Search

We present an extensive recent multiband optical photometric observations of the blazar S5 0716+714 carried out over 53 nights with two telescopes in India, two in Bulgaria, one in Serbia, and one in Egypt during 2019 November – 2022 December. We collected 1401, 689, 14726, and 165 photometric image frames in B, V, R , and I bands, respectively. We monitored the blazar quasi-simultaneously during three nights in B, V, R , and I bands; four nights in B, V , and R ; two nights in V, R , and I ; five nights in B and R ; and two nights in V and R bands. We also took 37 nights of data only in R band. Single band data are used to study intraday flux variability and two or more bands quasi-simultaneous observations allow us to search for colour variation in the source. We employ the power-enhanced F -test and the nested ANOVA test to search for genuine flux and colour variations in the light curves of the blazar on intraday time-scales. Out of 12, 11, 53, and 5 nights observations, intraday variations with amplitudes between ∼3 and ∼20 per cent are detected in 9, 8, 31 and 3 nights in B, V, R , and I bands, respectively, corresponding to duty cycles of 75, 73, 58, and 60 per cent. These duty cycles are lower than those typically measured at earlier times. On these time-scales colour variations with both bluer-when-brighter and redder-when-brighter are seen, though nights with no measurable colour variation are also present. We briefly discuss possible explanations for this observed intraday variability. [ABSTRACT FROM AUTHOR]

We report the first extensive optical flux and spectral variability study of the TeV blazar TXS 0506 + 056 on intranight to long-term time-scales using BVRI data collected over 220 nights between 2017 January 21 to 2022 April 9 using eight optical ground-based telescopes. In our search for intraday variability (IDV), we have employed two statistical analysis techniques, the nested ANOVA test and the power enhanced F -test. We found the source was variable in 8 nights out of 35 in the R -band and in 2 of 14 in the V -band yielding duty cycles (DC) of 22.8 per cent and 14.3 per cent, respectively. Clear colour variation in V − R was seen in only 1 out of 14 observing nights, but no IDV was found in the more limited B, I , and B − I data. During our monitoring period the source showed a 1.18 mag variation in the R -band and similar variations are clearly seen at all optical wavelengths. We extracted the optical (BVRI) SEDs of the blazar for 44 nights when observations were carried out in all four of those wavebands. The mean spectral index (α) was determined to be 0.897 ± 0.171. [ABSTRACT FROM AUTHOR]

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 (F ν∝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. [ABSTRACT FROM AUTHOR]

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 FROM AUTHOR]

We present a multiwavelength spectral and temporal investigation of OJ 287 emission during its strong optical-to-X-ray activity between 2016 July and 2017 July. The daily γ-ray fluxes from Fermi-Large Area Telescope (LAT) are consistent with no variability. The strong optical-to-X-ray variability is accompanied by a change in power-law spectral index of the X-ray spectrum from <2 to >2, with variations often associated with changes in optical polarization properties. Cross-correlations between optical-to-X-ray emission during four continuous segments show simultaneous optical-ultraviolet (UV) variations, while the X-ray and UV/optical are simultaneous only during the middle two segments. In the first segment, the results suggest X-rays lag the optical/UV, while in the last segment X-rays lead by ∼5–6 d. The last segment also shows a systematic trend with variations appearing first at higher energies followed by lower energy ones. The LAT spectrum before the very high-energy (VHE) activity is similar to preceding quiescent state spectrum, while it hardens during VHE activity period and is consistent with the extrapolated VHE spectrum during the latter. Overall, the broad-band spectral energy distributions (SEDs) during high-activity periods are a combination of a typical OJ 287 SED and a high-energy peaked (HBL) SED and can be explained in a two-zone leptonic model, with the second zone located at parsec scales, beyond the broad line region, being responsible for the HBL-like spectrum.The change of polarization properties from systematic to chaotic and back to systematic, before, during, and after the VHE activity, suggests dynamic roles for magnetic fields and turbulence. [ABSTRACT FROM AUTHOR]

We present a multiwavelength spectral and temporal analysis of the blazar OJ 287 during its recent activity between 2015 December and 2016 May, showing strong variability in the nearinfrared (NIR) to X-ray energies with detection at γ -ray energies as well. Most of the optical flux variations exhibit strong changes in polarization angle and degree. All the interband time lags are consistent with simultaneous emissions. Interestingly, on days with excellent data coverage in the NIR-UV bands, the spectral energy distributions (SEDs) show signatures of bumps in the visible-UV bands, never seen before in this source. The optical bump can be explained as accretion-disc emission associated with the primary black hole of mass ~1.8 × 1010 M☉ while the little bump feature in the optical-UV appears consistent with line emission. Further, the broad-band SEDs extracted during the first flare and during a quiescent period during this span show very different γ -ray spectra compared to previously observed flare or quiescent spectra. The probable thermal bump in the visible seems to have been clearly present since 2013 May, as found by examining all available NIR-optical observations, and favours the binary supermassive black hole model. The simultaneous multiwavelength variability and relatively weak γ -ray emission that shows a shift in the SED peak is consistent with γ -ray emission originating from inverse Compton scattering of photons from the line emission that apparently contributes to the little blue bump. [ABSTRACT FROM AUTHOR]

