We report the flux and spectral variability of PG 1553+113 on intra-night (IDV) to short-term timescales using BVRI data collected over 91 nights from 28 February to 8 November 2019 employing ten ...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%) 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 timescale. 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 to these SEDs of PG 1553+113. We found that the source follows a bluer-when-brighter trend on IDV timescales. We discuss possible physical causes of the observed spectral variability.
The BL Lac object 3C 371 has been observed by the Transiting Exoplanet Survey Satellite (\textit{TESS}) for approximately a year, between July 2019 and July 2020, with an unmatched 2-minute observing ...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. The goal of this study is evaluating the optical variability of 3C 371. Taking advantage of the remarkable cadence of \textit{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 is used to investigate long-term variability (LTV), including understanding spectral behaviour of the source and the polarization variability. Based on the derived characteristics, information on the origin of the variability on different timescales is extracted. We evaluated the variability applying the variability amplitude tool that quantifies how variable the emission is. Moreover, we employed common tools like 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. Short- and long-term colour behaviours have been evaluated to understand the spectral behaviour of the source. The polarized emission was analyzed, studying its variability and possible rotation patterns of the electric vector position angle (EVPA). Flux distributions of IDV and LTV were also studied with the aim of linking the flux variations to turbulent and/or accretion disc related processes.
In 2022 the BL Lac object S4 0954+65 underwent a major variability phase,
reaching its historical maximum brightness in the optical and $\gamma$-ray
bands. We present optical photometric and ...polarimetric data acquired by the
Whole Earth Blazar Telescope (WEBT) Collaboration from 2022 April 6 to July 6.
Many episodes of unprecedented fast variability were detected, implying an
upper limit to the size of the emitting region as low as $10^{-4}$ parsec. The
WEBT data show rapid variability in both the degree and angle of polarization.
We analyse different models to explain the polarization behaviour in the
framework of a twisting jet model, which assumes that the long-term trend of
the flux is produced by variations in the emitting region viewing angle. All
the models can reproduce the average trend of the polarization degree, and can
account for its general anticorrelation with the flux, but the dispersion of
the data requires the presence of intrinsic mechanisms, such as turbulence,
shocks, or magnetic reconnection. The WEBT optical data are compared to
$\gamma$-ray data from the Fermi satellite. These are analysed with both fixed
and adaptive binning procedures. We show that the strong correlation between
optical and $\gamma$-ray data without measurable delay assumes different slopes
in faint and high brightness states, and this is compatible with a scenario
where in faint states we mainly see the imprint of the geometrical effects,
while in bright states the synchrotron self-Compton process dominates.
Thanks to the advent of large-scale optical surveys, a diverse set of flares from the nuclear regions of galaxies has recently been discovered. These include the disruption of stars by supermassive ...black holes at the centers of galaxies - nuclear transients known as tidal disruption events (TDEs). Active galactic nuclei (AGN) can show extreme changes in the brightness and emission line intensities, often referred to as changing-look AGN (CLAGN). Given the physical and observational similarities, the interpretation and distinction of nuclear transients as CLAGN or TDEs remains difficult. One of the obstacles of making progress in the field is the lack of well-sampled data of long-lived nuclear outbursts in AGN. Here, we study PS16dtm, a nuclear transient in a Narrow Line Seyfert 1 (NLSy1) galaxy, which has been proposed to be a TDE candidate. Our aim is to study the spectroscopic and photometric properties of PS16dtm, in order to better understand the outbursts originating in NLSy1 galaxies. Our extensive multiwavelength follow-up that spans around 2000 days includes photometry and spectroscopy in the UV/optical, as well as mid-infrared (MIR) and X-ray observations. Furthermore, we improved an existing semiempirical model in order to reproduce the spectra and study the evolution of the spectral lines. The UV/optical light curve shows a double peak at \(\sim50\) and \(\sim100\) days after the first detection, and it declines and flattens afterward, reaching preoutburst levels after 2000 days of monitoring. The MIR light curve rises almost simultaneously with the optical, but unlike the UV/optical which is approaching the preoutburst levels in the last epochs of our observations, the MIR emission is still rising at the time of writing. The optical spectra show broad Balmer features and the strongest broad Fe II emission ever detected in a nuclear transient. abridged
We present the results of optical photometric observations of three extreme TeV blazars, 1ES 0229\(+\)200, 1ES 0414\(+\)009, and 1ES 2344\(+\)514, taken with two telescopes (1.3 m Devasthal Fast ...Optical Telescope, and 1.04 m Sampuranand Telescope) in India and two (1.4 m Milankovi\'{c} telescope and 60 cm Nedeljkovi\'{c} telescope) in Serbia during 2013--2019. We investigated their flux and spectral variability on diverse timescales. We examined a total of 36 intraday \(R-\)band light curves of these blazars for flux variations using the power-enhanced {\it F}-test and the nested ANOVA test. No significant intraday variation was detected on 35 nights, and during the one positive detection the amplitude of variability was only 2.26 per cent. On yearly timescales, all three blazars showed clear flux variations in all optical wavebands. The weighted mean optical spectral index (\(\alpha_{BR}\)), calculated using \(B - R\) color indices, for 1ES 0229\(+\)200 was 2.09 \(\pm\) 0.01. We also estimated the weighted mean optical spectral indices of 0.67 \(\pm\) 0.01 and 1.37 \(\pm\) 0.01 for 1ES 0414\(+\)009, and 1ES 2344\(+\)514, respectively, by fitting a single power-law (\(F_{\nu} \propto \nu^{-\alpha}\)) in their optical ({\it VRI}) spectral energy distributions. A bluer-when-brighter trend was only detected in the blazar 1ES 0414\(+\)009. We briefly discuss different possible physical mechanisms responsible for the observed flux and spectral changes in these blazars on diverse timescales.
