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We present a comprehensive analysis of the X-ray spectral properties of 198 newly identified active galactic nuclei (AGNs), leveraging archival data from the Chandra X-ray Observatory. All these AGNs exhibit a power-law spectral signature spanning a broad energy range of |$0.5{\!-\!}7.0$| keV, characterized by the photon index (⁠|$\Gamma$|⁠) values ranging from |$0.3^{+0.16}_{-0.14}$| to |$2.54^{+0.14}_{-0.13}$|⁠. Particularly, 76 of these AGNs display discernible levels of intrinsic absorption, after considering the Galactic absorption. The column densities associated with this local absorption (⁠|$n_{\rm H}^{\rm local}$|⁠) are within a range of |${\sim} 10^{19}{\!-\!}10^{22}\ {\rm cm^{-2}}$|⁠. We study the cosmological evolution of AGNs using the variation of |$n_{\rm H}^{\rm local}$| and |$\Gamma$| with their estimated redshift. The intrinsic spectral signature did not reveal any significant cosmological evolution; however, a deficit of hard sources at high redshift is possibly intrinsic. Our sample covers several orders of broad-band intrinsic luminosity (⁠|$L_{\rm B}^{\rm intr}$|⁠) ranging from |$4.59^{+0.41}_{-0.41} \times 10^{42}$| to |$2.4^{+0.12}_{-0.12} \times 10^{46}\, {\rm erg~s}^{-1}$| with peak at 1.84 redshift. We also investigate the hardness–luminosity diagram to further probe the AGNs. We conduct a sanity check by applying our findings to known AGNs, and the results are consistent with our observations. [ABSTRACT FROM AUTHOR]

Context. The evolution of active galactic nuclei (AGNs) is closely connected to their host galaxies and surroundings. Via feedback processes, AGNs can counteract the cooling of the intracluster medium (ICM) and suppress star formation in their host galaxies. Radio observations at low frequencies provide a glimpse into the history of AGN activity. The Virgo cluster is a substantial reservoir of nearby galaxies and provides an ideal laboratory for the study of AGNs as well as their feedback mechanisms. Aims. The aim of our work is to characterise the AGN population within the Virgo cluster down to low radio luminosities, constrain the AGN duty cycle, and investigate environmental feedback in cluster member galaxies. Methods. We analysed 144 MHz and 1.3 GHz radio observations of early-type galaxies from the ACS Virgo Cluster Survey (ACSVCS) taken with LOFAR and MeerKAT. Results. We detect 12 of these galaxies at 144 MHz, five of which show clearly extended radio emission. The radio luminosity shows a strong dependence on the stellar mass of the host galaxy, in agreement with previous results. As a notable outlier, the massive elliptical galaxy NGC 4365 (M* = 2.2 × 1011 M⊙) is not detected as compact source in the LOFAR observations. Instead, it is surrounded by diffuse, low-surface brightness emission, which hints towards a past phase of stronger nuclear activity. Furthermore, we find a cavity in NGC 4472 (=M 49) inflated by the wide-angle tail only visible in the LOFAR data, which implies that the cavity was created by a past outburst. The corresponding cavity power is of the same order of magnitude as the jet power in the present duty cycle of the AGN. [ABSTRACT FROM AUTHOR]

At optical/ultraviolet energies, blazars display an underlying thermal (unpolarized) contribution from the accretion disc, torus, and line-emitting regions, diluting the polarized emission from the jet-component. Optical polarimetry can be used to disentangle the thermal and non-thermal components, and place constraints on the particle populations and acceleration mechanisms responsible for the non-thermal emission. We present the results of a linear optical spectropolarimetric observing campaign of 18 blazars (six BL Lac-type objects and 12 flat-spectrum radio quasars) undertaken with the Southern African Large Telescope between 2016 and 2022. This was done to observe these systems during flaring states, as well as long-term monitoring of PKS1510-089, AP Lib, and PKS 1034-293. The observations traced the frequency dependence of the degree and angle of polarization, as well as changes in the spectral line strengths. We investigated possible correlations between the polarization and other observed characteristics for the sources. While an indication of correlation was found between the frequency dependence and the average level of polarization for some sources, a correlation was not found for the population as a whole. These results highlight that continuous observations and in-depth modelling of polarization and its frequency dependence is required to obtain a more holistic view of TeV blazars. [ABSTRACT FROM AUTHOR]

Context. Blazar flares provide a window onto the extreme physical processes occurring in relativistic outflows. Most numerical codes used for modelling blazar emission during flares use a simplified continuous-loss description of particle cooling due to the inverse Compton (IC) process, neglecting non-continuous (discrete) effects that arise in the Klein–Nishina (KN) regime. The significance of such effects has not yet been explored in detail. Aims. In this study, we investigate the importance of non-continuous Compton cooling losses and their impact on the electron spectrum and spectral energy distribution (SED) of blazars during high flux states (flares), as well as in the low state. Methods. We solve the full transport equation numerically, accounting for large relative jumps in energy by extending our existing blazar flare modelling code EMBLEM. We perform a detailed physical modelling of the brightest γ-ray flare of the archetypal flat-spectrum radio quasar (FSRQ) 3C 279 detected in June 2015. We then compare results obtained using the full cooling term and using the continuous-loss approximation. Results. We show that during flaring states of FSRQs characterised by high Compton dominance, the non-continuous cooling can lead to significant modification of the electron spectrum, introducing a range of distinct features, such as low-energy tails, hardening and/or softening, narrow and extended particle excesses, and shifts in the cooling break position. Such distortion translates to differences in the associated SED of up to ∼50%. This highlights the importance of non-continuous effects and the need to consider them in blazar emission models, particularly applied to extreme γ-ray flares. [ABSTRACT FROM AUTHOR]

