Search

In the period between 2009 and 2015, several very high-energy (VHE |$> 100$|  GeV) gamma-ray flaring events from the BL Lac object PKS 1424+240 were observed by the Cerenkov telescopes VERITAS and MAGIC. It had uncertain redshift (z) and using spectroscopical measurement, Paiano et al. (2017) found it to be |$z=0.604$|⁠. Using four different extragalactic background light (EBL) models and the photohadronic model, nine independently observed VHE gamma-ray spectra of PKS 1424+240 are analysed and a global |$\chi ^2$| fit is performed on all observations to estimate the best-fitting value for the redshift for each EBL model. Confidence levels (CL) intervals for the redshift are also estimated using all the EBL models. This method is tested by comparing our analysis with the observed value. It is shown that the photohadronic scenario provides an excellent description of all the observed spectra. It is found that the EBL model of Dominguez et al. (2011) is the one that provides the most restrictive limits on the redshift of PKS 1424+240, but in our analysis, |$z=0.604$| lies within the |$3\sigma$| CL interval of the EBL model of Saldana-Lopez et al. (2021). [ABSTRACT FROM AUTHOR]

The observation data of blazar 1ES 1426+42.8 were obtained using the 1.02 m optical telescope of Yunnan Observatories during 2021 to 2023. Intraday variability (IDV) is detected on seven nights. We use the turbulent model to investigate the mechanism of IDV in 1ES 1426+42.8. The fitting light curves match the actual IDV curves well. Using this model, we obtain the parameters such as the size of turbulent cells and the width of pulses in the jet. A possible short-lived quasi-periodic oscillation (QPO) of |$58.55 \pm 8.09$| min was detected on 2022 April 26 whose light curve exhibits eight cycles at |$\gt 3\sigma$| global significance and confirmed by several different techniques. Through a more detailed analysis of the light curve of this night, we find that the period is shortened from 54.23 min (⁠|$4\sigma$|⁠) to 29.71 min (⁠|$3\sigma$|⁠). The possible QPO and period shortening phenomenon are best explained by the processes of magnetic reconnections. [ABSTRACT FROM AUTHOR]

The Fermi -LAT has detected more than 7000 |$\gamma$| -ray sources which show emission above 50 MeV of which more than half are associated with blazars. However, the Fourth Fermi -LAT catalogue (4FGL-DR4) lists 1625 of these as blazar candidates of uncertain type. Increasing the number of classified Fermi -LAT sources is important for improving our understanding of extragalactic |$\gamma$| -ray sources and can be used to search for new classes of very high energy sources. We report on the optical spectroscopy of twelve blazar candidates with hard photon indices included in the Third Catalogue of Hard Fermi -LAT Sources during 2016 and 2017 using the SAAO 1.9-m telescope. We classify all the sources observed as BL Lac objects, and determine the potential spectroscopic redshift for seven of them. [ABSTRACT FROM AUTHOR]

We present the results of seven years of multicolour photometric monitoring of a sample of 31 |$\gamma$| -ray bright blazars using the RINGO3 polarimeter on the Liverpool Telescope from 2013–2020. We explore the relationships between simultaneous observations of flux in three optical wavebands along with Fermi |$\gamma$| -ray data in order to explore the radiation mechanisms and particle populations in blazar jets. We find significant correlations between optical and |$\gamma$| -ray flux with no detectable time lag, suggesting leptonic emission processes in the jets of these sources. Furthermore, we find the spectral behaviour against optical and |$\gamma$| -ray flux for many sources is best fit logarithmically. This is suggestive of a transition between bluer-/redder-when-brighter into stable-when-brighter behaviour during high activity states; a behaviour that might be missed in poorly sampled data, resulting in apparent linear relationships. [ABSTRACT FROM AUTHOR]

Blazars constitute the most numerous source class in the known extragalactic population of very high energy (VHE) gamma-ray sources. However, determining their redshifts is often challenging due to weak or non-existent emission lines in their spectra. This study focuses on two BL Lacs, KUV 00311−1938 and S2 0109+22, where previous attempts at redshift determination have faced difficulties. By combining spectroscopic observations with photometric redshift estimates, we tentatively assign a redshift of |$z=0.634$| to KUV 00311−1938 and a likely redshift of |$z=0.49$| to S2 0109+22. Establishing redshift estimates for high-redshift blazars is crucial for understanding extragalactic VHE gamma-ray sources and their interactions with the surrounding Universe. [ABSTRACT FROM AUTHOR]

The fourth-DR3 version (4FGL-DR3) of the Fermi -LAT catalogue of γ-ray sources contains ∼1000 objects at a galactic latitude | b | > 10° that are not identified with an optical counterpart. We performed a systematic study of these sources, focusing on 190 objects that have a unique X-ray counterpart in the available Swift /XRT observations. Optical counterparts were then selected, and optical spectra were found in the literature for 33 sources. We found that 21 could be classified as BL Lac objects. Among these, we were able to provide the redshift for eight of them, while for two others we established a lower limit to the redshift by detecting intervening absorption. The other 12 objects display optical spectra with prominent emission lines (0.036 <  z  < 1.65). These spectra are characterized by both broad and narrow emission lines, with the exception of three sources. One of these displays only broad emission lines, while the other two exclusively exhibit narrow lines. On the basis of the radio/optical flux ratio, all BL Lac objects in this study are radio-loud. Four sources out of the 12 with prominent emission lines can be classified as radio-loud, while at least five of the 12 sources with prominent lines are radio-quiet. This is somewhat unexpected, given the radio-loudness distribution of 4FGL-associated blazars. [ABSTRACT FROM AUTHOR]

Blazars present highly variable gamma-ray emission. This variability, which can range from a few minutes to several years, is also observed at other wavelengths across the entire electromagnetic spectrum. We make use of the first 12 yr of data from the Fermi Large Area Telescope, complemented with multiwavelength (MWL) archival data from different observatories and facilities in radio, infrared, and optical bands, to study the possible periodic emission from 19 blazars previously claimed as periodic candidates. A periodicity analysis is performed with a pipeline for periodicity searches. Moreover, we study the cross-correlations between the gamma-ray and MWL light curves. Additionally, we use the fractional variability and the structure function to evaluate the variability time-scales. We find five blazars showing hints of periodic modulation with ≥3.0σ (≈0σ post-trials), with periods ranging from 1.2 to 4 yr, both in their gamma-ray and MWL emission. The results provide clues for understanding the physical mechanisms generating the observed periodicity. [ABSTRACT FROM AUTHOR]

