 263001, India)
on 27 Feb 2024; 14:06 UT
Credential Certification: Dominik Elsaesser (dominik.elsaesser@tu-dortmund.de)

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The blazar Ton 599 (4C 29.45, PKS 1156+295; ICRS coordinates (at epoch = J2000): RA = 11 59 31.834, Dec = +29 14 43.827, z = 0.725) is a flat spectrum radio quasar (FSRQ). It reached an exceptionally high brightness level in the optical bands on the night of February 26, 2024.
The source has been in a relatively low to intermediate flux state during the last year. While the object has been showing an R-band optical magnitude between 15.5 and 17 mag in recent months, its brightness has risen strongly since the beginning of February this year and reached an exceptionally high magnitude of R = 13.289 mag in the night of February 26, 2024. This optical brightness is comparable to that achieved by this object just over a year ago, with values around 13.3 mag (R-band) (ATel #15870; ATel #15875; ATel #15893).

A year ago, the strong activity of this source was first noticed in the X-ray and gamma-ray energies (ATel # 15854; ATel #15853; ATel # 15859), and also in the radio range. The flux-density in the 1.3 mm radio band increased to about 5 Jy during this time (ATel #15873). In the 7 mm and 14 mm radio bands, Ton 599 showed a strong flux-density increase to 6.7 Jy at the same time as the optical maximum (ATel #15872).
According to recent TELAMON observations on February 4, 2024, between 19 GHz and 44 GHz, the source is currently not in an elevated radio state. Additional measurements in this frequency band will continue over the next months.

Here we report the following preliminary R-band magnitudes:

JD 2460273,6680: 16,986 ± 0,046
JD 2460297,4776: 16,054 ± 0,021
JD 2460309,6043: 16,141 ± 0,047
JD 2460318,5699: 16,347 ± 0,018
JD 2460319,5048: 16,253 ± 0,020
JD 2460320,5825: 16,057 ± 0,015
JD 2460336,5496: 15,520 ± 0,022
JD 2460337,4646: 15,955 ± 0,032
JD 2460338,5636: 15,856 ± 0,023
JD 2460353,6215: 14,828 ± 0,006
JD 2460365,6646: 13,657 ± 0,011
JD 2460366,2700: 13,321 ± 0,017
JD 2460366,2838: 13,290 ± 0,015
JD 2460366,3029: 13,289 ± 0,017

This object previously reached a similar optical brightness level in June 2022 with R = 13.39 mag (ATel #15441) and in the first half of 2021 with values of R = 14.9 to 13.5 mag (ATel #14353; #14391; #14547; #14696; #14727; #14775).

We encourage further multi-wavelength observations of this source.
The TELAMON programme 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.
The optical data were acquired through a Bessel R filter (Chroma), and observations were conducted in the framework provided by the WEBT Collaboration (https://www.oato.inaf.it/blazars/webt). The photometric sequence of WEBT for this source can be found under: https://www.oato.inaf.it/blazars/webt/1156295-4c-29-45/
These measurements are carried out as part of the long-term AGN monitoring programme of the Naturwissenschaftliches Labour fuer Schueler am Friedrich-Koenig-Gymnasium (FKG), the Universitaet Wuerzburg, and TU Dortmund with the 0.5m CDK-astrograph and a Moravian G4-16000 camera at the school and university observatory Hans-Haffner-Sternwarte in D-97265 Hettstadt-Germany (https://schuelerlabor-wuerzburg.de/en/observatory/).

Hans-Haffner-Sternwarte