A New Phase of Optical Activity of BL Lacertae in the Fall of 2024: Intra-Night Flux and Polarization Variations

TL;DR

This study analyzes the fall 2024 optical activity of BL Lacertae using high-cadence multi-band photometry and R-band polarimetry from three telescopes. It documents extreme intra-night flux variability with negligible inter-band delays, a persistent bluer-when-brighter color trend, and polarization behavior around ~10% with variable position angles. Using variability timescales and cross-band analysis, the paper constrains the emitting-region size to $R\lesssim 10^{15}$ cm and the magnetic-field strength to $B\sim 1$ G for a Doppler factor $\delta\sim 10$, suggesting rapid jet-based processes involving shocks and magnetic-field ordering. The findings support a curved-jet, Doppler-boost-driven scenario for high-activity episodes in BL Lacertae and provide quantitative constraints on jet microphysics relevant to blazar variability at optical wavelengths.

Abstract

BL Lacertae is not only archetypical of an entire class of jet-dominated active galactic nuclei, blazars, but also one of the most active and rapidly changing objects in this class. In the fall of 2024 (September--November), BL Lacertae underwent another episode of strong optical activity, reaching an R-band magnitude of about 12 and showing extremely rapid and large-amplitude inter- and intra-night flux and polarization variations. During this period, the object was monitored over 40 nights using telescopes with an aperture of up to 2 m at three observatories: Rozhen and Belogradchik in Bulgaria and Skinakas in Greece. The results from this study include some of the most spectacular intra-night variability episodes detected in a blazar. These rapid variations, combined with high photometric accuracy and high time resolution, allowed for confirmation of consistency between different optical bands with zero time delays, down to a minute scale. Unlike previous activity reports, polarization was relatively stable on these short time-scales. Possible connections between polarization, flux, and intra-night variability were explored in order to better model or constrain the physical processes and emission mechanisms in the relativistic jets.

Figures (6)

• Figure S1: Long-term photometric and polarimetric behavior of BL Lacertae between May 2024, and February 2025. From top to bottom: Light curves in BVRI, V-I colors, polarization degrees, and polarization angles (both measured in the R-band).
• Figure S2: Intra-night variability in two optical regions, divided by a beam splitter at 570 nm (see the text), obtained simultaneously using a 2 m Rozhen telescope over three different nights (indicated below the middle panels). The top panel for each night displays the light curves of the blazar (upper two curves) and a comparison star, C, (lower two curves), measured with respect to the other reference stars. Verical shifts of the light curves were applied for presentation purposes. The next two panels show the seeing during the night (in arcsec) and the (arbitrary) color changes of the blazar on intra-night time-scales. The bottom panel presents the interpolation cross-correlation as a function of time lag between the bands (a positive time peak means the blue range is leading). To assess the reality of the positive peak for the night of 6 August 2024 (left panels), an additional MC simulation was performed, and error bars were added (see the text). Note the extremely violent variability recorded during the other two nights, with virtually zero lag between the bands. Also note the color changes, which indicate BWB behavior on intra-night time-scales (middle panels).
• Figure S3: Quasi-simultaneous intra-night photometric and polarimetric monitoring of BL Lacertae for 3 nights (indicated above the bottom panels), performed with the 0.6 m Belogradchik telescope. The top panel for each night presents the light curves of the blazar and a comparison star (C measured with respect to B; see the text). Arbitrary magnitude shifts were applied to each light curve for clarity, and these shifts are indicated for BL Lacertae. The next 4 panels display the seeing variations during the night (in arcsec), the color changes, PD, and PA (measured in the R-band), from top to bottom. The lower panel shows the interpolation cross-correlation lags between the V and R bands (positive peaks indicate a leading V-band). Splines (in red) are provided to guide the eye. Linear interpolation was also used to plot the V-R color, where the small scale structures are likely an artefact of this interpolation. The typical photometric errors of the light curves are comparable to the symbol size (see the text).
• Figure S4: See Figure \ref{['f3a']}.
• Figure S5: See Figure \ref{['f3a']}.