Machine learning has emerged as a powerful tool in the field of gamma-ray astrophysics. The algorithms can distinguish between different source types, such as blazars and pulsars, and help uncover new insights into the high-energy universe. The Large Area Telescope onboard the Fermi gamma-ray telescope has significantly advanced our understanding of the Universe. The instrument has detected a large number of gamma-ray-emitting sources, among which a significant number of objects have been identified as active galactic nuclei. The sample is primarily composed of blazars; however, more than one-third of these sources are either of an unknown class or lack a definite association with a low-energy counterpart. In this work, we employ multiple machine learning algorithms to classify the sources based on their other physical properties. In particular, we utilized smart initialization techniques and self-supervised learning for classifying blazars into BL Lacertae (BL Lac, also BLL) objects and flat-spectrum radio quasars (FSRQs). The core advantage of the algorithm is its simplicity, usage of minimum number of features and easy deployment due to lesser number of parameters without compromising on the performance along with increase in inference speed (at least seven times more than existing algorithms). As a result, the best-performing model is deployed on multiple platforms so that any user irrespective of their coding background can use the tool. The model predicts that out of the 1115 sources of uncertain type in the 4FGL-DR3 catalogue, 820 can be classified as BL Lacs and 295 can be classified as FSRQs. [ABSTRACT FROM AUTHOR]

This document is a correction to a previous study on the multi-wavelength and neutrino spectral energy distributions (SEDs) of potential neutrino sources. The correction includes updated data and recalculations of fluxes and confidence bands. The study found that 27 out of 34 sources had a best-fit neutrino flux greater than 0, but 11 of those cases had a 68% confidence level compatible with no flux. The study also found a 2.2σ association between sources with matching gamma-ray and neutrino fluxes. However, the authors caution that these results are based on a small background sample and require confirmation with better statistics. [Extracted from the article]

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]

The extragalactic high-energy γ-ray sky is dominated by blazars, which are active galactic nuclei with their jets pointing towards us. Distance measurements are of fundamental importance yet for some of these sources are challenging because any spectral signature from the host galaxy may be outshone by the non-thermal emission from the jet. In this paper, we present a method to constrain redshifts for these sources that relies only on data from the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. This method takes advantage of the signatures that the pair-production interaction between photons with energies larger than approximately 10 GeV and the extragalactic background light leaves on γ-ray spectra. We find upper limits for the distances of 303 γ-ray blazars, classified as 157 BL Lacertae objects, 145 of uncertain class, and 1 flat-spectrum-radio quasar, whose redshifts are otherwise unknown. These derivations can be useful for planning observations with imaging atmospheric Cherenkov telescopes and also for testing theories of supermassive black hole evolution. Our results are applied to estimate the detectability of these blazars with the future Cherenkov Telescope Array , finding that at least 21 of them could be studied in a reasonable exposure of 20 h. [ABSTRACT FROM AUTHOR]

Context. Among the blazar class, extreme blazars have exceptionally hard intrinsic X-ray/TeV spectra, and extreme peak energies in their spectral energy distribution (SED). Observational evidence suggests that the non-thermal emission from extreme blazars is typically non-variable. All these unique features present a challenging case for blazar emission models, especially regarding those sources with hard TeV spectra. Aims. We aim to explore the X-ray and GeV observational features of a variety of extreme blazars, including extreme-TeV, extreme-synchrotron (extreme-Syn), and regular high-frequency-peaked BL Lac objects (HBLs). Furthermore, we aim to test the applicability of various blazar emission models that could explain the very hard TeV spectra. Methods. We conducted a detailed spectral analysis of X-ray data collected with AstroSat and Swift-XRT, along with quasi-simultaneous γ-ray data from Fermi-LAT, for five sources: 1ES 0120+340, RGB J0710+591, 1ES 1101−232, 1ES 1741+196, and 1ES 2322−409. We took three approaches to modelling the SEDs: (1) a steady-state one-zone synchrotron-self-Compton (SSC) code, (2) another leptonic scenario of co-accelerated electrons and protons on multiple shocks applied to the extreme-TeV sources only (e–p co-acceleration scenario), and (3) a one-zone hadro-leptonic (ONEHALE) code. The latter code is used twice to explain the γ-ray emission process: proton synchrotron and synchrotron emission of secondary pairs. Results. Our X-ray analysis provides well-constrained estimates of the synchrotron peak energies for both 1ES0120+340 and 1ES1741+196. These findings categorise these latter objects as extreme-synchrotron sources, as they consistently exhibit peak energies above 1 keV in different flux states. The multi-epoch X-ray and GeV data reveal spectral and flux variabilities in RGB J0710+591 and 1ES 1741+196, even on timescales of days to weeks. As anticipated, the one-zone SSC model adequately reproduces the SEDs of regular HBLs but encounters difficulties in explaining the hardest TeV emission. Hadronic models offer a reasonable fit to the hard TeV spectrum, though with the trade-off of requiring extreme jet powers. On the other hand, the lepto-hadronic scenario faces additional challenges in fitting the GeV spectra of extreme-TeV sources. Finally, the e–p co-acceleration scenario naturally accounts for the observed hard electron distributions and effectively matches the hardest TeV spectrum of RGB J0710+591 and 1ES 1101−232. [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]

In 2022 the BL Lac object S4 0954+65 underwent a major variability phase, reaching its historical maximum brightness in the optical and γ-ray bands. We present optical photometric and polarimetric data acquired by the Whole Earth Blazar Telescope (WEBT) Collaboration from 2022 April 6 to July 6. Many episodes of unprecedented fast variability were detected, implying an upper limit to the size of the emitting region as low as |$10^{-4}$| parsec. The WEBT data show rapid variability in both the degree and angle of polarization. We analyse different models to explain the polarization behaviour in the framework of a twisting jet model, which assumes that the long-term trend of the flux is produced by variations in the emitting region viewing angle. All the models can reproduce the average trend of the polarization degree, and can account for its general anticorrelation with the flux, but the dispersion of the data requires the presence of intrinsic mechanisms, such as turbulence, shocks, or magnetic reconnection. The WEBT optical data are compared to γ-ray data from the Fermi satellite. These are analysed with both fixed and adaptive binning procedures. We show that the strong correlation between optical and γ-ray data without measurable delay assumes different slopes in faint and high brightness states, and this is compatible with a scenario where in faint states we mainly see the imprint of the geometrical effects, while in bright states the synchrotron self-Compton process dominates. [ABSTRACT FROM AUTHOR]

