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Register a new userOptical Variability of Three Extreme TeV Blazars
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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_{ u} propto u^{-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.
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We present our optical photometric observations of three TeV blazars, PKS 1510-089, PG 1553+113 and Mrk 501 taken using two telescopes in India, one in Bulgaria, one in Greece and one in Serbia during 2012 - 2014. These observations covered a total of 95 nights with a total of 202 B filter frames, 247 images in V band, 817 in R band while 229 images were taken in the I filter. This work is focused on multi-band flux and colour variability studies of these blazars on diverse timescales which are useful in understanding the emission mechanisms. We studied the variability characteristics of above three blazars and found all to be active over our entire observational campaigns. We also searched for any correlation between the brightness of the sources and their colour indices. During the times of variability, no significant evidence for the sources to display spectral changes correlated with magnitude was found on timescales of a few months. We briefly discuss the possible physical mechanisms most likely responsible for the observed flux variability.
In this first systematic attempt to characterise the intranight optical variability (INOV) of TeV detected blazars, we have monitored a well defined set of 9 TeV blazars on total 26 nights during 2004-2010. In this R (or V)-band monitoring programme only one blazar was monitored per night for a minimum duration of 4 hours. Using the CCD, an INOV detection threshold of ~ 1-2 % was achieved in the densely sampled DLCs. We have further expanded the sample by including another 13 TeV blazars from literature. This enlarged sample of 22 TeV blazars, monitored on a total of 116 nights (including 55 nights newly reported here), has enabled us to arrive at the first estimate of the INOV duty cycle of TeV detected blazars. Applying the C-test, the INOV DC is found to be 59 %, which decreases to 47 % if only INOV fractional amplitudes above 3 % are considered. These observations also permit, for the first time, a comparison of the INOV characteristics of the two major subclasses of TeV detected BL Lacs, namely LBLs and HBLs, for which we find the INOV DCs to be ~ 63 % and ~ 38 %, respectively. This demonstrates that the INOV differential between LBLs and HBLs persists even when only their TeV detected subsets are considered. Despite dense sampling, the intranight light curves of the 22 TeV blazars have not revealed even a single feature on time scale substantially shorter than 1 hour, even though the inner jets of TeV blazars are believed to have exceptionally large bulk Lorentz factors (and correspondingly stronger time compression). An intriguing feature, clearly detected in the light curve of the HBL J1555+1111, is a 4 per cent `dip on a 1 hour timescale. This unique feature could have arisen from absorption in a dusty gas cloud, occulting a superluminally moving optical knot in the parsec scale jet of this relatively luminous BL Lacs object.
We present the results of photometric observations of three TeV blazars, 3C 66A, S5 0954+658 and BL Lacertae, during the period 2013--2017. Our extensive observations were performed in a total of 360 nights which produced $sim$6820 image frames in BVRI bands. We study flux and spectral variability of these blazars on these lengthy timescales. 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 timescales for two of them. The nature of the long-term variability of 3C 66A and S5 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 ~1.2 over the period of these observations.
We report the results of optical monitoring for a sample of 11 blazars including 10 BL Lacs and 1 Flat Spectrum Radio Quasar (FSRQ). We have measured the multiband optical flux and colour variations in these blazars on intra-day and short-term timescales of months and have limited data for 2 more blazars. These photometric observations were made during 2009 to 2011, using six optical telescopes, four in Bulgaria, one in Greece and one in India. On short-term timescales we found significant flux variations in 9 of the sources and colour variations in 3 of them. Intra-day variability was detected on 6 nights for 2 sources out of the 18 nights and 4 sources for which we collected such data. These new optical observations of these blazars plus data from our previous published papers (for 3 more blazars) were used to analyze their spectral flux distributions in the optical frequency range. Our full sample for this purpose includes 6 high-synchrotron-frequency-peaked BL Lacs (HSPs), 3 intermediate-synchrotron-frequency-peaked BL Lacs (ISPs) and 6 low-synchrotron-frequency-peaked BL Lacs (LSPs; including both BL Lacs and FSRQs). We also investigated the spectral slope variability and found that the average spectral slopes of LSPs show a good accordance with the Synchrotron Self-Compton (SSC) loss dominated model. Our analysis supports previous studies that found that the spectra of the HSPs and FSRQs have significant additional emission components. The spectra of all these HSPs and LSPs get flatter when they become brighter, while for FSRQs the opposite appears to hold. This supports the hypothesis that there is a significant thermal contribution to the optical spectrum for FSRQs.
We have examined 40 NuSTAR light curves (LCs) of five TeV emitting high synchrotron peaked blazars: 1ES 0229+200, Mrk 421, Mrk 501, 1ES 1959+650 and PKS 2155-304. Four of the blazars showed intraday variability in the NuSTAR energy range of 3-79 keV. Using an auto correlation function analysis we searched for intraday variability timescales in these LCs and found indications of several between 2.5 and 32.8 ks in eight LCs of Mrk 421, a timescale around 8.0 ks for one LC of Mrk 501, and timescales of 29.6 ks and 57.4 ks in two LCs of PKS 2155-304. The other two blazars LCs do not show any evidence for intraday variability timescales shorter than the lengths of those observations, however, the data was both sparser and noisier, for them. We found positive correlations with zero lag between soft (3-10 keV) and hard (10-79 keV) bands for most of the LCs, indicating that their emissions originate from the same electron population. We examined spectral variability using a hardness ratio analysis and noticed a general harder-when-brighter behavior. The 22 LCs of Mrk 421 observed between July 2012 and April 2013 show that this source was in a quiescent state for an extended period of time and then underwent an unprecedented double peaked outburst while monitored on a daily basis during 10 - 16 April 2013. We briefly discuss models capable of explaining these blazar emissions.
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