TY - JOUR

T1 - The optical behaviour of BL Lacertae at its maximum brightness levels: a blend of geometry and energetics

AU - Raiteri, C M

AU - Villata, M

AU - Jorstad, S G

AU - Marscher, A P

AU - Pulido, J A Acosta

AU - Carosati, D

AU - Chen, W P

AU - Joner, M D

AU - Kurtanidze, S O

AU - Lorey, C

AU - Marchini, A

AU - Matsumoto, K

AU - Mirzaqulov, D O

AU - Savchenko, S S

AU - Strigachev, A

AU - Vince, O

AU - Aceti, P

AU - Apolonio, G

AU - Arena, C

AU - Arkharov, A

AU - Bachev, R

AU - Bader, N

AU - Banfi, M

AU - Bonnoli, G

AU - Borman, G A

AU - Bozhilov, V

AU - Brown, L F

AU - Carbonell, W

AU - Carnerero, M I

AU - Damljanovic, G

AU - Dhiman, V

AU - Ehgamberdiev, S A

AU - Elsaesser, D

AU - Feige, M

AU - Gabellini, D

AU - Galán, D

AU - Galli, G

AU - Gaur, H

AU - Gazeas, K

AU - Grishina, T S

AU - Gupta, A C

AU - Hagen-Thorn, V A

AU - Hallum, M K

AU - Hart, M

AU - Hasuda, K

AU - Heidemann, K

AU - Horst, B

AU - Hou, W-J

AU - Ibryamov, S

AU - Ivanidze, R Z

AU - Jovanovic, M D

AU - Kimeridze, G N

AU - Kishore, S

AU - Klimanov, S

AU - Kopatskaya, E N

AU - Kurtanidze, O M

AU - Kushwaha, P

AU - Lane, D J

AU - Larionova, E G

AU - Leonini, S

AU - Lin, H C

AU - Mannheim, K

AU - Marino, G

AU - Minev, M

AU - Modaressi, A

AU - Morozova, D A

AU - Mortari, F

AU - Nazarov, S V

AU - Nikolashvili, M G

AU - Santos, J Otero

AU - Ovcharov, E

AU - Papini, R

AU - Pinter, V

AU - Privitera, C A

AU - Pursimo, T

AU - Reinhart, D

AU - Roberts, J

AU - Romanov, F D

AU - Rosenlehner, K

AU - Sakamoto, T

AU - Salvaggio, F

AU - Schoch, K

AU - Semkov, E

AU - Seufert, J

AU - Shakhovskoy, D

AU - Sigua, L A

AU - Singh, C

AU - Steineke, R

AU - Stojanovic, M

AU - Tripathi, T

AU - Troitskaya, Y V

AU - Troitskiy, I S

AU - Tsai, A

AU - Valcheva, A

AU - Vasilyev, A A

AU - Vrontaki, K

AU - Weaver, Z R

AU - Wooley, J H F

AU - Zaharieva, E

AU - Zhovtan, A V

N1 - stad942

PY - 2023/6

Y1 - 2023/6

N2 - 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 polarisation 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.

AB - 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 polarisation 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.

UR - http://arxiv.org/abs/2302.10555 http://dx.doi.org/10.1093/mnras/stad942

UR - https://www.mendeley.com/catalogue/3d1909d1-a272-375b-aa1a-d3084ce8342b/

U2 - 10.1093/mnras/stad942

DO - 10.1093/mnras/stad942

M3 - Article

VL - 522

SP - 102

EP - 116

JO - Monthly Notices of the Royal Astronomical Society

JF - Monthly Notices of the Royal Astronomical Society

SN - 0035-8711

IS - 1

ER -