We present results from the first dedicated study in the time domain of the hard X-ray variability behaviour of blazars on long time-scales based on ∼13 yr of continuous hard X-ray data in the 14–195 keV band. We use monthly binned data from the recent 157-month Swift -BAT (Burst Alert Telescope) catalogue to characterize the hard X-ray variability of 127 blazars and search for potential differences between the variability of BL Lacertae objects (BL Lacs) and flat-spectrum radio quasars (FSRQs). A significant portion of the blazars in the sample (∼37  per cent) do not show statistically significant hard X-ray variability on monthly time-scales, which is deeply at odds with previous studies that show that blazars are highly variable in the X-rays and other energy bands on a wide range of time-scales. We also find that, on average, the FSRQs and BL Lacs for which we do detect variability exhibit similar flux variability; this suggests that the variability in these FSRQs is not necessarily driven by variations in the source function of scattered external radiation arriving from extended regions, and that it is instead possibly driven by processes that lead to variations in particle injection. In addition, only five blazars in our sample show significant spectral variability in the long-term light curves. For three blazars, we find that a power law that changes slope on monthly time-scales is sufficient to characterize the variable hard X-ray spectrum, suggesting that, at least for some bright blazars, the long-term spectra in the hard X-rays may be described in a relatively simple fashion. [ABSTRACT FROM AUTHOR]

We calculate the jet power of the Blandford–Znajek (BZ) model and the hybrid model based on the self-similar solution of advection-dominated accretion flows (ADAFs). We study the formation mechanism of the jets of BL Lacertae (BL Lacs) with known redshifts detected by the Fermi satellite after 10 yr of data (4FGL-DR2). The kinetic power of the jets of Fermi BL Lacs is estimated through radio luminosity. The main results are as follows. (1) We find that the jet kinetic power of about 72 per cent intermediate peak frequency BL Lacs (IBL) and 94 per cent high-frequency peak BL Lacs (HBL) can be explained by the hybrid jet model based on ADAFs surrounding Kerr black holes. However, the jet kinetic power of about 74 per cent low-frequency peak BL Lacs (LBL) cannot be explained by the BZ jet model or the hybrid model. (2) The LBL has a higher accretion rate than IBL and HBL. About 14 per cent IBL and 62 per cent HBL have pure optically thin ADAFs. However, 7 per cent LBL may have a hybrid structure consisting of an standard thin disc (SS) plus optically thin ADAFs. (3) After excluding the redshift dependence, there is a weak correlation between the jet kinetic power and the accretion disc luminosity for Fermi BL Lacs. (4) There is a significant correlation between inverse-Compton luminosity and synchrotron luminosity for Fermi BL Lacs. The slope of the relation between inverse-Compton luminosity and synchrotron luminosity for Fermi BL Lacs is consistent with the synchrotron self-Compton (SSC) process. The result may suggest that the high-energy components of Fermi BL Lacs are dominated by the SSC process. [ABSTRACT FROM AUTHOR]

To demonstrate the magnetic energy dissipation via relativistic shocks, we carry out spherically symmetrical one-dimensional special relativistic magnetohydrodynamic simulations of highly magnetized outflows with an adaptive mesh refinement method. We first investigate the details of the dynamical energy dissipation via interaction between a single ejecta and an external medium. The energy dissipation time-scales, which affect the early behaviour of the afterglow emission in gamma-ray bursts, are estimated for a wide range of magnetization. In addition, we demonstrate the internal shock dissipation in multiple interactions between magnetically dominated relativistic ejecta and kinetically dominated non-relativistic winds. Our numerical results show that ∼10 per cent of the magnetic energy in the ejecta can be converted into the thermal energy of the relativistic and low-magnetized outflows via shocks in the rarefaction waves or the winds. Such hot and less magnetized outflows are relevant for observed non-thermal emissions in blazars or gamma-ray bursts. [ABSTRACT FROM AUTHOR]

We present hybrid spectral energy distributions, combining photon, and neutrino fluxes, for a sample of blazars, which are candidate IceCube neutrino sources. We furthermore check for differences in our sources' variability in the near-infrared, optical, X-ray, and γ-ray bands compared to a sample of non-neutrino source candidate blazars, and investigate the state of each blazar at the arrival time of high-energy neutrinos. We find no significant differences when comparing our sample with control sources, also in terms of their spectral energy distributions, and no correlation between flaring states and neutrino arrival times. Looking for signatures of hadronic production, we check for similar strengths of the γ-ray and neutrino fluxes and find a |$2.2\, \sigma$| signal for our source candidates. The hybrid spectral energy distributions assembled here will form the basis of the next step of our project, namely lepto-hadronic modelling of these blazars to assess the physical likelihood of a neutrino connection. [ABSTRACT FROM AUTHOR]

An equivalent–width-based classification may cause the erroneous judgement to the flat spectrum radio quasars (FSRQs) and BL Lacerate objects (BL Lac) due to the diluting the line features by dramatic variations in the jet continuum flux. To help address the issue, this work explores the possible intrinsic classification on the basis of a random forest supervised machine learning algorithm. In order to do so, we compile a sample of 1680 Fermi blazars that have both gamma-rays and radio-frequencies data available from the 4LAC-DR2 catalogue, which includes 1352 training and validation samples and 328 forecast samples. By studying the results for all of the different combinations of 23 characteristic parameters, we found that there are 178 optimal parameter combinations (OPCs) with the highest accuracy (≃98.89 per cent). Using the combined classification results from the nine combinations of these OPCs to the 328 forecast samples, we predict that there are 113 true BL Lacs (TBLs) and 157 false BL Lacs (FBLs) that are possible intrinsically FSRQs misclassified as BL Lacs. The FBLs show a clear separation from TBLs and FSRQs in the gamma-ray photon spectral index, Γph, and X -band radio flux, log FR , plot. Phenomenally, existence a BL Lac to FSRQ (B-to-F) transition zone is suggested, where the FBLs are in the stage of transition from BL Lacs to FSRQs. Comparing the LSP changing-look blazars (CLBs) reported in the literatures, the majority of LSP CLBs are located at the B-to-F zone. We argue that the FBLs located at B-to-F transition zone are the most likely candidates of CLBs. [ABSTRACT FROM AUTHOR]

We report the detection of a quasi-periodic oscillation (QPO) in the optical R -band light curve of the blazar S5 0716+714 spanning approximately 32 years. Using both the Weighted Wavelet Z-transform and Lomb-Scargle Periodogram methods, we identified a prominent QPO of 1060 ± 50 days at a 5 σ confidence level against the red noise background. Our findings align well with previous studies. We introduce a supermassive binary black hole (SMBBH) model to account for the observed QPO and estimate the binary system's Keplerian orbit to be 0.0053 parsec, indicating S5 0716+714 as a notable subject for upcoming nanohertz gravitational wave studies. [ABSTRACT FROM AUTHOR]

