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

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 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.

arXiv comment15 pages, 16 figures, submitted to MNRAS

References in corpus (66)

• Unified Schemes for Radio-Loud Active Galactic Nuclei
• Multi-messenger observations of a flaring blazar coincident with high-energy neutrino IceCube-170922A
• Neutrino emission from the direction of the blazar TXS 0506+056 prior to the IceCube-170922A alert
• Relativistic Jets in Active Galactic Nuclei
• Leptonic and Hadronic Modeling of Fermi-Detected Blazars
• Polarimetric Observations of 15 Active Galactic Nuclei at High Frequencies: Jet Kinematics from Bimonthly Monitoring with the Very Long Baseline Array
• Turbulent, Extreme Multi-Zone Model for Simulating Flux and Polarization Variability in Blazars
• Probing the Inner Jet of the Quasar PKS 1510-089 with Multi-waveband Monitoring during Strong Gamma-ray Activity
• Formation of Precessing Jets by Tilted Black-hole Discs in 3D General Relativistic MHD Simulations
• Instability of toroidal magnetic field in jets and plerions
• Internal shocks in the jets of radio-loud quasars
• Relativistic Jets Shine through Shocks or Magnetic Reconnection?
• Generating artificial light curves: Revisited and updated
• Blazar spectral variability as explained by a twisting inhomogeneous jet
• The WEBT BL Lac Campaign 2000
• High Resolution 3D Relativistic MHD Simulations of Jets
• Periodic Oscillations in the Intra-day Optical Light Curves of the Blazar S5 0716+714
• A method for the estimation of the significance of cross-correlations in unevenly sampled red-noise time series
• The WEBT BL Lacertae Campaign 2001 and its extension
• Results of WEBT, VLBA and RXTE monitoring of 3C 279 during 2006-2007
• Variability of Active Galactic Nuclei
• The awakening of BL Lacertae: observations by Fermi, Swift, and the GASP-WEBT
• The Outburst of the Blazar S5 0716+71 in 2011 October: Shock in a Helical Jet
• RoboPol: First season rotations of optical polarization plane in blazars
• Catching the Radio Flare in CTA 102 III. Core-Shift and Spectral Analysis
• Dissecting the regions around IceCube high-energy neutrinos: growing evidence for the blazar connection
• RoboPol: Connection between optical polarization plane rotations and gamma-ray flares in blazars
• Kink instabilities in jets from rotating magnetic fields
• Systematic search for gamma-ray periodicity in active galactic nuclei detected by the Fermi Large Area Telescope
• A 34.5 day quasi-periodic oscillation in gamma-ray emission from the blazar PKS 2247-131
• The correlated optical and radio variability of BL Lacertae. WEBT data analysis 1994-2005
• Anatomy of helical relativistic jets: The case of S5 0836+710
• The radio delay of the exceptional 3C 454.3 outburst. Follow-up WEBT observations in 2005-2006
• A multi-wavelength polarimetric study of the blazar CTA 102 during a Gamma-ray flare in 2012
• Large-Amplitude Blazar Polarization Angle Swing as a Signature of Magnetic Reconnection
• Structure and flux variability in the VLBI jet of BL Lacertae during the WEBT campaigns (1995--2004)
• Variability of the blazar 4C 38.41 (B3 1633+382) from GHz frequencies to GeV energies
• The long-term optical spectral variability of BL Lacertae
• Kink Instabilities In Relativistic Jets Can Drive Quasi-Periodic Radiation Signatures
• Exceptional outburst of the blazar CTA 102 in 2012: The GASP-WEBT campaign and its extension
• Kink-driven magnetic reconnection in relativistic jets: consequences for X-ray polarimetry of BL Lacs
• Multiwavelength Variability Signatures of Relativistic Shocks in Blazar Jets
• Quasi-periodic behaviour in the optical and γ-ray light curves of blazars 3C 66A and B2 1633+38
• The dual nature of blazar fast variability. Space and ground observations of S5 0716+714
• Multiwavelength behaviour of the blazar 3C279: decade-long study from $\gamma$-ray to radio
• Optical EVPA rotations in blazars: testing a stochastic variability model with RoboPol data
• Resolving the inner parsec of the blazar J1924-2914 with the Event Horizon Telescope
• Modeling Helical Structures in Relativistic Jets
• Detection of a quasi-periodic oscillation in gamma-ray light curve of the high redshift blazar B2 1520+31
• The nature of optical and near-infrared variability of BL Lacertae
• Radiation and Polarization Signatures from Magnetic Reconnection in Relativistic Jets--I. A Systematic Study
• Multiwavelength behaviour of the blazar OJ 248 from radio to γ-rays
• The complex variability of blazars: Time-scales and periodicity analysis in S4 0954+65
• Multiwavelength periodicity search in a sample of $\gamma$-ray bright blazars
• Plasmoid statistics in relativistic magnetic reconnection
• Synchrotron emission from the blazar PG 1553+113. An analysis of its flux and polarization variability
• Unravelling the Innermost Jet Structure of OJ 287 with the First GMVA+ALMA Observations
• Intraday Optical Variability of BL Lacertae
• Detection of a quasi-periodic oscillation in the optical light curve of the remarkable blazar AO 0235+164
• Multi-Wavelength Photometric and Polarimetric Observations of the Outburst of 3C 454.3 in Dec. 2009
• Stochastic model of optical variability of BL Lacertae
• The Time-Dependent Distribution of Optical Polarization Angle Changes in Blazars
• Numerical Analysis of Long-term Variability of AGN Jets through RMHD Simulations
• Study of correlation between optical flux and polarization variations in BL Lac objects
• Polarimetric Properties of Blazars Caught by the WEBT
• The microvariability and wavelength dependence of polarization degree/angle of BL Lacertae in the outburst 2020 to 2021

Cited by in corpus (9)

• Intra-night optical flux and polarization variability of BL~Lacertae during its 2020 $-$ 2021 high state
• Optical Quasi-periodic Oscillations in the TESS light curves of three blazars
• A wiggling filamentary jet at the origin of the blazar multi-wavelength behaviour
• Optical variability of the blazar 3C 371: from minute to year timescales
• Testing particle acceleration in blazar jets with continuous high-cadence optical polarization observations
• Multiband Optical Variability of the Blazar 3C 454.3 on Diverse Timescales
• Investigating the origin of optical flares from the TeV blazar S4 0954+65
• Multiband optical variability on diverse timescales of the blazar Ton 599 from 2011 to 2023
• Detectability of Polarized Gamma-ray Emission from Blazar Flares with COSI