Title: Evolution of the recent optical flare of the blazar BL Lacertae

ATel #16830; Alessandro Marchini (Astronomical Observatory, University of Siena - Italy); Leonardo Stiaccini, Gaia Verna (Department of Physical Sciences, Earth and Environment (DSFTA), University of Siena - Italy); Simone Leonini, Massimo Conti, Paolo Rosi, Luz Marina Tinjaca Ramirez (Montarrenti Observatory, Sovicille (Siena - Italy)); Ivo Peretto, Giovanni Furlato, Stefano Lora (MarSEC, Marana di Crespadoro (Vicenza - Italy)); Pietro Aceti, Massimo Banfi (Seveso Astronomical Observatory, Seveso (Monza e Brianza - Italy)); Giacomo Bonnoli (INAF - Brera Astronomical Observatory)
on 26 Sep 2024; 08:43 UT
Credential Certification: Giacomo Bonnoli (giacomo.bonnoli@inaf.it)

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The blazar BL Lacertae (RA: 22 02 43.29 Dec: +42 16 39.98 J2000.0) at redshift z=0.069 (Miller, J. S., French, H. B., & Hawley, S. A. 1978, ApJ, 219, L85) is one of the main targets of our joint blazar monitoring program. The observations have been lately intensified in response to ATels#16806,#16810, which reported an optical flare of the source. The main outcome of our follow-up is that the outburst phase is still ongoing, with other noticeable brightness maxima grazing R=12, such as R=12.07(0.01) measured on 2024 Sep 20.839. The following table reports a selection of our R (Johnson-Cousins) measurements:

Civil Date(UT) Mag (dMag) Observatory
2024 Sep. 01.84113 13.12 (0.01) MarSEC
2024 Sep. 02.89441 12.76 (0.01) MarSEC
2024 Sep. 03.83179 12.74 (0.04) MarSEC
2024 Sep. 03.93861 12.94 (0.01) Montarrenti
2024 Sep. 04.87553 13.30 (0.01) Montarrenti
2024 Sep. 06.88051 12.58 (0.01) Montarrenti
2024 Sep. 06.91549 12.64 (0.02) MarSEC
2024 Sep. 06.94699 12.67 (0.01) Montarrenti
2024 Sep. 07.91110 11.91 (0.01) MarSEC
2024 Sep. 09.92433 12.56 (0.01) MarSEC
2024 Sep. 10.86061 12.42 (0.01) MarSEC
2024 Sep. 10.93328 12.38 (0.01) Montarrenti
2024 Sep. 11.81968 12.15 (0.01) Montarrenti
2024 Sep. 13.04028 12.22 (0.01) Seveso
2024 Sep. 13.96506 12.35 (0.01) Montarrenti
2024 Sep. 15.87174 12.69 (0.01) Seveso
2024 Sep. 15.89347 12.72 (0.01) Montarrenti
2024 Sep. 15.93020 12.77 (0.01) Seveso
2024 Sep. 17.81762 12.73 (0.01) MarSEC
2024 Sep. 20.83947 12.07 (0.01) Montarrenti
2024 Sep. 21.82896 12.94 (0.07) MarSEC
2024 Sep. 21.90111 12.93 (0.01) Montarrenti


Quoted uncertainties are statistical only.

The full dataset, comprising R-band time series with a resolution of a few minutes, and continuously covering up to few hours, as well as interleaved observations in the V (Montarrenti, Seveso) and in the B, I filters (Montarrenti) is under further study and can be made available upon request. A plot is linked at the bottom of this ATel.

Any enquiry on these observations can be addressed either to Alessandro Marchini (marchini@unisi.it) or to Giacomo Bonnoli (giacomo.bonnoli@inaf.it). We will continue monitoring the source in the following nights. Multi-wavelength follow-up is strongly encouraged.

We acknowledge excellent scientific cooperation with, and valuable support from, the WEBT Collaboration.

This monitoring has been carried out with the 0.30 m telescope at the Astronomical Observatory of the University of Siena (K54), the 0.53 m telescope at Montarrenti Observatory (C88), the 0.30 m telescope at Seveso Astronomical Observatory (C24), and the 0.36 m telescope at MarSEC.

Brief descriptions of the instrumental setups used for the reported observations are available at the official webpage of the Astronomical Observatory of the University of Siena, on the website of the Unione Astrofili Senesi, on the website of the MarSEC and on the page of the Seveso Astronomical Observatory within the website of the Whole Earth Blazar Telescope .

Optical multi-band photometric light-curve of the BL Lac flare in September 2024