Haritma Gaur

We present the most densely sampled multiband optical photometric observations of the peculiar BL Lacertae object OJ 287 from 2015 to 2025 with a focus on its optical activity on diverse timescales. We present a total of 2296, 10,927, 11,484, and 2982 data points in B , V , R , and I bands, respectively. The densely sampled observations allow us to keep track of the source evolution that it has exhibited since the start of the predicted major optical flaring activity at the end of 2015. The study reveals clear and persistent bluer-when-brighter trends in both the long-term and short-term variations. Different bands were cross-correlated with discrete correlation functions, which peak at zero lag, implying cospatial emission. Using eight optical spectra in the low-flux states of OJ 287 taken from 2017 October 21 to 2017 November 22, from Steward Observatory, we estimate the central black hole mass to be at least 3.89 × 10 ⁹ M _⊙ from the [O III ] line width. The emission mechanism of the binary black hole blazar, and its possible implication in various aspects of multimessenger astronomy, are briefly discussed.

We present the results of blazar 3C 273 from simultaneous observations obtained using the XMM-Newton and NuSTAR satellites during the period 2015–2019 in five epochs. When the spectra are modeled with a power law, significant residuals arise below ∼2 keV and in the energy range of 30–78 keV in NuSTAR data. Residuals in the lower energy band represent soft X-ray excess, while at higher energies it likely represents a Compton reflection hump, which might be a weak component arising from dense and cold material. A faint iron line is present in XMM-Newton observations. We interpret such features as being attributed to the coronal emission plus those produced through reflection from an accretion disk. We model the spectral energy distributions (SEDs) with the single-zone inverse Compton (IC) jet model based on synchrotron self-Compton and external Compton phenomena. It is found that a one-zone synchrotron+IC model explains the SEDs quite well, but the jet component alone fails to fit the multiband X-ray emission for the low flux state of this object in 2018 and 2019, which arises owing to spectral flattening at low-energy X-rays, indicating that an additional Seyfert-like thermal component must be present at X-rays. This is further supported by a big blue bump present in the optical/UV band in all SEDs. Finally, we analyzed all the epochs using the relxill model to incorporate relativistic reflection and to model those residuals of soft excess and Compton hump in the X-ray bands.

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.

We present an extensive recent multi-band 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 montiored the blazar quasi-simultaneously during 3 nights in B, V, R, and I bands; 4 nights in B, V, and R; 2 nights in V, R, and I; 5 nights in B and R; and 2 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 color variations in the light curves of the blazar on intraday timescales. Out of 12, 11, 53, and 5 nights observations, intraday variations with amplitudes between ~3% and ~20% 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%. These duty cycles are lower than those typically measured at earlier times. On these timescales color 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.

We report the first extensive optical flux and spectral variability study of the TeV blazar TXS 0506+056 on intra-night to long-term timescales using BVRI data collected over 220 nights between January 21, 2017 to April 9, 2022 using 8 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% and 14.3%, 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

We study the optical flux and polarization variability of the binary black hole blazar OJ 287 using quasi-simultaneous observations from 2015 to 2023 carried out using telescopes in the USA, Japan, Russia, Crimea, and Bulgaria. This is one of the most extensive quasi-simultaneous optical flux and polarization variability studies of OJ 287. OJ 287 showed large amplitude, ∼3.0 mag flux variability, large changes of ∼37% in degree of polarization, and a large swing of ∼215° in the angle of the electric vector of polarization. During the period of observation, several flares in flux were detected. Those flares are correlated with a rapid increase in the degree of polarization and swings in electric vector of polarization angle. A peculiar behavior of anticorrelation between flux and polarization degree, accompanied by a nearly constant polarization angle, was detected from JD 2,458,156 to JD 2,458,292. We briefly discuss some explanations for the flux and polarization variations observed in OJ 287.

We report the first extensive optical flux and spectral variability study of the TeV blazar TXS 0506+056 on intra-night to long-term timescales using BVRI data collected over 220 nights between January 21, 2017 to April 9, 2022 using 8 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% and 14.3%, 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.

High synchrotron energy peaked blazar 1ES 1959+650 was studied with the Swift and XMM-Newton satellites in a total of 127 observations during the period 2018 June–2020 December. We extensively studied its flux and spectral variability on intraday and long-term timescales. Discrete correlation function analysis between soft and hard X-ray bands indicates soft as well as hard lags. The results were used to constrain the magnetic field of the emitting region, which was found to be 0.64 ± 0.05 G. On long-term timescales, the distribution of fluxes shows lognormal behavior, which could be attributed to the minijets-in-a-jet model or might be due to the propagation of relativistic shocks down the jet. The spectral energy distribution around the synchrotron peak is well described by the log-parabola model. Spectral parameters like the peak energy E _p , curvature β, and peak luminosity L _p were derived from spectral analysis. Their correlations were studied to constrain the acceleration processes of the emitting particles. E _p shows strong correlation with L _p during the high state of the source, which indicates spectral changes might be caused by the variations of the average electron energy. Low values of the curvature parameter β and a weak correlation between E _p and β indicate the coexistence of stochastic/statistical acceleration of electrons in the emitting region. Implications of other results are also discussed.