Publications

Abstract: Flat spectrum radio quasars (FSRQs) have been detected at TeV energies by ground-based atmospheric Cherenkov telescopes mainly during flaring states. VERITAS is carrying out the first systematic and unbiased search for TeV emissions from a set of FSRQs. Fermi-LAT-detected FSRQs with positive declinations and extrapolated fluxes from the 3FHL catalog exceeding 1% Crab at >200 GeV after correcting for EBL absorption were selected for this survey, resulting in eight targets. Additionally, four FSRQs that were already detected at TeV energies are also included in this survey. In an unbiased fashion, the observations of 12 FSRQs, even without detection, will provide the first constraints on their duty cycle of TeV emission. We report the results from four of the 12 FSRQs observed during 2020-21 season in this work. For these sources, we also show the results from nearly simultaneous \textit{Fermi}-LAT observations.

Abstract: The regular monitoring of flat-spectrum radio quasars (FSRQs) in γ-rays by Fermi-LAT (Large Area Telescope) since past 12 yr indicated six sources who exhibited extreme γ-ray outbursts crossing daily flux of 10^-5 photons cm^-2 s^-1. We obtained nearly simultaneous multiwavelength data of these sources in radio to γ-ray waveband from OVRO (Owens Valley Radio Observatory), Steward Observatory, SMARTS (Small and Moderate Aperture Research Telescope System), Swift-UVOT (UV/Optical Telescope), Swift-XRT (X-ray Telescope), and Fermi-LAT. The time-averaged broad-band spectral energy distributions (SEDs) of these sources in quiescent states were studied to get an idea about the underlying baseline radiation processes. We modelled the SEDs using one-zone leptonic synchrotron and inverse Compton emission scenario from broken power-law electron energy distribution inside a spherical plasma blob, relativistically moving down a conical jet. The model takes into account inverse Compton scattering of externally and locally originated seed photons in the jet. The big blue bumps visible in quiescent state SEDs helped to estimate the accretion disc luminosities and central black hole masses. We found a correlation between the magnetic field inside the emission region and the ratio of emission region distance to disc luminosity, which implies that the magnetic field decreases with an increase in emission region distance and decrease in disc luminosity, suggesting a disc-jet connection. The high-energy index of the electron distribution was also found to be correlated with observed γ-ray luminosity as γ-rays are produced by high-energy particles. In most cases, kinetic power carried by electrons can account for jet radiation power as jets become radiatively inefficient during quiescent states.

Abstract: Blazars, a class of highly variable active galactic nuclei, sometimes exhibit Orphan γ-ray flares. These flares having high flux only in γ-ray energies do not show significant variations in flux at lower energies. We study the temporal and spectral profile of these Orphan γ-ray flares in detail from three γ - ray bright blazars, 3C 273, PKS 1510-089 and 3C 279 and also their simultaneous broadband emissions. We find that the variability timescales of the Orphan γ-ray flares were (0.96 ± 0.28) days, (3.12 ± 2.40) hr and (2.16 ± 0.72) hr, for 3C 273, PKS 1510-089 and 3C 279, respectively. The broadband spectral energy distributions (SEDs) during these flares have been modelled with a leptonic model from two emission regions. This model suggests that Orphan γ-ray flares might have originated from inverse Compton scattering of relativistic electrons by the seed photons from the broad-line region or dusty torus, which is the first region. While the second broader region, lying further down the jet, could be responsible for X-ray and radio emissions. The possible locations of these emission regions in the jets of the three sources have been estimated from SED modelling.

Abstract: A study of blazar PKS 1424-418 was carried out using multi-waveband data collected by the Fermi Large Area Telescope (Fermi-LAT), Swift X-Ray Telescope (Swift-XRT), Swift UltraViolet and Optical Telescope (Swift-UVOT), and Small and Moderate Aperture Research Telescope System (SMARTS) between MJD 56000 and MJD 56600 (2012 March 14-2013 November 4). Two flaring episodes were identified by analysing the gamma-ray light curve. Simultaneous multi-waveband spectral energy distributions (SEDs) were obtained for those two flaring periods. A cross-correlation analysis of infrared (IR)-optical and γ-ray data suggested that the emission originated from the same region. We have set a lower limit for the Doppler factor using the highest energy photon observed from this source during the flaring periods, which should be >12. The broad-band emission mechanism was studied by modelling the SED using the leptonic emission mechanism.

Abstract: The flat-spectrum radio quasar Ton 599 attained its highest ever γ-ray flux state during the first week of 2017 November. Observations of the source by the Swift satellite during this period made it possible to generate a simultaneous high flux state broad-band spectral energy distribution (SED). The high flux state activity of Ton 599 is modelled in this work for the first time. We modelled one high flux state and one quiescent state of the source in order to characterize the evolution of SEDs covering the entire dynamic range of γ-ray flux observed by Fermi-LAT. An attempt was made to model the 2017 November state of the source using an external Compton (EC) model in the leptonic scenario. We reproduce the broad-band flaring state SED using a two-component leptonic emission model. We considered one component as an EC+synchrotron self-Compton (SSC) component and the other as pure SSC, lying further down in the jet. The EC+SSC component was located outside the broad-line region (BLR). It mainly reproduces the GeV emission by an EC process with a dusty torus (DT) photon field providing seed photons. We reproduce the broad-band emission from Ton 599 satisfactorily during its peculiar flaring state with a leptonic two-component model. Besides this, we compare the model parameters of a quiescent-state SED with the available average state model parameters in the literature.

