Intra-night optical multiband variability of BL Lacertae during the 2011 outburst

Abstract

Aims. We present our optical multiband (B and R) observations of BL Lacertae, which were acquired over 20 nights from May 26 to August 3, 2011, and analyse our data to study the optical emission variability properties of BL Lac. Methods. The simultaneous photometric observations were carried out using the 0.8 m Tsinghua-NAOC Telescope (TNT) at the Xinglong Observatory of National Astronomical Observatories, Chinese Academy of Sciences (NAOC). TNT is equipped with the Princeton Instrument CCD and standard Johnson-Cousin UBVRI filters. Exposure times were well chosen to make a good compromise between high precision and high temporal density. Results. A multipeak outburst was observed in 2011 observing season. The brightest magnitudes for BL Lac during our observations were B = 14.48 mag and R = 12.89 mag at JD = 2 455 709.2810 (May 27). One of the most rapid variations was detected on July 9, when a flux increase of about 0.34 mag occurred over 2.8 h. In general, both the amplitude of variability and the variation rate are higher at the shorter wavelength. A colour analysis shows that the colour variability processes are apparently complicated. A bluerwhen-brighter trend was detected during the outburst. These trends may be the interactive contribution from a "mildly-chromatic" component and a "strongly-chromatic" one. Meanwhile, intra-night variability in B and R bands was found to be correlated, without any time delay exceeding 2.5 min.

Microvariability in BL Lacertae: “Zooming” into the Innermost Blazar Regions

“…Then, the combined effects of the cooling (synchrotron and adiabatic expansion) on the accelerated particles and the light crossing timescales contribute in shaping the observed fast variability. The frequency dependent variability amplitude i.e., larger variability amplitude at higher frequency,-also reported in previous works on intranight variability (e.g., [31,32,34])-could also be the result of synchrotron emission; assuming constant magnetic field and isotropic distribution of pitch angles, the synchrotron emission (by particles with Lorentz factor γ) together with −dγ/dt ∝ γ 2 and t syn ∝ 1/γ could result dissipation larger amount of energy in shorter timescales. For such calculations, BL Lac being a LSP source, it is also considered that most of the synchrotron radiation is emitted around optical frequency (as characteristic frequency).…”

“…Because of its highly pronounced optical variability, it has been one of the most favorable targets for a large number of multifrequency studies (e.g., [24][25][26][27][28]). On the intranight timescales, the source was found to show rapid variability [29]; and on similar timescales, rapid flux and color variations with a trend to become bluer when brighter have been reported by several authors (e.g., [30][31][32][33][34][35][36]). The optical power spectral density (PSD) in the intranight timescale can be characterized as red noise behavior with a slope index of 2 [37].…”

“…In particular, bluer-when-brighter (alternatively, harder-when-brighter) trend has been frequently observed in the source during intranight observations (see [34,37]). Such an achromatic behavior, both bluer-when-brighter and redder-when-brighter, have been frequently observed in the blazars (for further discussion see [2,19]).…”

Intra-night variability of the blazar CTA 102 during its 2012 and 2016 giant outbursts

“…Similar results (lags generally consistent with zero considering the errors) were reported for BL Lacertae (e.g. Papadakis et al 2003;Stalin et al 2006;Zhai & Wei 2012) and S4 0954+65 (Papadakis et al 2004;Bachev et al 2016). A possible lag was found for one night of observations of 3C 454.3 during two different campaigns (Zhai et al 2011;Bachev et al 2011) and for all the other nights the lag was practically consistent with zero.…”

“…In this paper, we concentrate on the multicolour intra-night variability of CTA 102 during its 2012 and 2016 maxima. So far, several objects have been multicolour (quasi)-simultaneously monitored on intranight time scales, among which S4 0954+65 (Papadakis et al 2004;Bachev et al 2016), 3C 454.3 (Bachev et al 2011Zhai et al 2011), BL Lacertae (Zhai & Wei 2012), S5 0716+714 (Stalin et al 2009;Wu et al 2012), etc. To the best of our knowledge, this is the first such study for CTA 102.…”