The extragalactic background light (EBL) that spans the ultraviolet-infrared (UV-IR) band originates from direct and dust-reprocessed starlight integrated over the history of the Universe. EBL measurements are very challenging due to foreground emission like the zodiacal light and interplanetary dust emission. Indeed, some optical/NIR (near infrared) direct measurements overpredict EBL models based on galaxy counts. On the other hand, there is some debate on possible additional components of the optical-NIR photon density, e.g. population-III stars, axion-photon decay, direct collapse of black holes, intrahalo light, etc. Owing to the absorption of very high energy (VHE) gamma-rays by interaction with EBL photons, we study the prospects of accommodating an additional population of EBL sources in the optical-NIR band on top of the standard galaxy-count–based component. To this aim we use 105 VHE spectra of 37 blazars with known redshifts, 0.03 < z < 0.94. We correct the observed spectra for absorption by our model EBL. By requiring the intrinsic spectra to be non-concave and with a VHE spectral index >1.5, we estimate, at different wavelengths, upper limits to the additional low-energy photon fields that would contribute to the absorption of gamma-rays. Considering these limits, we suggest that there is room for photons from Pop III stars and axion-like particle annihilation. However, these additional hypothetical photon fields are bound to fall significantly below direct published EBL measurements by several instruments, and therefore, our limits are either in tension or even inconsistent with such measurements. [ABSTRACT FROM AUTHOR]

BL Lac objects detected at TeV energies preferentially belong to the subclass called 'high-frequency-peaked' BL Lacs (HBLs). Parsec-scale radio jets in these TeV-HBLs often show dominant, slow-moving radio knots that are at most mildly superluminal. We report the first systematic campaign to characterize the intranight optical variability (INOV) of TeV-HBLs using a representative sample of six such sources, all showing a fairly high degree of optical polarization. Our campaign consists of high-sensitivity monitoring of this sample in 24 sessions of more than 3 h duration each. For these TeV-HBLs, we find a striking lack of INOV and based on this, we discuss the importance of superluminal motion of the radio knots vis-a-vis the optical polarization, as the key diagnostic for INOV detection. [ABSTRACT FROM AUTHOR]

Using stacking of images obtained at different epochs, we studied the variability properties of linear polarization of active galactic nucleus (AGN) jets on parsec-scales. Our sample is drawn from the MOJAVE programme, and consists of 436 AGNs manifesting core-jet morphology and having at least five VLBA observing epochs at 15 GHz from 1996 January through 2019 August, with some additional archival VLBA data reduced by us. We employed a stacking procedure and constructed maps of (i) standard deviation of fractional polarization and electric vector position angle (EVPA) over epochs as the measure of variability and (ii) median polarization degree to quantify typical values in time. The distributions of these values along and across the jet were analysed for the whole sample for the first time. We found that core EVPA variability is typically higher than that of the jet, presumably due to component blending and outflow bends in the core. The BL Lacertae object cores have lower EVPA variability, compared to that of quasars, possibly due to lower Faraday rotation measure, suggesting a stronger ordered magnetic field component. The EVPA becomes more stable down the jet. Most of the sources showing this trend have a time coverage of more than 12 yr and at least 15 epochs. The possible cause could be the increase of stability in the magnetic field direction, reflecting an increase in the fraction of the magnetic field that is ordered. There are no significant optical-class-dependent or spectral-class-dependent relations in the EVPA variability properties in AGN jets. [ABSTRACT FROM AUTHOR]

Observation of several very high energy (VHE) flaring events of the BL Lac object VER J0521+211 was reported by the VERITAS and MAGIC collaborations between 2009 and 2014. The redshift of this source is uncertain, and several analyses have derived different limits for it. In the framework of the photohadronic model and using three different extragalactic background light (EBL) models, we analyse seven independent VHE spectra of VER J0521+211 and determine the limiting values on its redshift. It is observed that the photohadronic scenario provides excellent fits to the reported observations. It is further observed that the photohadronic scenario, along with the EBL model of Domínguez et al. puts the most restrictive limits on the redshift z of VER J0521+211: 0.29 ≤ z ≤ 0.31 from the confidence level (CL) intervals at 2σ, or a more conservative 0.28 ≤ z ≤ 0.33 at 3σ. [ABSTRACT FROM AUTHOR]

We explore the potential of the ongoing Zwicky Transient Facility (ZTF) survey for studying intranight optical variability (INOV) of active galactic nuclei (AGNs), in particular for picking rare events of large INOV amplitudes, whose detection may require extensive temporal coverage. For this, we have used the available high cadence subsets of the ZTF data base to build a well-defined large sample of 53 blazars and another sample of 132 radio-quiet quasars (RQQs), matched to the blazar sample in the redshift−magnitude plane. High-cadence ZTF monitoring of these two matched samples is available, respectively, for 156 and 418 intranight sessions. Median durations for both sets of sessions are 3.7 h. The two classes of powerful AGNs monitored in these sessions represent opposite extremes of jet activity. The present analysis of their ZTF light curves has revealed some strong INOV events that, although not exceptionally rare for blazars, are indeed so for RQQs, and their possible nature is briefly discussed. [ABSTRACT FROM AUTHOR]

In this work, we report the presence of rapid intra-night optical variations in both flux and polarization of the blazar BL Lacertae during its unprecedented 2020–2021 high state of brightness. The object showed significant flux variability and some colour changes, but no firmly detectable time delays between the optical bands. The linear polarization was also highly variable in both polarization degree and angle (electric vector polarization angle). The object was observed from several observatories throughout the world, covering a total of almost 300 h during 66 nights. Based on our results, we suggest that the changing Doppler factor of an ensemble of independent emitting regions, travelling along a curved jet that at some point happens to be closely aligned with the line of sight, can successfully reproduce our observations during this outburst. This is one of the most extensive variability studies of the optical polarization of a blazar on intra-night time-scales. [ABSTRACT FROM AUTHOR]

In 2021 BL Lacertae underwent an extraordinary activity phase, which was intensively followed by the Whole Earth Blazar Telescope (WEBT) Collaboration. We present the WEBT optical data in the BVRI bands acquired at 36 observatories around the world. In mid-2021 the source showed its historical maximum, with R  = 11.14. The light curves display many episodes of intraday variability, whose amplitude increases with source brightness, in agreement with a geometrical interpretation of the long-term flux behaviour. This is also supported by the long-term spectral variability, with an almost achromatic trend with brightness. In contrast, short-term variations are found to be strongly chromatic and are ascribed to energetic processes in the jet. We also analyse the optical polarimetric behaviour, finding evidence of a strong correlation between the intrinsic fast variations in flux density and those in polarization degree, with a time delay of about 13 h. This suggests a common physical origin. The overall behaviour of the source can be interpreted as the result of two mechanisms: variability on time-scales greater than several days is likely produced by orientation effects, while either shock waves propagating in the jet, or magnetic reconnection, possibly induced by kink instabilities in the jet, can explain variability on shorter time-scales. The latter scenario could also account for the appearance of quasi-periodic oscillations, with periods from a few days to a few hours, during outbursts, when the jet is more closely aligned with our line of sight and the time-scales are shortened by relativistic effects. [ABSTRACT FROM AUTHOR]

