ABSTRACT
We present the results of photometric observations of three TeV blazars, 3C 66A, S4 0954+658, and BL Lacertae (BL Lac), during the period 2013–2017. Our extensive observations were performed ...in a total of 360 nights which produced ∼6820 image frames in BVRI bands. We study flux and spectral variability of these blazars on these lengthy time-scales. We also examine the optical spectral energy distributions of these blazars, which are crucial in understanding the emission mechanism of long-term variability in blazars. All three TeV blazars exhibited strong flux variability during our observations. The colour variations are mildly chromatic on long time-scales for two of them. The nature of the long-term variability of 3C 66A and S4 0954+658 is consistent with a model of a non-thermal variable component that has a continuous injection of relativistic electrons with power-law distributions around 4.3 and 4.6, respectively. However, the long-term flux and colour variability of BL Lac suggests that these can arise from modest changes in velocities or viewing angle toward the emission region, leading to variations in the Doppler boosting of the radiation by a factor of ∼1.2 over the period of these observations.
ABSTRACT
Blazar S5 0716+714 is well-known for its short-term variability, down to intraday time-scales. We here present the 2-min cadence optical light curve obtained by the TESS space telescope in ...2019 December–2020 January and analyse the object fast variability with unprecedented sampling. Supporting observations by the Whole Earth Blazar Telescope Collaboration in B, V, R, and I bands allow us to investigate the spectral variability during the TESS pointing. The spectral analysis is further extended in frequency to the UV and X-ray bands with data from the Neil Gehrels Swift Observatory. We develop a new method to unveil the shortest optical variability time-scales. This is based on progressive de-trending of the TESS light curve by means of cubic spline interpolations through the binned fluxes, with decreasing time bins. The de-trended light curves are then analysed with classical tools for time-series analysis (periodogram, autocorrelation, and structure functions). The results show that below 3 d there are significant characteristic variability time-scales of about 1.7, 0.5, and 0.2 d. Variability on time-scales $\lesssim 0.2$ d is strongly chromatic and must be ascribed to intrinsic energetic processes involving emitting regions, likely jet substructures, with dimension less than about 10−3 pc. In contrast, flux changes on time-scales $\gtrsim 0.5$ d are quasi-achromatic and are probably due to Doppler factor changes of geometric origin.
We report on a recent multiband optical photometric and polarimetric observational campaign of the blazar OJ 287 that was carried out during 2016 September-2017 December. We employed nine telescopes ...in Bulgaria, China, Georgia, Japan, Serbia, Spain, and the United States. We collected over 1800 photometric image frames in BVRI bands and over 100 polarimetric measurements over ∼175 nights. In 11 nights with many quasi-simultaneous multiband (V, R, I) observations, we did not detect any genuine intraday variability in flux or color. On longer timescales, multiple flaring events were seen. Large changes in color with respect to time and in a color-magnitude diagram were seen, and while only a weak systematic variability trend was noticed in color with respect to time, the color-magnitude diagram shows a bluer-when-brighter trend. Large changes in the degree of polarization and substantial swings in the polarization angle were detected. The fractional Stokes parameters of the polarization showed a systematic trend with time in the beginning of these observations, followed by chaotic changes and then an apparently systematic variation at the end. These polarization changes coincide with the detection and duration of the source at very high energies as seen by VERITAS. The spectral index shows a systematic variation with time and V-band magnitude. We briefly discuss possible physical mechanisms that could explain the observed flux, color, polarization, and spectral variability.
