BL Lac still in optical high state

ATel #14854; M. Minev (Sofia University; IA and NAO, Bulgaria), A. Valcheva (Sofia University, Bulgaria), A. Kostov (IA and NAO, BAS, Bulgaria), R. Vasileva, R. Mitrev
on 12 Aug 2021; 12:39 UT
Credential Certification: Evgeni Ovcharov (evgeni@phys.uni-sofia.bg)

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We perform optical monitoring of the flaring blazar BL Lacertae (ICRS coords. (ep=J2000): RA 22 02 43.291 Dec +42 16 39.978). It is still in a high optical state (as reported in Atel #14820) with slightly decreasing flux in the last five days and rapid intranight flux variability. The maximum R-band magnitude we have measured is 11.28 ± 0.01 mag on 2021 Aug 06.934 UT (JD 2459433.431). Our whole night monitoring from 2021 Aug 9.834 (20:01 UT – JD 2459436.3341) to 2021 Aug 10.051 (01:13 UT – JD 2459436.5509) show increase in R-band brightness of 0.27 ± 0.04 mag (from 11.70 ± 0.03 mag to 11.43 ± 0.03 mag) and in V-band of 0.29 ± 0.04 mag (from 12.29 ± 0.03 mag to 12.00 ± 0.03 mag).

Here we report the following brightness values:

B - band magnitudes:
2021 Aug 06.941 (JD 2459433.441) 12.72 ± 0.02 mag
2021 Aug 08.907 (JD 2459435.407) 13.11 ± 0.02 mag
V - band magnitudes:
2021 Aug 06.937 (JD 2459433.437) 11.86 ± 0.01 mag
2021 Aug 08.898 (JD 2459435.398) 12.23 ± 0.02 mag
2021 Aug 09.830 (JD 2459436.330) 12.29 ± 0.03 mag
2021 Aug 10.053 (JD 2459436.553) 12.00 ± 0.03 mag
2021 Aug 10.952 (JD 2459437.452) 12.28 ± 0.03 mag
2021 Aug 11.047 (JD 2459437.547) 12.32 ± 0.03 mag
R - band magnitudes:
2021 Aug 06.931 (JD 2459433.431) 11.28 ± 0.01 mag
2021 Aug 08.893 (JD 2459435.393) 11.62 ± 0.02 mag
2021 Aug 09.834 (JD 2459436.334) 11.70 ± 0.03 mag
2021 Aug 10.051 (JD 2459436.551) 11.43 ± 0.03 mag
2021 Aug 10.958 (JD 2459437.458) 11.68 ± 0.03 mag
2021 Aug 11.042 (JD 2459437.542) 11.71 ± 0.03 mag
I - band magnitudes:
2021 Aug 06.934 (JD 2459433.434) 10.55 ± 0.01 mag
2021 Aug 08.895 (JD 2459435.395) 10.88 ± 0.02 mag

Our photometric data are obtained with the 50/70-cm Schmidt telescope at Rozhen NAO, Bulgaria and 20-cm Newton telescope at the "Beli Brezi" Astronomy Summer School near Kardzhali, Bulgaria both equipped with standard Johnson-Cousins photometric filters. For the aperture photometry, we have used the specifications of the GASP list of WEBT (ahttps://www.oato.inaf.it/blazars/webt/gasp/fc/2200fc.html).

The work was partially supported by the Ministry of Education and Science of the Republic of Bulgaria, National RI Roadmap Project DO1-383/18.12.2020, NSF Bulgaria grant DN18/10-11.12.2017 and America for Bulgaria Foundation.