The optical behaviour of BL Lacertae at its maximum brightness levels: a blend of geometry and energetics
C. M. RaiteriINAF, Osservatorio Astrofisico di Torino , via Osservatorio 20, I-10025 Pino Torinese, ItalyM. VillataINAF, Osservatorio Astrofisico di Torino , via Osservatorio 20, I-10025 Pino Torinese, ItalySvetlana G. JorstadAstronomical Institute, St Petersburg State University , 28 Universitetskiy Prospekt, Peterhof, St Petersburg 198504, RussiaAlan P. MarscherInstitute for Astrophysical Research, Boston University , 725 Commonwealth Avenue, Boston, MA 02215, USAJ. A. Acosta‐PulidoDepartamento de Astrofisica, Universidad de La Laguna (ULL) , E-38206 Tenerife, SpainD. CarosatiEPT Observatories, Tijarafe , La Palma, E-38780, SpainWen-Ping ChenInstitute of Astronomy, National Central University , Taoyuan 32001, TaiwanM. D. JonerDepartment of Physics and Astronomy, Brigham Young University , N283 ESC, Provo, UT 84602, USAS. O. KurtanidzeAbastumani Observatory , Mt. Kanobili, 0301 Abastumani, GeorgiaC. LoreyAlessandro MarchiniAstronomical Observatory, Department of Physical Sciences, Earth and Environment, University of Siena , Siena, I-53100, ItalyKatsura MatsumotoAstronomical Institute, Osaka Kyoiku University , Kashiwara, 582-8582, JapanD. O. MirzaqulovUlugh Beg Astronomical Institute , Astronomy Street 33, Tashkent 100052, UzbekistanС. С. СавченкоAstronomical Institute, St Petersburg State University , 28 Universitetskiy Prospekt, Peterhof, St Petersburg 198504, RussiaA. StrigachevInstitute of Astronomy and National Astronomical Observatory, Bulgarian Academy of Sciences , 72 Tsarigradsko Shosse Blvd., 1784 Sofia, BulgariaO. VinceAstronomical Observatory , Volgina 7, 11060 Belgrade, SerbiaPietro AcetiDepartment of Aerospace Science and Technology, Politecnico di Milano , Milano, I-20156, ItalyG. ApolonioDepartment of Physics and Astronomy, Brigham Young University , N283 ESC, Provo, UT 84602, USAC. ArenaGruppo Astrofili Catanesi (GAC) , Catania, I-95125, ItalyA. A. ArkharovPulkovo Observatory , St Petersburg 196140, RussiaR. BachevInstitute of Astronomy and National Astronomical Observatory, Bulgarian Academy of Sciences , 72 Tsarigradsko Shosse Blvd., 1784 Sofia, BulgariaN BaderM. BanfiG. BonnoliINAF, Osservatorio Astronomico di Brera , via E. Bianchi 46, I-23807 Merate, ItalyГ. А. БорманCrimean Astrophysical Observatory of the Russian Academy of Sciences , P/O Nauchny 298409, RussiaV. BozhilovDepartment of Astronomy, Faculty of Physics, Sofia University ‘St. Kliment Ohridski’ , 5 James Bourchier Blvd., BG-1164 Sofia, BulgariaL. F. BrownDepartment of Physics, Astronomy and Geophysics, Connecticut College , New London, CT 06320, USAW CarbonellDepartment of Physics, Astronomy and Geophysics, Connecticut College , New London, CT 06320, USAM. I. CarnereroINAF, Osservatorio Astrofisico di Torino , via Osservatorio 20, I-10025 Pino Torinese, ItalyG. DamljanovićAstronomical Observatory , Volgina 7, 11060 Belgrade, SerbiaVinit DhimanAryabhatta Research Institute of Observational Sciences (ARIES) , Manora Peak, Nainital 263001, IndiaSh. A. EhgamberdievNational University of Uzbekistan , Tashkent 100174, UzbekistanD. ElsäesserFakultät Physik, TU Dortmund , D-44227 Dortmund, GermanyM. FeigeD. GabelliniD. GalánDepartamento de Astrofisica, Universidad de La Laguna (ULL) , E-38206 Tenerife, SpainG GalliHaritma GaurAryabhatta Research Institute of Observational Sciences (ARIES) , Manora Peak, Nainital 263001, IndiaK. GazeasSection of Astrophysics, Astronomy and Mechanics, Department of Physics, National and Kapodistrian University of Athens , GR-15784 Zografos, Athens, GreeceT. S. GrishinaAstronomical Institute, St Petersburg State University , 