The blazar 3C 454.3 exhibited a strong flare seen in γ-rays, X-rays and optical/near-infrared bands during 2009 December 3-12. Emission in the V and J bands rose more gradually than did the γ-rays and soft X-rays, though all peaked at nearly the same time. Optical polarization measurements showed dramatic changes during the flare, with a strong anticorrelation between optical flux and degree of polarization (which rose from ~3 to ~20 per cent) during the declining phase of the flare. The flare was accompanied by large rapid swings in polarization angle of ~170°. This combination of behaviours appears to be unique. We have cm-band radio data during the same period but they show no correlation with variations at higher frequencies. Such peculiar behaviour may be explained using jet models incorporating fully relativistic effects with a dominant source region moving along a helical path or by a shock-in-jet model incorporating three-dimensional radiation transfer if there is a dominant helical magnetic field. We find that spectral energy distributions at different times during the flare can be fit using modified one-zone models where only the magnetic field strength and particle break frequencies and normalizations need change. An optical spectrum taken at nearly the same time provides an estimate for the central black hole mass of ~2.3 × 109M☉.We also consider two weaker flares seen during the ~200 d span over which multiband data are available. In one of them, the V and J bands appear to lead the γ-ray and X-ray bands by a few days; in the other, all variations are simultaneous. [ABSTRACT FROM AUTHOR]

We obtained and analysed more than 100 h of multicolour optical time series of the blazar CTA 102 during its 2012 and 2016 outbursts. The object reached almost 11th mag at the end of 2016, which is perhaps the brightest blazar state ever observed! During both outbursts, CTA 102 showed significant and rapid variability on intra-night time-scales, reaching up to 0.2mag for 30 min on some occasions. The 'rms-flux' relation, built for all data sets, shows a large scatter and no apparent saturation on the magnitude scale. The ensemble structure function of the light curves can be fitted well with a straight line of a slope of ~0.4. The time lags between the different optical bands appear to be consistent with zero, taking into account our time resolution. We discuss different variability scenarios and favour the changing Doppler factor of the emitting blobs as the most plausible one to account for the observed intra-night variability. [ABSTRACT FROM AUTHOR]

We present recent optical photometric observations of the blazar OJ 287 taken during 2015 September-2016 May. Our intense observations of the blazar started in 2015 November and continued until 2016 May and included detection of the large optical outburst in 2015 December that was predicted using the binary black hole model for OJ 287. For our observing campaign, we used a total of nine ground-based optical telescopes of which one is in Japan, one is in India, three are in Bulgaria, one is in Serbia, one is in Georgia, and two are in the USA. These observations were carried out in 102 nights with a total of ~1000 image frames in BVRI bands, though the majority were in the R band. We detected a second comparably strong flare in 2016 March. In addition, we investigated multiband flux variations, colour variations, and spectral changes in the blazar on diverse time-scales as they are useful in understanding the emission mechanisms. We briefly discuss the possible physical mechanisms most likely responsible for the observed flux, colour, and spectral variability. [ABSTRACT FROM AUTHOR]

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 multiband flux and colour variability studies of these blazars on diverse time-scales 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 time-scales of a few months. We briefly discuss the possible physical mechanisms most likely responsible for the observed flux variability. [ABSTRACT FROM AUTHOR]

We analysed the multiband optical behaviour of the BL Lacertae object, S5 0716+714, during its outburst state from 2014 November to 2015 March. We took data on 23 nights at three observatories, one in India and two in Bulgaria, making quasi-simultaneous observations in B, V, R, and I bands. We measured multiband optical fluxes, colour, and spectral variations for this blazar on intraday and short time-scales. The source was in a flaring state during the period analysed and displayed intense variability in all wavelengths. R-band magnitude of 11.6 was attained by the target on 2015 January 18, which is the brightest value ever recorded for S5 0716+714. The discrete correlation function method yielded good correlation between the bands with no measurable time lags, implying that radiation in these bands originate from the same region and by the same mechanism. We also used the structure function technique to look for characteristic time-scales in the light curves. During the times of rapid variability, no evidence for the source to display spectral changes with magnitude was found on either of the time-scales. The amplitude of variations tends to increase with increasing frequency with a maximum of ~22 per cent seen during flaring states in B band. A mild trend of larger variability amplitude as the source brightens was also found. We found the duty cycle of our source during the analysed period to be ~90 per cent. We also investigated the optical spectral energy distribution of S5 0716+714 using B, V, R, and I data points for 21 nights. We briefly discuss physical mechanisms most likely responsible for its flux and spectral variations. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

