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We monitored BL Lacertae simultaneously in the optical B, V, R and I bands for 13 nights during the period 2012-2016. The variations were well correlated in all bands and the source showed significant intraday variability (IDV). We also studied its optical flux and colour behaviour, and searched for inter-band time lags. A strong bluer-when-brighter chromatism was found on the intra-night time-scale. The spectral changes are not sensitive to the host galaxy contribution. Cross-correlation analysis revealed possible time delay of about 10 min between variations in the V and R bands. We interpreted the observed flares in terms of the model consisting of individual synchrotron pulses.
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We monitored BL Lacertae for 13 nights in optical B, V, R, and I bands during October and November 2014 including quasi-simultaneous observations in V and R bands using two optical telescopes in India. We have studied multi-band optical flux variations, colour variation and spectral changes in this blazar. Source was found to be active during the whole monitoring period and showed significant intraday variability on 3 nights in V and R filters while displayed hints of variability on 6 other dates in R passband and on 2 nights in V filter. From the colour-magnitude analysis of the source we found that the spectra of the target gets flatter as it becomes brighter on intra-night timescale. Using discrete correlation technique, we found that intraday light curves in both V and R filters are almost consistent and well correlated with each other. We also generated spectral energy distribution (SED) of the target using the B, V, R, and I data sets for all 13 nights which could help us investigate the physical process responsible for the observed variations in BL Lacertae objects. We also discuss possible physical causes of the observed spectral variability.
We present a study of the intraday variability behaviour of two samples of x-ray selected BL Lac objects, the EMSS and EXOSAT samples consisting of 22 and 11 sources, respectively. In both samples we were able to detect intraday variability in less than 40% of the sources only. The duty cycle (the fraction of time, when a BL Lac object is variable) in x-ray selected BL Lac objects is 0.4 or less. The typical peak-to-peak amplitudes of the variability are 10%. Typical time-scales and an activity parameter for our variable BL Lac objects were inferred from structure function and autocorrelation function analyses. In only 4 BL Lac objects we were able to measure a characteristic time-scale, which was in the range between 1.3 and 2.7 days. Comparison with our previous study of a complete sample of radio-selected BL Lac objects from the 1 Jy catalogue shows that x-ray and radio-selected BL Lac objects differ in their duty cycle by a factor of 2 and the typical peak-to-peak amplitudes by a factor of 3. The observed time-scales are similar. We also found that the same mechanism may be responsible for the observed variability in the x-ray selected and radio-selected BL Lac objects. The expectations of the various schemes linking x-ray selected and radio-selected BL Lac objects have been compared to our observations. Consistency is found for a scenario, where x-ray selected BL Lac objects have on average stronger magnetic fields and are seen under relatively larger viewing angles than the radio-selected BL Lac objects. However, the suggestion that x-ray selected BL Lac objects have decelerating jets and radio-selected BL Lac objects accelerating jets can also not be ruled out.
We monitored BL Lacertae in the B, V, R and I bands for 14 nights during the period of 2016-2018. The source showed significant intraday variability on 12 nights. We performed colour-magnitude analysis and found that the source exhibited bluer-when-brighter chromatism. This bluer-when-brighter behavior is at least partly caused by the larger variation amplitude at shorter wavelength. The variations at different wavelengths are well correlated and show no inter-band time lag.
In an effort to locate the sites of emission at different frequencies and physical processes causing variability in blazar jets, we have obtained high time-resolution observations of BL Lacertae over a wide wavelength range: with the emph{Transiting Exoplanet Survey Satellite} (TESS) at 6,000-10,000 AA with 2-minute cadence; with the Neil Gehrels emph{Swift} satellite at optical, UV, and X-ray bands; with the Nuclear Spectroscopic Telescope Array at hard X-ray bands; with the emph{Fermi} Large Area Telescope at $gamma$-ray energies; and with the Whole Earth Blazar Telescope for measurement of the optical flux density and polarization. All light curves are correlated, with similar structure on timescales from hours to days. The shortest timescale of variability at optical frequencies observed with TESS is $sim 0.5$ hr. The most common timescale is $13pm1$~hr, comparable with the minimum timescale of X-ray variability, 14.5 hr. The multi-wavelength variability properties cannot be explained by a change solely in the Doppler factor of the emitting plasma. The polarization behavior implies that there are both ordered and turbulent components to the magnetic field in the jet. Correlation analysis indicates that the X-ray variations lag behind the $gamma$-ray and optical light curves by up to $sim 0.4$ days. The timescales of variability, cross-frequency lags, and polarization properties can be explained by turbulent plasma that is energized by a shock in the jet and subsequently loses energy to synchrotron and inverse Compton radiation in a magnetic field of strength $sim3$ G
We present the results of the first systematic long-term multi-color optical monitoring of the BL Lacertae object 1ES 0806+52.4. The monitoring was performed in multiple passbands with a 60/90 cm Schmidt telescope from December 2005 to February 2011. The overall brightness of this object decreased from 2005 December to 2008 December, and regained after that. A sharp outburst probably occurred around the end of our monitoring program. Overlapped on the long-term trend are some short-term small-amplitude oscillations. No intra-night variability was found in the object, which is in accord with the historical observations before 2005. By investigating the color behavior, we found strong bluer-when-brighter chromatism for the long-term variability of 1ES 0806+52.4. The total amplitudes at the c, i and o bands are 1.18, 1.12, and 1.02 mags, respectively. The amplitudes tend to increase toward shorter wavelength, which may be the major cause of bluer-when-brighter. Such bluer-when-brighter is also found in other blazars like S5 0716+714, OJ 287, etc. The hard X-ray data collected from the Swift/BAT archive was correlated with our optical data. No positive result was found, the reason of which may be that the hard X-ray flux is a combination of the synchrotron and inverse Compton emission but with different timescales and cadences under the leptonic Synchrotron-Self-Compton (SSC) model.
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