28 Universitetskiy Prospekt, Peterhof, St Petersburg 198504, RussiaAlok C. GuptaAryabhatta Research Institute of Observational Sciences (ARIES) , Manora Peak, Nainital 263001, IndiaV. A. Hagen-ThornAstronomical Institute, St Petersburg State University , 28 Universitetskiy Prospekt, Peterhof, St Petersburg 198504, RussiaM. K. HallumInstitute for Astrophysical Research, Boston University , 725 Commonwealth Avenue, Boston, MA 02215, USAMurdock HartInstitute for Astrophysical Research, Boston University , 725 Commonwealth Avenue, Boston, MA 02215, USAK. HasudaDepartment of Physical Science, Aoyama Gakuin University , 5-10-1 Fuchinobe, Chuo-ku, Sagamihara-shi, Kanagawa 252-5258, JapanK HeidemannB HorstW-J HouInstitute of Astronomy, National Central University , Taoyuan 32001, TaiwanS. IbryamovDepartment of Physics and Astronomy, Faculty of Natural Sciences, University of Shumen , 115 Universitetska Str, 9712 Shumen, BulgariaR. Z. IvanidzeAbastumani Observatory , Mt. Kanobili, 0301 Abastumani, GeorgiaM. D. JovanovićAstronomical Observatory , Volgina 7, 11060 Belgrade, SerbiaG. N. KimeridzeAbastumani Observatory , Mt. Kanobili, 0301 Abastumani, GeorgiaShubham KishoreAryabhatta Research Institute of Observational Sciences (ARIES) , Manora Peak, Nainital 263001, IndiaS. A. KlimanovPulkovo Observatory , St Petersburg 196140, RussiaE. N. KopatskayaAstronomical Institute, St Petersburg State University , 28 Universitetskiy Prospekt, Peterhof, St Petersburg 198504, RussiaO. M. KurtanidzeAbastumani Observatory , Mt. Kanobili, 0301 Abastumani, GeorgiaPankaj KushwahaAryabhatta Research Institute of Observational Sciences (ARIES) , Manora Peak, Nainital 263001, IndiaDavid J. LaneAbbey Ridge Observatory , CanadaЕ. Г. ЛарионоваAstronomical Institute, St Petersburg State University , 28 Universitetskiy Prospekt, Peterhof, St Petersburg 198504, RussiaS. LeoniniH. C. LinInstitute of Astronomy, National Central University , Taoyuan 32001, TaiwanK. MannheimHans-Haffner-Sternwarte, Naturwissenschaftliches Labor für Schüler am FKG, Friedrich-Koenig-Gymnasium , D-97082 Würzburg, GermanyG. MarinoGruppo Astrofili Catanesi (GAC) , Catania, I-95125, ItalyM. MinevDepartment of Astronomy, Faculty of Physics, Sofia University ‘St. Kliment Ohridski’ , 5 James Bourchier Blvd., BG-1164 Sofia, BulgariaA ModaressiDepartment of Physics, Astronomy and Geophysics, Connecticut College , New London, CT 06320, USAD. A. MorozovaAstronomical Institute, St Petersburg State University , 28 Universitetskiy Prospekt, Peterhof, St Petersburg 198504, RussiaF. MortariS. V. NazarovCrimean Astrophysical Observatory of the Russian Academy of Sciences , P/O Nauchny 298409, RussiaM. G. NikolashviliAbastumani Observatory , Mt. Kanobili, 0301 Abastumani, GeorgiaJorge Otero-SantosDepartamento de Astrofisica, Universidad de La Laguna (ULL) , E-38206 Tenerife, SpainE. OvcharovDepartment of Astronomy, Faculty of Physics, Sofia University ‘St. Kliment Ohridski’ , 5 James Bourchier Blvd., BG-1164 Sofia, BulgariaR. PapiniV. PintérDepartment of Physics and Astronomy, Aarhus University , Munkegade 120, DK-8000 Aarhus C, DenmarkC A PriviteraAstronomical Observatory, Department of Physical Sciences, Earth and Environment, University of Siena , Siena, I-53100, ItalyT. PursimoDepartment of Physics and Astronomy, Aarhus University , Munkegade 120, DK-8000 Aarhus C, DenmarkDavid ReinhartJason L. RobertsDepartment of Physics and Astronomy, Brigham Young University , N283 ESC, Provo, UT 84602, USAF. D. RomanovAAVSO Observer , RussiaK RosenlehnerT. SakamotoDepartment of Physical Science, Aoyama Gakuin University , 5-10-1 Fuchinobe, Chuo-ku, Sagamihara-shi, Kanagawa 252-5258, JapanF. SalvaggioGruppo Astrofili Catanesi (GAC) , Catania, I-95125, ItalyK SchochE. SemkovInstitute of Astronomy and National Astronomical Observatory, Bulgarian Academy of Sciences , 72 Tsarigradsko Shosse Blvd., 1784 Sofia, BulgariaJ SeufertD. N. ShakhovskoyCrimean Astrophysical Observatory of the Russian Academy of Sciences , P/O Nauchny 298409, RussiaL. A. SiguaAbastumani Observatory , Mt. Kanobili, 0301 Abastumani, GeorgiaChandra B. SinghDepartment of Physics, Astronomy and Geophysics, Connecticut College , New London, CT 06320, USAR. SteinekeM. StojanovićAstronomical Observatory , Volgina 7, 11060 Belgrade, SerbiaTushar TripathiAryabhatta Research Institute of Observational Sciences (ARIES) , Manora Peak, Nainital 263001, IndiaYu. V. TroitskayaAstronomical Institute, St Petersburg State University , 28 Universitetskiy Prospekt, Peterhof, St Petersburg 198504, RussiaI. S. TroitskiyAstronomical Institute, St Petersburg State University , 28 Universitetskiy Prospekt, Peterhof, St Petersburg 198504, RussiaAn-Li TsaiInstitute of Astronomy, National Central University , Taoyuan 32001, TaiwanA. ValchevaDepartment of Astronomy, Faculty of Physics, Sofia University ‘St. Kliment Ohridski’ , 5 James Bourchier Blvd., BG-1164 Sofia, BulgariaA. A. VasilyevAstronomical Institute, St Petersburg State University , 28 Universitetskiy Prospekt, Peterhof, St Petersburg 198504, RussiaK VrontakiSection of Astrophysics, Astronomy and Mechanics, Department of Physics, National and Kapodistrian University of Athens , GR-15784 Zografos, Athens, GreeceZ. R. WeaverInstitute for Astrophysical Research, Boston University , 725 Commonwealth Avenue, Boston, MA 02215, USAJ H F WooleyDepartment of Physics and Astronomy, Brigham Young University , N283 ESC, Provo, UT 84602, USAE. ZaharievaDepartment of Astronomy, Faculty of Physics, Sofia University ‘St. Kliment Ohridski’ , 5 James Bourchier Blvd., BG-1164 Sofia, BulgariaА. В. ЖовтанCrimean Astrophysical Observatory of the Russian Academy of Sciences , P/O Nauchny 298409, Russia
ABI
Аннотация
ABSTRACT In 2021 BL Lacertae underwent an extraordinary activity phase, which was intensively followed by the Whole Earth Blazar Telescope (WEBT) Collaboration. We present the WEBT optical data in the BVRI bands acquired at 36 observatories around the world. In mid-2021 the source showed its historical maximum, with R = 11.14. The light curves display many episodes of intraday variability, whose amplitude increases with source brightness, in agreement with a geometrical interpretation of the long-term flux behaviour. This is also supported by the long-term spectral variability, with an almost achromatic trend with brightness. In contrast, short-term variations are found to be strongly chromatic and are ascribed to energetic processes in the jet. We also analyse the optical polarimetric behaviour, finding evidence of a strong correlation between the intrinsic fast variations in flux density and those in polarization degree, with a time delay of about 13 h. This suggests a common physical origin. The overall behaviour of the source can be interpreted as the result of two mechanisms: variability on time-scales greater than several days is likely produced by orientation effects, while either shock waves propagating in the jet, or magnetic reconnection, possibly induced by kink instabilities in the jet, can explain variability on shorter time-scales. The latter scenario could also account for the appearance of quasi-periodic oscillations, with periods from a few days to a few hours, during outbursts, when the jet is more closely aligned with our line of sight and the time-scales are shortened by relativistic effects.
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