Here we present the first optical photometric monitoring results of a sample of 12 newly discovered blazars from the ICRF – Gaia CRF astrometric link. The observations were performed from April 2013 to August 2019 using eight telescopes located in Europe. For a robust test for the brightness and colour variability, we use Abbé criterion and F-test. Moreover, linear fittings are performed to investigate the relation in the colour-magnitude variations of the blazars. Variability was confirmed in the case of 10 sources; two sources, 1429+249 and 1556+335 seem to be possibly variable. Three sources (1034+574, 1722+119, and 1741+597) have displayed large amplitude brightness change of more than one magnitude. We found that the seven sources displayed bluer-when-brighter variations, and one source showed redder-when-brighter variations. We briefly explain the various AGN emission models which can explain our results. [ABSTRACT FROM AUTHOR]

Yes 4FGL J0847.2 + 1134 RX J0847.1 + 1133 08:47:13 + 11:33:50 0.198 ISP 0.990 SDS Yes Yes 4FGL J0850.5 + 3455 RX J0850.5 + 3455 08:50:36 + 34:55:23 0.145 HSP 0.994 SDS Yes Yes 4FGL J0854.0 + 2753 SDSS J085410.16 + 275421.7 08:54:10 + 27:54:22 0.494 ISP 0.991 SDS Unc. Yes 4FGL J0847.2 + 1134 RX J0847.1 + 1133 08:47:13 + 11:33:50 0.198 ISP 0.990 SDS Yes Yes 4FGL J0850.5 + 3455 RX J0850.5 + 3455 08:50:36 + 34:55:23 0.145 HSP 0.994 SDS Yes Yes 4FGL J0854.0 + 2753 SDSS J085410.16 + 275421.7 08:54:10 + 27:54:22 0.494 ISP 0.991 SDS Unc. Yes 4FGL J1340.8-0409 NVSS J134042-041006 13:40:42 -04:10:07 0.223 HSP 0.698 ZBL Yes Yes 4FGL J1341.6 + 5515 SBS 1339 + 554 13:41:36 + 55:14:37 0.207 ISP 0.994 SDS Yes Yes 4FGL J1353.3 + 1434 OP 186 13:53:23 + 14:35:39 0.808 LSP 0.994 SDS Unc. [Extracted from the article]

The extragalactic background light (EBL) in the infrared to ultraviolet bands partly absorbs very high energy (VHE; E ≥ 100 GeV) γ-ray photons travelling over cosmological distances via pair production. In this paper, to get stronger constraints on EBL, we use the deliberate selection of the EBL model and data of five BL Lacs with better statistics and the harder spectra to limit the EBL density and the radiation mechanism of BL Lacs. We constrain the upper limit of the EBL density by fitting the spectral energy distributions of TeV BL Lacs and find that our results are compatible with the published measurement, reaching 50 |$\rm {nW \,m^{-2} \,sr^{-1}}$|⁠. We also obtain that the EBL is not necessarily transparent to VHE photons. We fix the intrinsic spectral index Γ i of TeV BL Lacs as 1.0 and 1.5 under observation evidence and model assumption. Comparing the EBL density given by galaxy count and Spitzer observations, we then obtain that 1ES 1101−232 has Γ i ≤ 1.0 and 1ES 0229+200 should have Γ i not harder than 1.0. We demonstrate that the common radiation assumption of BL Lacs, in which the Γ i is softer than 1.5, should be revisited. Furthermore, we propose that the upper EBL density could be given by fitting the hardest energy spectra of TeV BL Lacs. [ABSTRACT FROM AUTHOR]

A correlation between astrophysical high-energy neutrinos and blazars has been suggested by various authors. In particular, a likely association between IceCube events and intermediate- and high-energy peaked BL Lac objects has led to a sample of 47 objects having a high probability of being neutrino sources. In the first paper of this series we reported optical spectroscopy of 17 objects, which together with data taken from the literature covered 80 per cent of the sample. Here, we present spectroscopy obtained at large aperture telescopes of a further 17 objects (plus four additional targets), which completes the sample coverage. For 12 objects we are able to determine the redshift (0.07 < z < 1.6), while for the others we set a lower limit on it, based on either the robust detection of intervening absorption systems or on an estimation derived from the absence of spectral signatures of the host galaxy. With these new data we expand and reinforce the main results of our previous papers, namely the fact that in terms of their broad-band properties our sources appear to be indistinguishable from the rest of the blazar population and the relatively large (>34 per cent and possibly as high as 80 per cent) fraction of masquerading BL Lac objects, for which the low equivalent width of the emission lines is due to the brightness of the boosted continuum, rather than being an intrinsic property, in our sample. [ABSTRACT FROM AUTHOR]

We analysed the parsec-scale linear polarization properties of 436 active galactic nuclei (AGNs) based on 15 GHz polarimetric Very Long Baseline Array observations. We present polarization and total intensity images averaged over at least five epochs since 1996 January 19 through 2019 August 4. Stacking improves the image sensitivity down to ∼30 μJy beam−1 and effectively fills out the jet cross-section both in total intensity and linear polarization. It delineates the long-term persistent magnetic field configuration and its regularity by restoring spatial distributions of the electric vector position angle (EVPA) and fractional polarization, respectively. On average, about 10 yr of stacking period is needed to reveal the stable and most-complete polarization distribution of a source. We find that the degree of polarization significantly increases down and across the jet towards its edges, typically manifesting U or W-shaped transverse profiles, suggesting a presence of a large-scale helical magnetic field associated with the outflow. In some AGN jets, mainly BL Lacs, we detect quasi-constant fractional polarization profiles across the jet, accompanied by EVPAs that closely follow the outflow. BL Lacs show higher fractional polarization values in their cores and jets than those in quasars up to hectoparsec de-projected scales, while on larger scales, they become comparable. High-synchrotron-peaked BL Lac jets are found to be less polarized than intermediate and low-synchrotron-peaked BL Lacs. The spatial distribution of the EVPAs in BL Lacs tend to align with the local jet direction, while quasars show an excess of orthogonal polarization orientation. [ABSTRACT FROM AUTHOR]

