Strong increase of the optical brightness of BL Lac observed at the Hans-Haffner-Sternwarte

ATel #14548; Kilian Schoch, Niels Bader, Felix Hemrich, Ben Horst, Johannes Knoett, Kai Koeberlein, David Reinhart, Korbinian Rosenlehner, Lukas Schneider, Remco Steineke, Dirk Seifert, Anton Stoz, Lukas Waller, Nils Zottmann, Martin Feige, Christian Lorey (all Friedrich-Koenig-Gymnasium / Hans-Haffner-Sternwarte), Lukas Kunkel, Adrian Scherbantin, Karl Mannheim (all Universitaet Wuerzburg), Dominik Elsaesser (TU Dortmund)
on 14 Apr 2021; 15:45 UT
Credential Certification: Dominik Elsaesser (dominik.elsaesser@tu-dortmund.de)

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BL Lacertae (ICRS coords. (ep=J2000): RA 22 02 43.291 Dec +42 16 39.978) is the prototypical BL Lac object and has been showing exceptional activity for months.
After this object had reached historic optical brightness values in midsummer and autumn 2020 (ATel#13930, ATel#13956, ATel# 13958, ATel# 14065, ATel# 14069, ATel #14081, ATel #14096) and again in the beginning of this year (ATel #14328 & #14329, ATel #14334, ATel #14343, ATel #14356, ATel #14467), we were able to measure another strong increase in the brightness of BL Lac in the optical range in the last night.
Our optical measurements were taken using Bessel filters. Photometry was done using the specifications of the GASP list of WEBT (https://www.oato.inaf.it/blazars/webt/gasp/fc/2200fc.html).
Since the last measurements 4 nights ago (due to the wintry weather in Germany no further measurements were possible in between) the brightness in the R-band increased by about a magnitude. With a brightness of 11.78 this outburst comes close to the maximum from autumn 2020 (ATel #14081: MJD 59127.450; R=11.73) and the all-time high from the beginning of this year (ATel #14328 & #14329: 2021 Jan. 17.87; R=11.45).

R - band magnitudes:

JD 2459305.5237: 12.88 +/- 0.01
JD 2459308.5557: 12.81 +/- 0.01
JD 2459309.5283: 12.91 +/- 0.01
JD 2459313.5594: 12.83 +/- 0.01
JD 2459318.5210: 11.80 +/- 0.01
JD 2459318.5494: 11.78 +/- 0.01
JD 2459318.5548: 11.78 +/- 0.01

V - band magnitudes:

JD 2459305.5256: 13.45 +/- 0.02
JD 2459308.5575: 13.43 +/- 0.01
JD 2459309.5302: 13.52 +/- 0.02
JD 2459313.5612: 13.43 +/- 0.02
JD 2459318.5228: 12.35 +/- 0.01
JD 2459318.5512: 12.32 +/- 0.01
JD 2459318.5566: 12.33 +/- 0.01

B - band magnitudes:

JD 2459305.5274: 14.69 +/- 0.05
JD 2459308.5593: 14.56 +/- 0.02
JD 2459309.5321: 14.61 +/- 0.06
JD 2459313.5630: 14.59 +/- 0.07
JD 2459318.5246: 13.44 +/- 0.02
JD 2459318.5530: 13.38 +/- 0.02
JD 2459318.5584: 13.39 +/- 0.02


These measurements are carried out as part of the long-term AGN monitoring program of the Naturwissenschaftliches Labor fuer Schueler am Friedrich-Koenig-Gymnasium (FKG), the Universitaet Wuerzburg, and TU Dortmund with the 0.5m CDK-astrograph at the school and university observatory Hans-Haffner-Sternwarte in D-97265 Hettstadt (Germany).

Naturwissenschaftliches Labor fuer Schueler am Friedrich-Koenig-Gymnasium (FKG)