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We present a comprehensive variability study on three blazars, S4 0954+65, PKS 0903−57, and 4C +01.02, covering a mass range of log(M /M⊙) = 8–9, by using ∼15-yr-long γ-ray LCs from Fermi -LAT. The variability level is characterized by the fractional variability amplitude that is higher for γ-rays compared with optical/UV and X-rays emissions. A power spectral density (PSD) study and damped random walk (DRW) modelling are done to probe the characteristic time-scale. The PSD is fitted with a single power law (PL) and bending PL models and the corresponding success fraction was estimated. In the case of PKS 0903−57, We observed a break in the γ-ray PSD at 256 d, which is comparable to the viscous time-scale in the accretion disc, suggesting a possible disc–jet coupling. The non-thermal damping time-scale from the DRW modelling is compared with the thermal damping time-scale for AGNs including our three sources. Our sources lie on the best fit of the |$\mathrm{\tau ^{\rm rest}_{\rm damping}}\!-\!M_{\rm BH}$| plot derived for AGN, suggesting a possible accretion disc–jet connection. If the jet's variability is linked to the disc's variability, we expect a log-normal flux distribution, often connected to the accretion disc's multiplicative processes. Our study observed a double log-normal flux distribution, possibly linked to long- and short-term variability from the accretion disc and the jet. In summary, PSD and DRW modelling results for these three sources combined with blazars and AGNs studied in literature favour a disc–jet coupling scenario. However, more such studies are needed to refine this understanding. [ABSTRACT FROM AUTHOR]

We report on a multiwavelength study of the high-synchrotron-peaked BL Lac 1ES 1218+304 using near-simultaneous data obtained during the period from 2018 January 1 to 2021 May 31 (MJD 58119–59365) from various instruments, including Fermi–LAT, Swift–XRT, AstroSat , and optical data from Swift-UVOT and the TUBITAK observatory in Turkey. The source was reported to be flaring in the TeV γ-ray band during 2019, but no significant variation was observed with Fermi–LAT. A sub-hour variability is seen in the SXT light curve, suggesting a compact emission region for the variability. However, hour-scale variability is observed in the γ-ray light curve. A 'softer-when-brighter' trend is observed in the γ-ray, and an opposite trend is seen in the X-ray, suggesting that the two emissions are produced through two different processes, as expected from a high-frequency-peaked BL Lac source. We have chosen the two epochs in 2019 January to study and compare their physical parameters. A joint fit of SXT and LAXPC provides a constraint on the synchrotron peak, estimated to be ∼1.6 keV. A clear shift in the synchrotron peak is observed from ∼1 keV to above 10 keV, revealing its extreme nature or behaviour like an extreme blazar-type source. The optical observation provides a colour-index variation as 'blue-when-brighter'. The broad-band spectral energy distribution is fitted with a single-zone synchrotron-self Compton model, and their parameters are discussed in the context of a TeV blazar and the possible mechanism behind the broad-band emission. [ABSTRACT FROM AUTHOR]

Detection of delayed sub-TeV photons from gamma-ray bursts (GRBs) by MAGIC and HESS has proven the promising future of GRB afterglow studies with the Cherenkov Telescope Array (CTA), the next-generation gamma-ray observatory. With the unprecedented sensitivity of CTA, afterglow detection rates are expected to increase dramatically. In this paper, we explore the multidimensional afterglow parameter space to see the detectability of sub-TeV photons by CTA. We use a one-zone electron synchrotron and synchrotron self-Compton model to obtain the spectral energy distribution. We consider bursts going off in a medium of homogenous density. The blast wave is assumed to be radiatively inefficient and evolving adiabatically. Considering that the electron acceleration is not efficient if the acceleration time-scale exceeds the radiative cooling time-scale, we find that the sub-TeV emission is always due to the self-Compton process. We find that jets with high kinetic energy or large bulk Lorentz factor decelerating into a dense ambient medium offer better detection prospects for CTA. For relatively lower values of the downstream magnetic field, electrons are slow-cooling, and the emitted radiation is positively correlated with the magnetic field. For larger magnetic fields, the electron population enters the fast-cooling phase where the radiated flux is inversely proportional to the magnetic field. We apply our results in the context of bright TeV afterglows detected in recent years. Our results indicate that cosmological short GRBs have only moderate prospects of detection by CTA while local neutron star merger counterparts can be detected if the jet is launched towards the observer. [ABSTRACT FROM AUTHOR]