Abstract
In this work, we present 8523 pairs of
R
-band optical photometry observations for the quasar 3C 454.3 made during the period of 2006 October–2018 February on the 70 cm meniscus telescope at ...Abastumani Observatory, Georgia, to study its intraday variabilities (IDVs) and long-term variations, and we have come to the following results. (1) We detected 10 outbursts, a Δ
R
= 3.825 mag variation, and some IDVs. The IDV timescales are from 4.1 to 285 minutes, with the corresponding variability amplitude being
A
= 2.9%–43.67%. The amplitude increases with IDV timescale. (2) The largest variation over a 1 day timescale is Δ
R
= 1.38 mag. (3) The IDV timescales suggest that the emission sizes are from 8.9 × 10
13
cm to 6.20 × 10
15
cm, and the magnetic field strengths are
B
= 0.18–0.79 G. (4) Period analysis results show three possible long-term periods,
p
= 3.04 ± 0.02 yr,
p
= 1.66 ± 0.06 yr, and
p
= 1.20 ± 0.03 yr in the optical light curve. We adopted the accretion disk models and the lighthouse models to period
p
= 3.04 ± 0.02 yr: in the accretion disk models, the binary black holes have masses
M
= 1.17 × 10
9
M
⊙
; in the lighthouse models, we used two boosted jet flux densities to fit the observational light curve. (5) WWZ analysis gives some short-period (high-frequency) signals associated with strong bursts (JD 2,454,302 and JD 2,454,521) with variable frequencies and lasting for the entire observation time span (11.3 yr).
Abstract
Variability is one of the main observational characteristics of blazars. Studying variability is an efficient method to reveal the nature of active galactic nuclei. In the present work, we ...report optical
R
-band photometry observations of a TeV blazar, 1ES 2344 + 514, carried out with a 70 cm telescope in the period of 1998 July–2017 November at Abastumani Observatory, Georgia. Based on the optical
R
-band observations, the optical variation behaviors on both short timescales and long timescales are investigated. Three methods (Jurkevich, discrete correlation function, and power spectrum analysis) are used to investigate periodicity in the light curve. In addition, combined with multiwavelength data, the jet physical properties are discussed. The following conclusions are drawn: (1) A variability of Δ
R
= 0.155 mag (15.356 − 15.201 mag) over a timescale of Δ
T
= 12.99 minutes is detected during our 628 days of monitoring. (2) According to the Kelvin–Helmholtz thermal instability, if the magnetic field intensity (
B
) for the source is greater than a critical value (
B
c
), it will reduce the incidence of intraday variations in the light curves. (3) The physical parameters of the dissipation region are obtained by fitting the spectral energy distribution with a one-zone synchrotron self-Compton model for the average and flare states. (4) The three methods show that there are periods of
P
= 2.72 ± 0.47 yr,
P
= 1.61 ± 0.18 yr,
P
= 1.31 ± 0.17 yr, and
P
= 1.05 ± 0.07 yr. When a binary black hole system is adopted with a period of
P
= 2.72 ± 0.41 yr, we obtain the orbital parameters for the binary black hole system as follows:
M
= 8.08 × 10
9
M
⊙
, the sum of the semiaxes is
r
= 7.18 × 10
16
cm, and the lifetime of the binary black hole is
τ
merge
= 6.24 × 10
2
yr.
Context. The BL Lac object 3C 371 was observed by the Transiting Exoplanet Survey Satellite (TESS) for approximately a year, between July 2019 and July 2020, with an unmatched two-minute imaging ...cadence. In parallel, the Whole Earth Blazar Telescope (WEBT) Collaboration organized an extensive observing campaign, providing three years of continuous optical monitoring between 2018 and 2020. These datasets allow for a thorough investigation of the variability of the source. Aims. The goal of this study is to evaluate the optical variability of 3C 371. Taking advantage of the remarkable cadence of TESS data, we aim to characterize the intra-day variability (IDV) displayed by the source and identify its shortest variability timescale. With this estimate, constraints on the size of the emitting region and black hole mass can be calculated. Moreover, WEBT data are used to investigate long-term variability (LTV), including in terms of the spectral behavior of the source and the polarization variability. Based on the derived characteristics, we aim to extract information on the origin of the variability on different timescales. Methods. We evaluated the variability of 3C 371 by applying the variability amplitude tool, which quantifies variability of the emission. Moreover, we employed common tools, such as ANOVA (ANalysis Of VAariance) tests, wavelet and power spectral density (PSD) analyses to characterize the shortest variability timescales present in the emission and the underlying noise affecting the data. We evaluated the short- and long-term color behavior to understand its spectral behavior. The polarized emission was analyzed, studying its variability and possible rotation patterns of the electric vector position angle (EVPA). Flux distributions of the IDV and LTV were also studied with the aim being to link the flux variations to turbulent and/or accretion-disk-related processes. Results. Our ANOVA and wavelet analyses reveal several entangled variability timescales. We observe a clear increase in the variability amplitude with increasing width of the time intervals evaluated. We are also able to resolve significant variations on timescales of as little as ∼0.5 h. The PSD analysis reveals a red-noise spectrum with a break at IDV timescales. The spectral analysis shows a mild bluer-when-brighter (BWB) trend on long timescales. On short timescales, mixed BWB, achromatic and redder-when-brighter signatures can be observed. The polarized emission shows an interesting slow EVPA rotation during the flaring period, where a simple stochastic model can be excluded as the origin with a 3 σ significance. The flux distributions show a preference for a Gaussian model for the IDV, and suggest it may be linked to turbulent processes, while the LTV is better represented by a log-normal distribution and may have a disk-related origin.