We present the results of our analysis of Gaia19dke, an extraordinary microlensing event in the Cygnus constellation that was first spotted by the {\gaia} satellite. This event featured a strong ...microlensing parallax effect, which resulted in multiple peaks in the light curve. We conducted extensive photometric, spectroscopic, and high-resolution imaging follow-up observations to determine the mass and the nature of the invisible lensing object. Using the Milky Way priors on density and velocity of lenses, we found that the dark lens is likely to be located at a distance of \(D_L =(3.05^{+4.10}_{-2.42})\)kpc, and has a mass of \(M_L =(0.51^{+3.07}_{-0.40}) M_\odot\). Based on its low luminosity and mass, we propose that the lens in Gaia19dke event is an isolated white dwarf.
We report the results of our optical (VRI) photometric observations of the TeV blazar 1ES 0806\(+\)524 on 153 nights during 2011-2019 using seven optical telescopes in Europe and Asia. We ...investigated the variability of the blazar on intraday as well as on long-term timescales. We examined eighteen intraday light curves for flux and color variations using the most reliable power-enhanced F-test and the nested ANOVA test. Only on one night was a small, but significant, variation found, in both \(V\) band and \(R\) band light curves. The \(V-R\) color index was constant on every one of those nights. Flux density changes of around 80 % were seen over the course of these eight years in multiple bands. We found a weighted mean optical spectral index of 0.639\(\pm\)0.002 during our monitoring period by fitting a power law (\(F_{\nu} \propto \nu^{-\alpha}\)) in 23 optical (\(VRI\)) spectral energy distributions of 1ES 0806\(+\)524. We discuss different possible mechanisms responsible for blazar variability on diverse timescales.
We report on the results of a multiwavelength monitoring campaign of the bright, nearby Seyfert galaxy, Ark 120 using a ~50-day observing programme with Swift and a ~4-month co-ordinated ground-based ...observing campaign, predominantly using the Skynet Robotic Telescope Network. We find Ark 120 to be variable at all optical, UV, and X-ray wavelengths, with the variability observed to be well-correlated between wavelength bands on short timescales. We perform cross-correlation analysis across all available wavelength bands, detecting time delays between emission in the X-ray band and the Swift V, B and UVW1 bands. In each case, we find that the longer-wavelength emission is delayed with respect to the shorter-wavelength emission. Within our measurement uncertainties, the time delays are consistent with the \tau ~ \lambda^{4/3} relation, as predicted by a disc reprocessing scenario. The measured lag centroids are \tau_{cent} = 11.90 +/- 7.33, 10.80 +/- 4.08, and 10.60 +/- 2.87 days between the X-ray and V, B, and UVW1 bands, respectively. These time delays are longer than those expected from standard accretion theory and, as such, Ark 120 may be another example of an active galaxy whose accretion disc appears to exist on a larger scale than predicted by the standard thin-disc model. Additionally, we detect further inter-band time delays: most notably between the ground-based I and B bands (\tau_{cent} = 3.46 +/- 0.86 days), and between both the Swift XRT and UVW1 bands and the I band (\tau_{cent} = 12.34 +/- 4.83 and 2.69 +/- 2.05 days, respectively), highlighting the importance of co-ordinated ground-based optical observations.
PDF