To search for optical variability on awide range of time-scales, we have carried out photometric 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. [ABSTRACT FROM AUTHOR]

ABSTRACT 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 quasi-periodic 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. [ABSTRACT FROM AUTHOR]

ABSTRACT 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. [ABSTRACT FROM AUTHOR]

ABSTRACT We present the results of our study of spectral energy distributions (SEDs) of a sample of 10 low- to intermediate-synchrotron-peaked blazars. We investigate some of the physical parameters most likely responsible for the observed short-term variations in blazars. To do so, we focus on the study of changes in the SEDs of blazars corresponding to changes in their respective optical fluxes. We model the observed spectra of blazars from radio to optical frequencies using a synchrotron model that entails a log-parabolic distribution of electron energies. A significant correlation among the two fitted spectral parameters ( a, b) of log-parabolic curves and a negative trend among the peak frequency and spectral curvature parameter, b, emphasize that the SEDs of blazars are fitted well by log-parabolic curves. On considering each model parameter that could be responsible for changes in the observed SEDs of these blazars, we find that changes in the jet Doppler factors are most important. [ABSTRACT FROM AUTHOR]

We have measured multiband optical flux and colour variations for a sample of 12 low-energy peaked blazars (LBLs) on short, day-to-month, time-scales. Our sample contains six BL Lacertae objects (BL Lacs) and six flat spectrum radio quasars (FSRQs). These photometric observations, made during 2008 September to 2009 June, used five optical telescopes, one in India and four in Bulgaria. We detected short-term flux variations in 11 of these blazars and colour variability in eight of them. Our data indicate that six blazars (3C 66A, AO 164, S 714, PKS 178, OJ 287 and 3C 454.3) were observed in pre- or post-outburst states, five (PKS 014, 4C 29.45, 3C 279, PKS 089 and BL Lac) were in a low state, while one (3C 273) was in an essentially steady state. The duty cycles for flux and colour variations on short time-scales in these LBLs are ∼92 and ∼33 per cent, respectively. The colour versus magnitude correlations seen here support the hypothesis that BL Lac objects tend to become bluer with increase in brightness; however, FSRQs may show the opposite trend, and there are exceptions to these trends in both categories of blazar. We briefly discuss emission models for active galactic nuclei that might explain our results. [ABSTRACT FROM AUTHOR]

We present the results from a monitoring campaign of the Narrow-Line Seyfert 1 galaxy PG 1211+143. The object was monitored with ground-based facilities ( UBVRI photometry; from 2007 February to July) and with Swift[X-ray photometry/spectroscopy and ultraviolet (UV)/optical photometry; between 2007 March and May]. We found PG 1211+143 in a historical low X-ray flux state at the beginning of the Swift monitoring campaign in 2007 March. It is seen from the light curves that while violently variable in X-rays, the quasar shows little variations in optical/UV bands. The X-ray spectrum in the low state is similar to other narrow-line Seyfert 1 galaxies during their low states and can be explained by a strong partial covering absorber or by X-ray reflection on to the disc. With the current data set, however, it is not possible to distinguish between both scenarios. The interband cross-correlation functions indicate a possible reprocessing of the X-rays into the longer wavelengths, consistent with the idea of a thin accretion disc, powering the quasar. The time lags between the X-ray and the optical/UV light curves, ranging from ∼2 to ∼18 d for the different wavebands, scale approximately as , but appear to be somewhat larger than expected for this object, taking into account its accretion disc parameters. Possible implications for the location of the X-ray irradiating source are discussed. [ABSTRACT FROM AUTHOR]

In this paper we present the results of a search for short-term variability in the optical band of selected high-luminosity, high-redshift radio-quiet quasars. Each quasar has been monitored typically for 2–4 h with a time resolution of 2–5 min and a photometric accuracy of about 0.01–0.02 mag. As a result of the significant redshift, the covered wavelength range falls into the ultraviolet region (typically 1500–2500Å). We have found no statistical evidence for any continuum variations larger than 0.01–0.02 mag for any of the monitored objects. Our results suggest that the presence of a short-term variability in radio-quiet quasars is unlikely even in the ultraviolet region, contrary to reports by other authors. This conclusion holds true at least for high-luminosity (large black hole mass and accretion rate?) objects. The results are consistent with the idea that significant short-term (less than 1 h) variations in active galactic nuclei, where observed, should be attributed primarily to processes in a relativistic jet. [ABSTRACT FROM AUTHOR]