ABSTRACT
We present a multiwavelength study of the flat-spectrum radio quasar CTA 102 during 2013–2017. We use radio-to-optical data obtained by the Whole Earth Blazar Telescope, 15 GHz data from the ...Owens Valley Radio Observatory, 91 and 103 GHz data from the Atacama Large Millimeter Array, near-infrared data from the Rapid Eye Monitor telescope, as well as data from the Swift (optical-UV and X-rays) and Fermi (γ-rays) satellites to study flux and spectral variability and the correlation between flux changes at different wavelengths. Unprecedented γ-ray flaring activity was observed during 2016 November–2017 February, with four major outbursts. A peak flux of (2158 ± 63) × 10−8 ph cm−2 s−1, corresponding to a luminosity of (2.2 ± 0.1) × 1050 erg s−1, was reached on 2016 December 28. These four γ-ray outbursts have corresponding events in the near-infrared, optical, and UV bands, with the peaks observed at the same time. A general agreement between X-ray and γ-ray activity is found. The γ-ray flux variations show a general, strong correlation with the optical ones with no time lag between the two bands and a comparable variability amplitude. This γ-ray/optical relationship is in agreement with the geometrical model that has successfully explained the low-energy flux and spectral behaviour, suggesting that the long-term flux variations are mainly due to changes in the Doppler factor produced by variations of the viewing angle of the emitting regions. The difference in behaviour between radio and higher energy emission would be ascribed to different viewing angles of the jet regions producing their emission.
ABSTRACT
We report the flux and spectral variability of PG 1553 + 113 on intra-night (IDV) to short-term time-scales using BVRI data collected over 91 nights from 28 February to 8 November 2019 ...employing 10 optical telescopes: three in Bulgaria, two each in India and Serbia, and one each in Greece, Georgia, and Latvia. We monitored the blazar quasi-simultaneously for 16 nights in the V and R bands and 8 nights in the V, R, I bands and examined the light curves (LCs) for intra-day flux and colour variations using two powerful tests: the power-enhanced F-test and the nested ANOVA test. The source was found to be significantly (>99 per cent) variable in 4 nights out of 27 in R-band, 1 out of 16 in V-band, and 1 out of 6 nights in I-band. No temporal variations in the colours were observed on IDV time-scale. During the course of these observations the total variation in R-band was 0.89 mag observed. We also investigated the spectral energy distribution (SED) using B-, V-, R-, and I-band data. We found optical spectral indices in the range of 0.878 ± 0.029 to 1.106 ± 0.065 by fitting a power law (Fν∝ν−α) to these SEDs of PG 1553 + 113. We found that the source follows a bluer-when-brighter trend on IDV time-scales. We discuss possible physical causes of the observed spectral variability.
Variability is one of the extreme observational properties of BL Lacertae objects. AO 0235+164 is a well-studied BL Lac throughout all electromagnetic wavebands. In the present work, we show its ...optical R-band photometric observations carried out during the period from 2006 November to 2012 December using the Ap6E CCD camera attached to the primary focus of the 70 cm meniscus telescope at Abastumani Observatory, Georgia. During our monitoring period, it showed a large variation of (14.19-19.07 mag) and a short timescale of . During the period of 2006 December to 2009 November, we made radio observations of the source using the 25 m radio telescope at Xinjiang Astronomical Observatory. By adopting a discrete correlation function to the optical and radio observations we found that the optical variation leads a radio variation of 23.2 12.9 days.
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