Context. The detection of quasi-periodic oscillations (QPOs) in the light curves of active galactic nuclei (AGNs) can provide insights into the physics of the super-massive black holes (SMBHs) powering these systems and could represent a signature of the existence of SMBH binaries, setting fundamental constraints on SMBH evolution in the Universe. Aims. The identification of long-term QPOs, characterized by periods on the order of several months to years, is particularly challenging and can only be achieved via all-sky monitoring instruments that provide unbiased, continuous light curves of astrophysical objects. The Fermi-LAT satellite, thanks to its monitoring observing strategy, is an ideal instrument for such a goal. Here, we aim to identify QPOs in the-ray light curves of the brightest AGNs within the Fermi-LAT catalog. Methods. We analyzed the light curves of the 35 brightest Fermi-LAT AGNs, including data from the beginning of the Fermi mission (August 2008) to April 2021, with energies from 100MeV to 300 GeV. Two time binnings were investigated: 7 and 30 days. The search for quasi-periodic features was then performed using the continuous wavelet transform. The significance of the result was tested via Monte Carlo simulations of artificial light curves with the same power spectral density and probability distribution function as the original light curves. The significances were then corrected for the look-elsewhere effect and provided as post-trials. Results. We identified 24 quasars with candidate QPOs. Several of our candidates coincide with previous claims in the literature, namely: PKS 0537441, S5 0716+714, Mrk 421, B2 1520+31, and PKS 2247131. All our candidates are transient. The most significant multi-year QPO, with a period of about 1100 days, was observed in the quasar S5 1044+71. It is reported here for the first time. [ABSTRACT FROM AUTHOR]

The correlation between the kinetic jet power P jet, intrinsic γ-ray luminosity (L int), and accretion (L disc) may reveal the underlying jet physics in various black hole systems. We study the relation between kinetic jet power, intrinsic γ-ray luminosity, and accretion by using a large sample of jetted active galactic nuclei (AGNs), including flat-spectrum radio quasars (FSRQs), BL Lacertae objects (BL Lacs), γ-ray narrow-line Seyfert 1 galaxies (γNLS1s), and radio galaxies. Our main results are as follows: (1) The slope indices of the relation between P jet and L int are 0.85 ± 0.01 for the whole sample, 0.70 ± 0.02 for the FSRQs, 0.83 ± 0.03 for the BL Lacs, 0.68 ± 0.11 for the γNLS1s, and 0.93 ± 0.09 for the radio galaxies, respectively. The jets in γNLS1s and radio galaxies almost follow the same P jet– L int correlation that was obtained for Fermi blazars. (2) The slope indices of the relation between L int and L disc are 1.05 ± 0.02 for the whole sample, 0.94 ± 0.05 for the FSRQs, 1.14 ± 0.05 for the BL Lacs, and 0.92 ± 0.18 for the γNLS1s, respectively. The γNLS1s and radio galaxies almost also follow the L int– L disc correlation derived for Fermi blazars. (3) The jet power is larger than the luminosity of accretion discs for almost all jetted AGNs. Jet power depends on both the Eddington ratio and black hole mass. We obtain log  P jet ∼ (1.00 ± 0.02)log  L disc for the whole sample, which is consistent with the theoretically predicted coefficient. These results may imply that the jets of jetted AGNs are powered by the Blandford–Znajek mechanism. [ABSTRACT FROM AUTHOR]

We explore the possible quasi-periodic oscillations (QPOs) of the blazar PKS 1510−089, utilizing the ∼38-yr Metsähovi flux density observations at 37 GHz and over 10-yr Fermi Large Area Telescope observations in the 0.1–300 GeV energy range. Both the flux distributions of the radio and γ-ray light curves suggest a lognormal distribution, and power spectral densities of both light curves can be well represented by a bending power-law model. Comparing the results from the weighted wavelet Z-transform and Lomb–Scargle periodogram approaches, a quasi-period of 1330 ± 55 d at a 99.80 per cent (>3σ) significance against the red noise background is observed in the radio light curve, whereas no similar modulation is seen in the γ-ray light curve. We briefly discuss the research actuality of blazar QPO phenomena and explain the observed radio QPO as the result of Keplerian motion in a supermassive binary black hole system and periodic modulation induced by the Lense–Thirring precession of the innermost portions of the accretion disc. [ABSTRACT FROM AUTHOR]

To unravel the dominant cause of the weak emission line in a subset of optically selected radio-quiet 'weak emission line quasars' (WLQs), we have investigated the possibility of an underdeveloped broad line region (BLR). For this, we have modelled spectral energy distributions (SED) of 61 WLQs by using their optical and infrared (IR) photometric observations from Sloan Digital Sky Survey (SDSS) and WISE, respectively. SED fit consists of various emission components, including the luminosity from the dusty torus (L tor). For comparison with the normal quasar, we have used a control sample of 55 Quasi-Stellar Objects (QSOs) for each WLQ matching in emission redshift and SDSS r-band. Based on our measurement of L tor, we found a decrement of 42 ± 2 per cent in IR luminosity in WLQs with respect to the control sample of normal QSOs. Using L tor/ L bol as the measure of torus covering factor (CFtor), we found a similar decrement in WLQ covering factor, with their CFtor distribution being significantly different with respect to the normal QSOs with a KS test P null of 4.27 × 10−14. As dusty torus and BLR covering factors are expected to be of a similar order in active galactic nucleus (AGN), our results suggest that the BLR in the WLQs is underdeveloped and could be a dominant cause of the weakness of their emission line. As a result, our analysis gives support to the models of WLQs based on the evolution scenario being in an early stage of AGNs. [ABSTRACT FROM AUTHOR]

The orientation of the jet axis to the line of sight of the observer plays a major role in explaining the phenomena observed from blazars and radio galaxies. In the γ-ray band, only a handful of radio galaxies have been identified, all being located in the nearby Universe (z < 0.5). Here, we report the identification of 4FGL J1435.5+2021, associated with TXS 1433+205, as a Fanaroff–Riley type II (FR II) radio galaxy at a considerably higher redshift of z = 0.748, thereby making it the most distant γ-ray detected radio galaxy known as of now. The Very Large Array Sky Survey data at 3 GHz resolves the source morphology into a bright core, a jet and two hotspots, with a total end-to-end projected length between lobe extremities of ∼170 kpc. The optical and radio properties of this enigmatic object suggest it to be a high-excitation FR II radio galaxy. The multiwavelength behaviour of TXS 1433+205 is found to be similar to other γ-ray detected FR II sources but is at the high-luminosity end. We suggest that the ongoing and upcoming high-resolution radio surveys will lead to the identification of many more high-redshift radio galaxies in the γ-ray sky, thus allowing comprehensive studies of misaligned relativistic jets. [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]