In this study, we carry out a comprehensive variability analysis of the BL Lac object 4FGL J2202.7+4216 detected by the Fermi- LAT, over a period of more than 3 yr, from 2019 April 27 to 2022 August 09. We detect the presence of quasi-periodic fluctuations with a period of ∼100 d with a confidence level exceeding 4σ. We also tentatively detect two other quasi-periodic fluctuations with a period of ∼75 and ∼50 d, which persist over the entire duration of observation. This is the first time such a variability feature pertaining to this source is being reported. We propose that the observed quasi-periodic oscillations may be related to the precession of the blazar jet with a high Lorentz factor or to the motion of a plasma blob through the helical structure of the jet. The harmonicity among the detected periodicities suggests the global p-mode oscillations of the thick disc to be a plausible mechanism as well. For a decisive conclusion on the physical origin of such fluctuation, further multiwavelength complementary observations, especially very long baseline interferometric observations, would be required. [ABSTRACT FROM AUTHOR]

We have presented a multiwavelength temporal and spectral study of the blazar PKS 0346-27 for the period 2019 January–2021 December (MJD 58484–59575) using data from Fermi -LAT (γ-rays), Swift -XRT (X-rays), and Swift -UVOT (ultraviolet and optical). We identified multiple flaring episodes by analysing the gamma-ray light curve generated from the Fermi -LAT data over a 2-yr period. The light curves of these individual gamma-ray flares with 1-d binning were then modelled using a sum-of-exponentials fit. We found the minimum variability times for the gamma-ray light curve to be 1.34 ± 0.3 d and a range of 0.1–3.2 d for the Swift wavelengths, suggesting the compactness of the source. The broad-band emission mechanism was studied by modelling the simultaneous multiwaveband spectral energy distributions (SEDs) using the one-zone leptonic emission mechanism. We found that the optical-UV and X-ray data can be explained by the synchrotron and synchrotron self-Compton (SSC) emissions. However, the disc component of the external Compton (EC) radiation is dominant at higher energies with contributions from the EC broad-line region component and SSC. Further, we performed a power spectral density analysis with data from the gamma-ray light curve using the power spectrum response method. With the power-law model, the best-fitting slope of 2.15 ± 0.87 was found. This source could be a promising target for the upcoming Cherenkov telescope array for its harder spectrum at lower energies (tens of GeV). [ABSTRACT FROM AUTHOR]

We attempt to present a multiwavelength variability and correlation study as well as detailed multiwaveband spectral characteristics of the May 2021 gamma-ray flare of the blazar source TXS 1700+685. The multiwavelength observation from Fermi -LAT, Swift -XRT/UVOT as well as radio archival data are used for our spectro-temporal investigation. We estimate the variability time-scale of the source from the flux doubling time in different flaring region detected in Fermi -LAT observation and the shortest variability time is used to put a constraint on the minimum Doppler factor and on the size of the emission region. We have detected a statistically significant quasi-periodic oscillation feature at ∼17 d. The broad-band emission is satisfactorily represented during its flaring state with a leptonic synchrotron and inverse Compton component. From the broad-band spectral modelling, we observe the external Comptonization of the seed photons originating in the broad-line region to be dominant compared to the dusty torus. The equipartition value implies the energy density of the magnetic field in the jet comoving frame is weak. In order to produce the high-energy hump, we need the injection of a large population of high-energy electrons and/or the presence of strong magnetic field; and we observe the later component to be subdominant in our case. The gamma-ray spectral energy distribution shows the flat rising and steep falling profile, as well as the break or spectral curvature at ∼1 GeV, which has been seen for other flat-spectrum radio quasar sources before. [ABSTRACT FROM AUTHOR]

Blazars are a class of AGN, one of their jets is pointed towards the earth. Here, we report about the multiwavelength study for blazar S5 1803+78 between MJD 58727 and MJD 59419. We analysed gamma-ray data collected by Fermi-LAT, X-ray data collected by Swift-XRT & NuSTAR, and optical photons detected by Swift-UVOT & TUBITAK observatory in Turkey. Three flaring states are identified by analysing the gamma-ray light curve. A day-scale variability is observed throughout the flares with the similar rise and decay times suggesting a compact emission region located close to the central engine. Cross-correlation studies are carried out between gamma-ray, radio, and X-ray bands, and no significant correlation is detected. The gamma-ray and optical emission are significantly correlated with zero time lag suggesting a co-spatial origin of them. The broad-band spectral energy distribution (SED) modelling was performed for all the flaring episodes as well as for one quiescent state for comparison. SEDs are best fitted with the synchrotron-self Compton (SSC) model under a one-zone leptonic scenario. The SED modelling shows that to explain the high flaring state, strong Doppler boosting is required. [ABSTRACT FROM AUTHOR]