Abstract: We performed a long-term optical (B, V, R bands), infrared (J and K bands), and radio band (15, 22, 37 GHz band) study of the flat spectrum radio quasar, 3C 454.3, using data collected over a period of more than 8 yr (MJD 54500─57500). The temporal variability, spectral properties, and interwaveband correlations were studied by dividing the available data into smaller segments with more regular sampling. This helped us constrain the size and the relative locations of the emission regions for different wavebands. Spectral analysis of the source revealed the interplay between the accretion disk and jet emission. The source predominantly showed a redder-when-brighter trend, though we observed a bluer-when-brighter trend at high flux levels, which could be a signature of particle acceleration and radiative cooling. Significant correlations with near-zero lag were seen between various optical and infrared bands, indicating that these emission regions are cospatial. Correlations with a time lag of about 10─100 days are seen between the optical/infrared and radio bands indicating these emissions arise from different regions. We also observe the DCF peak lag change from year to year. We try to explain these differences using a curved jet model where the different emission regions have different viewing angles resulting in a frequency-dependent Doppler factor. This variable Doppler factor model explains the variability timescales and the variation in DCF peak lag between the radio and optical emissions in different segments. Lags of 6─180 days are seen between emissions in various radio bands, indicating a core-shift effect.

Abstract: The TeV blazar Ton 599 has exhibited a peculiar flare in 2017 November. The temporal variation of the source is studied using simultaneous γ-ray data from the Fermi Large Area Telescope and radio data from the Owens Valley Radio Observatory’s 40 m telescope, over the period of 9 yr. Four major flaring periods are observed in the γ-ray energy band of 0.1-300 GeV. These periods are studied on a shorter timescale and modeled with a time-dependent function containing exponential rising and decaying components. The physical parameters of the jet are estimated numerically and compared with those reported in the literature. During the fourth flare, a bunch of high-energy photons (>10 GeV) were detected. The two highest-energy photons, with energies of 76.9 and 61.9 GeV, are detected on MJD 58,059.0 and 58,073.3, respectively. This observation possibly constrains the γ-ray emission region to lie near the outer edge or outside the broad-line region of size ∼0.08 pc. The variation of equivalent width of an Mg II line is studied using the spectroscopic data from Steward Observatory. It was observed that the equivalent width of the line varies inversely with the underlying power-law continuum.

Aims: The nearby TeV blazar 1ES 1959+650 (z = 0.047) was reported to be in flaring state during June-July 2016 by Fermi-LAT, FACT, MAGIC and VERITAS collaborations. We studied the spectral energy distributions (SEDs) in different states of the flare during MJD 57530-57589 using simultaneous multiwaveband data with the aim of understanding the possible broadband emission scenario during the flare.

Methods: The UV-optical and X-ray data from UVOT and XRT respectively on board Swift and high energy γ-ray data from Fermi-LAT were used to generate multiwaveband lightcurves as well as to obtain high flux states and quiescent state SEDs. The correlation and lag between different energy bands was quantified using discrete correlation function. The synchrotron self-Compton (SSC) model was used to reproduce the observed SEDs during flaring and quiescent states of the source.

Results: A good correlation is seen between X-ray and high energy γ-ray fluxes. The spectral hardening with increase in the flux is seen in X-ray band. The power law index vs. flux plot in γ-ray band indicates the different emission regions for 0.1-3 GeV and 3-300 GeV energy photons. Two zone SSC model satisfactorily fits the observed broadband SEDs. The inner zone is mainly responsible for producing synchrotron peak and high energy γ-ray part of the SED in all states. The second zone is mainly required to produce less variable optical-UV and low energy γ-ray emission.

Conclusions: Conventional single zone SSC model does not satisfactorily explain broadband emission during observation period considered. There is an indication of two emission zones in the jet which are responsible for producing broadband emission from optical to high energy γ-rays.

Abstract: The flat-spectrum radio quasar CTA 102 experienced a prolonged state of enhanced activity across the entire observed electromagnetic spectrum during 2016-2017, most pronounced during a major outburst between 2016 December and 2017 May. Fermi-LAT observed a flux of (2.2 ± 0.2) × 10^-5 photons cm^-2 s^-1 at energies above 100 MeV on 2017 April 19 during a single orbit. We report here the detection of significant (4.7σ) flux variations down to timescales of ∼5 minutes during this orbit. The measured variability timescale is much shorter than the light-travel time across the central black hole (∼70 minutes) indicating a very compact emission region within the jet, similar to that seen in IC 310, Mrk 501, or PKS 1222+21 from MAGIC observations. This short-timescale variability is unexpected since the γ-ray spectrum shows no sign of attenuation due to pair creation in interactions with photons from the broad emission line region, and therefore must be assumed to originate far from the black hole. The observed fast variability could either indicate the dissipation of magnetic islands or protons in a collimated beam from the base of the jet encountering the turbulent plasma at the end of the magnetic nozzle.