Extreme high-frequency peaked BL Lacs (EHBLs) are characterized by a synchrotron peak frequency exceeding 1017 Hz and a second peak that can be in the energy range of few GeVs to several TeVs. The Major Atmospheric Gamma Imaging Cherenkov Telescope (MAGIC) detected multi-TeV gamma-rays on 2018 April 19 for the first time from the EHBL PGC 2402248, which was simultaneously observed in multiwavelength by several other instruments. The broad-band spectral energy distribution of the source is conventionally modelled using the leptonic and the hadronic models. Due to the success of the photohadronic model in interpreting the enigmatic very high energy (VHE) flaring events from many high-energy blazars, we extend this model to explain the VHE events from PGC 2402248 observed by MAGIC telescopes and compare our results with other models. We conclude that the photohadronic fits are comparable and even fare better than most other models. Furthermore, we show that the spectrum is not hard and is in a low-emission state. The estimated bulk Lorentz factor for this flaring event is found to be ≲34. [ABSTRACT FROM AUTHOR]

We present the results of a long-term periodicity search in a sample of γ-ray blazars within a multiwavelength context. These blazars have been selected from the Steward Observatory sample as part of its optical monitoring program between 2008 and 2018. We study 15 sources with a temporal coverage in their optical total and polarized emission sufficiently large (>9 yr) to perform a reliable long-term periodicity analysis. We collect data from several observatories to extend the coverage, enabling the search of longer periods. In addition, data are also gathered in the high-energy (E > 100 MeV) γ-ray band from the Fermi Large Area Telescope; and in the 15 GHz radio band from the Owens Valley Radio Observatory. We identify 5 promising candidates to host quasi-periodic emission, AO 0235+164, PKS 1222+216, Mrk 501, BL Lacertae, and 1ES 2344+514 with periods in one or more bands and statistical significances ∼3σ after trial factor correction. AO 0235+164 shows a period of ∼8.2 yr in the R band; PKS 1222+216 has a quasi-periodic modulation in its total and polarized optical emission of ∼1.6 yr; Mrk 501 displays a ∼5 yr quasi-periodicity in optical and radio wavelengths; BL Lacertae presents a period of ∼1.8 yr in its polarized emission; and 1ES 2344 + 514 shows a hint of a ∼5.5 yr period in its optical R band. We interpret these results in the framework of the most common models and scenarios, namely the presence of a binary supermassive black hole system; or geometrical effects like helical or precessing jets. [ABSTRACT FROM AUTHOR]

The Fermi source 4FGL J1848.7–0129 has been historically related to the globular cluster GLIMPSE-C01 since its very first detection. Although this association is widely accepted, as it appears in the most recent Fermi catalogue, it deserves to be revisited given the multiwavelength evidences and the recent discovery of variable X-ray sources in the Fermi source region. In particular, low frequency radio maps from the Giant Metre Radio Telescope in Pune (India) have been carefully inspected which, together with X-ray data re-analysis from Chandra, lead us to get a deep insight into the candidates to be associated to 4FGL J1848.7–0129. This results in the discovery of a new X-ray variable point source coincident with an unreported non-thermal radio emitter, both of them well inside the 4FGL J1848.7–0129 error ellipse. We analyse and discuss all these observational facts, and we propose now a newly discovered blazar candidate as the most promising responsible for the gamma-ray emission in the Fermi source. If confirmed, this result would set constrains on the number of millisecond pulsars in GLIMPSE-C01 or their gamma-ray emission properties. [ABSTRACT FROM AUTHOR]

Active galactic nuclei (AGNs) make up about 35 per cent of the more than 250 sources detected in very high-energy (VHE) gamma rays to date with the imaging atmospheric Cherenkov telescopes. Apart from four nearby radio galaxies and two AGNs of unknown type, all known VHE AGNs are blazars. Knowledge of the cosmological redshift of gamma-ray blazars is key to enabling the study of their intrinsic emission properties, as the interaction between gamma rays and the extragalactic background light (EBL) results in a spectral softening. Therefore, the redshift determination exercise is crucial to indirectly placing tight constraints on the EBL density, and to studying blazar population evolution across cosmic time. Due to the powerful relativistic jets in blazars, most of their host galaxies' spectral features are outshined, and dedicated high signal-to-noise (S/N) spectroscopic observations are required. Deep medium- to high-resolution spectroscopy of 33 gamma-ray blazar optical counterparts was performed with the European Southern Observatory, New Technology Telescope, Keck II telescope, Shane 3-metre telescope, and the Southern African Large Telescope. From the sample, spectra from 25 objects display spectral features or are featureless and have high S/N. The other eight objects have low-quality featureless spectra. We systematically searched for absorption and emission features and estimated, when possible, the fractional host galaxy flux in the measured total flux. Our measurements yielded 14 firm spectroscopic redshifts, ranging from 0.0838 to 0.8125, one tentative redshift, and two lower limits: one at |$z > 0.382$| and the other at z > 0.629. [ABSTRACT FROM AUTHOR]

We present a first systematic time series study of a sample of blazars observed by the Transiting Exoplanet Survey Satellite (TESS). By cross matching the positions of the sources in the TESS observations with those from Roma-BZCAT, 29 blazars including both BL Lacerate objects and flat-spectrum radio quasars were identified. The observation lengths of the 79 light curves of the sources, across all sectors on which the targets of interest have been observed by TESS , range between 21.25 and 28.2 d. The light curves were analysed using various methods of time series analysis. The results show that the sources exhibit significant variability with fractional variability spanning between 1.41 per cent and 53.84 per cent. The blazar flux distributions were studied by applying normal and log-normal probability density function models. The results indicate that optical flux histogram of the sources are consistent with normal probability density function with most of them following bimodal distribution as opposed to unimodal distribution. This suggests that the days-time-scale optical variability is contributed either by two different emission zones or two distinct states of short-term activity in blazars. Power spectral density analysis was performed by using the power spectral response method and the true power spectra of unevenly sampled light curves were estimated. The power spectral slopes of the light curves ranged from 1.7 to 3.2. [ABSTRACT FROM AUTHOR]

Blazars whose synchrotron spectral energy distribution (SED) peaks at X-ray energies need to accelerate electrons to energies in the >100 GeV range in relativistic plasma jets at distances of parsecs from the central engine. Compton scattering by the same electrons can explain high luminosities at very high photon energies (>100 GeV) from the same objects. Turbulence combined with a standing conical shock can accomplish this. Such a scenario can also qualitatively explain the level and variability of linear polarization observed at optical frequencies in these objects. Multi-wavelength polarization measurements, including those at X-ray energies by the Imaging X-ray Polarimetry Explorer (IXPE), find that the degree of polarization is several times higher at X-ray than at optical wavelengths, in general agreement with the turbulence-plus-shock picture. Some detailed properties of the observed polarization can be naturally explained by this scenario, while others pose challenges that may require modifications to the model. [ABSTRACT FROM AUTHOR]

