Haritma Gaur

Quasi-simultaneous two-band optical variability of the blazars 1ES 1959+650 and 1ES 2344+514

We report the results of quasi-simultaneous two-filter optical monitoring of two high-energy peaked blazars, 1ES 1959+650 and 1ES 2344+514, to search for microvariability and short-term variability (STV). We carried out optical photometric monitoring of these sources in an alternating sequence of B and R passbands, and have 24 and 19 nights of new data for these two sources, respectively. No genuine microvariability (intranight variability) was detected in either of these sources. This non-detection of intranight variations is in agreement with the conclusions of previous studies that high-energy peaked BL Lacs are intrinsically less variable than low-energy peaked BL Lacs in the optical bands. We also report the results of STV studies for these two sources between 2009 July and 2010 August. Genuine STV is found for the source 1ES 1959+650 but not for 1ES 2344+514. We briefly discuss possible reasons for the difference between the intranight variability behaviour of high- and low-energy peaked blazars.

Optical flux and spectral variability of blazars

We report the results of optical monitoring for a sample of 11 blazars including 10 BL Lacertae objects (BL Lacs) and one flat spectrum radio quasar (FSRQ). We have measured the multiband optical flux and colour variations in these blazars on intraday and short-term time-scales of months and have limited data for two more blazars. These photometric observations were made during 2009–2011, using six optical telescopes, four in Bulgaria, one in Greece and one in India. On short-term time-scales we found significant flux variations in nine of the sources and colour variations in three of them. Intraday variability was detected on six nights for two sources out of the 18 nights and four sources for which we collected such data. These new optical observations of these blazars plus data from our previous published papers (for three more blazars) were used to analyse their spectral flux distributions in the optical frequency range. Our full sample for this purpose includes six high-synchrotron-frequency-peaked BL Lacs (HSPs), three intermediate-synchrotron-frequency-peaked BL Lacs (ISPs) and six 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 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.

MULTIWAVELENGTH VARIABILITY OF THE BLAZARS Mrk 421 AND 3C 454.3 IN THE HIGH STATE

We report the results of photometric observations of the blazars Mrk 421 and 3C 454.3 designed to search for intraday variability (IDV) and short-term variability (STV). Optical photometric observations were spread over 18 nights for Mrk 421 and 7 nights for 3C 454.3 during our observing run in 2009-2010 at the 1.04 m telescope at Aryabhatta Research Institute of Observational Sciences, India. Genuine IDV is found for the source 3C 454.3 but not for Mrk 421. Genuine STV is found for both sources. Mrk 421 was revealed by the Monitor of All-sky X-ray Image (MAXI) X-ray detector on the International Space Station to be in an exceptionally high flux state in 2010 January-February. We performed a correlation between the X-ray and optical bands to search for time delays and found a weak correlation with higher frequencies leading the lower frequencies by about 10 days. The blazar 3C 454.3 was found to be in a high flux state in 2009 November-December. We performed correlations in optical observations made at three telescopes, along with X-ray data from the MAXI camera and public release γ-ray data from the Fermi space telescope. We found strong correlations between the γ-ray and optical bands at a time lag of about four days, but the X-ray flux is not correlated with either. We briefly discuss the possible reasons for the time delays between these bands within the framework of existing models for X-ray and γ-ray emission mechanisms.

The nature of the intra‐night optical variability in blazars

In this paper we present the results of a short-term optical monitoring program of 13 blazars. The objects were monitored mostly in the R band for a total of ∼160 h between 2006 and 2011. We study the nature of the short-term variations and show that most of them could be described as slow, smooth and (almost) linear changes of up to ∼0.1 mag h −1 , but that many objects show no short-term variations at all. In fact, we found only a ∼2 per cent chance of observing variability of more than 0.1 mag h −1 for the sample we observed. Hints of quasiperiodic oscillations at very low-amplitude levels are also found for some objects. We briefly discuss some of the possible mechanisms for generating the intra-night variability and the quasi-periodic oscillations.

Multiband optical–NIR variability of blazars on diverse time-scales

Tosearchforopticalvariabilityonawiderangeoftime-scales,wehavecarriedoutphotometric monitoring of two flat spectrum radio quasars, 3C 454.3 and 3C 279, plus one BL Lac, S5 0716+714, all of which have been exhibiting remarkably high activity and pronounced variability at all wavelengths. CCD magnitudes in B, V, R, and I passbands were determined for ∼7000 new optical observations from 114 nights made during 2011–2014, with an average length of ∼4 h each, at seven optical telescopes: four in Bulgaria, one in Greece, and two in India. We measured multiband optical flux and colour variations on diverse time-scales. Discrete correlation functions were computed among B, V, R, and I observations, to search for any time delays. We found weak correlations in some cases with no significant time lags. The structure function method was used to estimate any characteristic time-scales of variability. We also investigated the spectral energy distribution of the three blazars using B, V, R, I, J, and K passband data. We found that the sources almost always follow a bluer-when-brighter trend. We discuss possible physical causes of the observed spectral variability.

Anti-correlated Optical Flux and Polarization Variability in BL Lac

We present the results of photometric (V band) and polarimetric observations of the blazar BL Lac during 2008-2010 using TRISPEC attached to the KANATA 1.5 m telescope in Japan. The data reveal a great deal of variability ranging from days to months with detection of strong variations in fractional polarization. The V band flux strongly anticorrelates with the degree of polarization during the first of two observing seasons but not during the second. The direction of the electric vector, however, remained roughly constant during all of our observations. These results are consistent with a model with at least two emission regions being present, with the more variable component having a polarization direction nearly perpendicular to that of the relatively quiescent region so that a rising flux can produce a decline in degree of polarization. We also computed models involving helical jet structures and single transverse shocks in jets and show that they might also be able to agree with the anticorrelations between flux and fractional polarization.

Nature of intranight optical variability of BL Lacertae

We present the results of extensive multiband intranight optical monitoring of BL Lacertae during 2010-2012. BL Lacertae was very active in this period and showed intense variability in almost all wavelengths. We extensively observed it for a total for 38 nights; on 26 of them, observations were done quasi-simultaneously in B, V, R and I bands (totalling 113 light curves), with an average sampling interval of around 8 min. BL Lacertae showed significant variations on hour-like time-scales in a total of 19 nights in different optical bands. We did not find any evidence for periodicities or characteristic variability time-scales in the light curves. The intranight variability amplitude is generally greater at higher frequencies and decreases as the source flux increases. We found spectral variations in BL Lacertae in the sense that the optical spectrum becomes flatter as the flux increases but in several flaring states, deviates from the linear trend suggesting different jet components contributing to the emission at different times.

Multiband optical variability of three TeV blazars on diverse time-scales

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.

Peak of spectral energy distribution plays an important role in intra-day variability of blazars?

Blazars can be divided into two sub-classes namely high energy and low energy peaked blazars. In spectral energy distribution, the first synchrotron hump of the former class peaks in UV/X-rays and in IR/optical bands for the latter class. The peak of the spectral energy distribution seems to be responsible for variability properties of these classes of blazars in X-ray and optical bands. Since, in low energy peaked blazars, the X-ray bands lies well below the synchrotron hump, one expects that the highest energy electrons available for the synchrotron emission would have slower effect of variability on X-ray intra-day timescale. In this paper, by taking the advantage of a sample of 12 low energy peaked blazars with total 50 observations from XMM

Multiband optical variability of the blazar OJ 287 during its outbursts in 2015 – 2016

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