The long-term optical spectral variability of BL Lacertae

Abstract

We present the results from a study of the long-term optical spectral variations of BL Lacertae, using the long and well-sampled B and R-band light curves of the Whole Earth Blazar Telescope (WEBT) collaboration, binned on time intervals of 1 day. The relation between spectral slope and flux (the spectrum gets bluer as the source flux increases) is well described by a power-law model, although there is significant scatter around the best-fitting model line. To some extent, this is due to the spectral evolution of the source (along well-defined loop-like structures) during low-amplitude events, which are superimposed on the major optical flares, and evolve on time scales of a few days. The "bluer-when-brighter" mild chromatism of the long-term variations of the source can be explained if the flux increases/decreases faster in the B than in the R band. The B and R-band variations are well correlated, with no significant, measurable delays larger than a few days. On the other hand, we find that the spectral variations lead those in the flux light curves by ~ 4 days. Our results can be explained in terms of Doppler factor variations due to changes in the viewing angle of a curved and inhomogeneous emitting jet.

7 pages, 5 figures, accepted for publication in A&A

References in corpus (4)

• The unprecedented optical outburst of the quasar 3C 454.3. The WEBT campaign of 2004-2005
• Testing the inverse-Compton catastrophe scenario in the intra-day variable blazar S5 0716+71. I. Simultaneous broadband observations during November 2003
• Multifrequency variability of the blazar AO 0235+164. The WEBT campaign in 2004-2005 and long-term SED analysis
• Optical multi-band observations of BL Lacertae during the outburst of 2005

Cited by in corpus (34)

• Photopolarimetric Monitoring of Blazars in the Optical and Near-Infrared Bands with the Kanata Telescope. I. Correlations between Flux, Color, and Polarization
• Results of WEBT, VLBA and RXTE monitoring of 3C 279 during 2006-2007
• The awakening of BL Lacertae: observations by Fermi, Swift, and the GASP-WEBT
• A new activity phase of the blazar 3C 454.3. Multifrequency observations by the WEBT and XMM-Newton in 2007-2008
• The correlated optical and radio variability of BL Lacertae. WEBT data analysis 1994-2005
• The dual nature of blazar fast variability. Space and ground observations of S5 0716+714
• The WEBT Campaign on the Intermediate BL Lac Object 3C66A in 2007-2008
• Rapid optical variability in blazar S5 0716+71 during 2010 March
• Two Flares with One Shock: the Interesting Case of 3C 454.3
• The nature of optical and near-infrared variability of BL Lacertae
• Long term variability and correlation study of the blazar 3C 454.3 in radio, NIR and optical wavebands
• Simultaneous MITSuME gRI monitoring of S5 0716+714
• Multi-band optical-NIR variability of blazars on diverse timescales
• Multicolor Optical Monitoring of the Blazar S5 0716+714 from 2017 to 2019
• Correlation between optical and $\gamma$-ray flux variations in bright flat spectrum radio quasars
• Peak of spectral energy distribution play an important role in intra-day variability of Blazars?
• Intraday Optical Variability of BL Lacertae
• Multi-color optical monitoring of ten blazars from 2005 to 2011
• Long term optical and infrared variability characteristics of Fermi Blazars
• The optical behaviour of BL Lacertae at its maximum brightness levels: a blend of geometry and energetics
• Multi-band behaviour of the TeV blazar PG 1553+113 in optical range on diverse timescales
• The variability of blazars throughout the electromagnetic spectrum
• Mid-infrared Variability of gamma-ray emitting blazars
• Correlation between optical and $\gamma$-ray flux variations in BL Lacs
• Optical Spectral Variations of a Large Sample of Fermi Blazars
• Optical flux and spectral variability of BL Lacertae during its historical high outburst in 2020
• Analysis of the intra-night variability of BL Lacertae during its August 2020 flare
• Optical Microvariability in Quasars: Spectral Variability
• Intra-day variability of BL Lacertae from 2016 to 2018
• Multi-band Optical Variability of the TeV Blazar PG 1553+113 in 2019
• Study of variability in long-term multiwavelength optical lightcurves of blazar AO 0235+164
• Optical monitoring in southern blazars. Analysis of variability and spectral colour behaviours
• Optical Monitoring and Long-term Optical Spectral Variability of BL Lacertae Object S5 0716+714
• Long-term Mid-infrared Color Variations of Narrow-Line Seyfert 1 Galaxies