In this paper we present the RATAN-600 multi-frequency catalogue of blazars, an updated version of the BLcat: the RATAN-600 multi-frequency catalogue of BL Lacertae objects. The main novelty in the catalogue is an extension of the sample with flat-spectrum radio quasars (FSRQs), thus currently it contains more than 1700 blazars of different types. The main feature of the BLcat is a compilation of radio continuum data for blazars based on the RATAN-600 quasi-simultaneous measurements at frequencies of 1.2, 2.3, 4.7, 7.7/8.2, 11.2, and 21.7/22.3 GHz. We additionally supplement the catalogue with the radio data from external sources to provide an opportunity to more complete study of radio spectra and radio light curves. For the convenience of users, we developed tools to calculate the spectral index, variability index, and radio luminosity. We briefly describe basic radio properties of blazar subsamples of the catalogue: spectral classification, spectral indices, flux density variability, and radio luminosity. [ABSTRACT FROM AUTHOR]

Blazars are the most numerous type of observed high-energy gamma-ray emitters. However, their emission mechanisms and population properties are still not well-understood. Crucial to this understanding are their cosmological redshifts, which are often not easy to obtain. This presents a great challenge to the next-generation ground-based observatory for very-high-energy gamma rays, the Cherenkov Telescope Array (CTA), which aims to detect a large number of distant blazars to study their intrinsic emission properties and to place tight constraints on the extragalactic background light density, amongst others. The successful investigation of these subjects needs a precise redshift determination. Motivated by these challenges, the CTA redshift task force initiated more than 3 years ago a spectroscopic observing program using some of the largest optical and infrared telescopes to measure the redshifts of a large fraction of blazars that are likely to be detected with CTA. In this proceedings, we give an overview of the CTA redshift task force, discuss some of the difficulties associated with measuring the redshifts of blazars and present our sample selection and observing strategies. We end the proceedings with reporting selected results from the program, the on-going collaborative efforts and our plans for the future. [ABSTRACT FROM AUTHOR]

The classification of γ-ray-detected blazar candidates of uncertain type (BCU) is a relevant problem in extragalactic γ-ray astronomy. Here, we report the optical spectroscopic characterization, using two 3–4 m class telescopes, Telescopio Nazionale Galileo and Devasthal Optical Telescope , of 27 BCUs detected with the Fermi  Large Area Telescope. Since the identification of emission lines is easier in broad-line blazars, which usually exhibit low frequency peaked (synchrotron peak frequency ≤1014 Hz) spectral energy distribution, we primarily target such BCUs. We found that 8 out of 27 sources exhibit broad emission lines in their optical spectra, 3 of them have redshifts >1 and the farthest one is at z = 2.55. The optical spectra of 2 of the 19 remaining objects are dominated by the absorption spectra of the host galaxy, and there is a tentative detection of the Lyman-α absorption feature in one source. The spectra of the remaining 16 objects, on the other hand, are found to be featureless. [ABSTRACT FROM AUTHOR]

We re-examine possible dependencies on redshift of the spectral parameters of blazars observed at very-high energies (VHEs) with Imaging Atmospheric Cherenkov telescopes. This is relevant to assess potential effects with the source distance of the photon to axion-like particle mixing that would deeply affect the propagation of VHE photons across the Universe. We focus our spectral analysis on 38 BL Lac objects (32 high-peaked and 6 intermediate-peaked) up to redshift z ≃ 0.5, and a small sample of 5 Flat Spectrum Radio Quasars up to z  = 1 treated independently to increase the redshift baseline. The 78 independent spectra of these sources are first of all carefully corrected for the gamma–gamma interaction with photons of the Extragalactic Background Light that are responsible for the major redshift-dependent opacity effect. Then, the corrected spectra are fitted with simple power laws to infer the intrinsic spectral indices Γem at VHE, to test the assumption that such spectral properties are set by the local rather than the global cosmological environment. We find some systematic anticorrelations with redshift of Γem that might indicate, although with low-significance, a spectral anomaly potentially requiring a revision of the photon propagation process. More conclusive tests with higher statistical significance will require the observational improvements offered by the forthcoming new generation of Cherenkov arrays (CTA, ASTRI, LHAASO). [ABSTRACT FROM AUTHOR]

Since its discovery in 1995, the high-energy peaked blazar 1ES 2344+514 has undergone several episodes of GeV–TeV flaring and has been observed in the multiwavelength by several telescopes. The observed X-ray spectrum of 1996 and the flaring event of 2016 establish that 1ES 2344+514 has a temporary behaviour like that of an extremely high-energy peaked BL Lacertae object (EHBL). Such behaviour has also been observed in several nearby high-energy peaked blazars. We use the photohadronic model to account for the GeV–TeV flaring observed events of 1995 and 2007. Also, a recently proposed two-zone photohadronic model, which is successful in explaining the multi-TeV flaring events of many transient EHBL-like sources, is employed to explain the GeV–TeV flaring spectra of MJD 57611 and MJD 57612. We find that the zone-2 parameters of the two-zone photohadronic model play a central role in explaining these spectra. This is probably an indication of a new type of transient EHBL-like source. We find that our fits to the observed spectra are comparable or better than the other leptonic and hadronic models employed in the literature to address the same issue. [ABSTRACT FROM AUTHOR]

The relativistic jets produced by some Active Galactic Nuclei (AGNs) are among the most efficient persistent sources of non-thermal radiation and represent an ideal laboratory for studying high-energy interactions. In particular, when the relativistic jet propagates along the observer's line of sight, the beaming effect produces dominant signatures in the observed spectral energy distribution (SED), from the radio domain up to the highest energies, with the further possibility of resulting in radiation-particle multimessenger associations. In this work, we investigate the relationships between the emission of γ rays and the optical spectra of a sample of AGN, selected from BL Lac sources detected by the Fermi Large Area Telescope (Fermi -LAT). We find that there is a close relationship between the optical and γ-ray spectral indices. Despite all the limitations due to the non-simultaneity of the data, this observation strongly supports a substantial role of Synchrotron-Self Compton (SSC) radiation in a single zone leptonic scenario for most sources. This result simplifies the application of theoretical models to explore the physical parameters of the jets in this type of sources. [ABSTRACT FROM AUTHOR]