A substantial fraction of quasars display broad absorption lines (BALs) in their rest-frame ultraviolet spectra. While the origin of BALs is thought to be related to the accretion disc wind, it remains unclear whether the observed ratio of BAL to non-BAL quasars is a result of orientation. We conducted observations of 48 BAL quasars and the same number of non-BAL quasars at 322 MHz using the Giant Metrewave Radio Telescope. Combined with previous flux measurements ranging from MHz to GHz frequencies, we compared continuum radio spectra between the two quasar groups. These data offer insights into low-frequency radio properties that have been difficult to investigate with previous observations only at GHz frequencies. Our results show that 73 ± 13 per cent of the BAL quasars exhibit steep or peaked spectra, a higher proportion than the 44 ± 14 per cent observed in the non-BAL quasars. In contrast, there are no discernible differences between the two quasar groups in the radio luminosity, peak frequency, and spectral index distributions of sources with steep or peaked spectra and sources with flat or inverted spectra. Generally, as the jet axis and line of sight become closer to parallel, quasars exhibit flat or inverted spectra rather than steep or peaked spectra. Therefore, these results suggest that BAL quasars are more frequently observed farther from the jet axis than non-BAL quasars. However, given that a certain proportion of BAL quasars exhibit flat or inverted spectra, more than the simple orientation scenario is required to elucidate the radio properties of BAL quasars. [ABSTRACT FROM AUTHOR]

Aims. We present the first multiwavelength study of Mrk 501 that contains simultaneous very-high-energy (VHE) γ-ray observations and X-ray polarization measurements from the Imaging X-ray Polarimetry Explorer (IXPE). Methods. We used radio-to-VHE data from a multiwavelength campaign carried out between March 1, 2022, and July 19, 2022 (MJD 59639 to MJD 59779). The observations were performed by MAGIC, Fermi-LAT, NuSTAR, Swift (XRT and UVOT), and several other instruments that cover the optical and radio bands to complement the IXPE pointings. We characterized the dynamics of the broadband emission around the X-ray polarization measurements through its multiband fractional variability and correlations, and compared changes observed in the polarization degree to changes seen in the broadband emission using a multi-zone leptonic scenario. Results. During the IXPE pointings, the VHE state is close to the average behavior, with a 0.2–1 TeV flux of 20%–50% of the emission of the Crab Nebula. Additionally, it shows low variability and a hint of correlation between VHE γ-rays and X-rays. Despite the average VHE activity, an extreme X-ray behavior is measured for the first two IXPE pointings, taken in March 2022 (MJD 59646 to 59648 and MJD 59665 to 59667), with a synchrotron peak frequency > 1 keV. For the third IXPE pointing, in July 2022 (MJD 59769 to 59772), the synchrotron peak shifts toward lower energies and the optical/X-ray polarization degrees drop. All three IXPE epochs show an atypically low Compton dominance in the γ-rays. The X-ray polarization is systematically higher than at lower energies, suggesting an energy stratification of the jet. While during the IXPE epochs the polarization angles in the X-ray, optical, and radio bands align well, we find a clear discrepancy in the optical and radio polarization angles in the middle of the campaign. Such results further support the hypothesis of an energy-stratified jet. We modeled broadband spectra taken simultaneous to the IXPE pointings, assuming a compact zone that dominates in the X-rays and the VHE band, and an extended zone stretching farther downstream in the jet that dominates the emission at lower energies. NuSTAR data allow us to precisely constrain the synchrotron peak and therefore the underlying electron distribution. The change between the different states observed in the three IXPE pointings can be explained by a change in the magnetization and/or the emission region size, which directly connects the shift in the synchrotron peak to lower energies with the drop in the polarization degree. [ABSTRACT FROM AUTHOR]

Context. Very high-energy (VHE, E > 100 GeV) observations of the blazar Mrk 501 with MAGIC in 2014 provided evidence for an unusual narrow spectral feature at about 3 TeV during an extreme X-ray flaring activity. The one-zone synchrotron-self Compton scenario, widely used in blazar broadband spectral modeling, fails to explain the narrow TeV component. Aims. Motivated by this rare observation, we propose an alternative model for the production of narrow features in the VHE spectra of flaring blazars. These spectral features may result from the decay of neutral pions (π0 bumps) that are in turn produced via interactions of protons (of tens of TeV energy) with energetic photons, whose density increases during hard X-ray flares. Methods. We explored the conditions needed for the emergence of narrow π0 bumps in VHE blazar spectra during X-ray flares reaching synchrotron energies ∼100 keV using time-dependent radiative transfer calculations. We focused on high-synchrotron peaked (HSP) blazars, which comprise the majority of VHE-detected extragalactic sources. Results. We find that synchrotron-dominated flares with peak energies ≳100 keV can be ideal periods for the search of π0 bumps in the VHE spectra of HSP blazars. The flaring region is optically thin to photopion production, its energy content is dominated by the relativistic proton population, and the inferred jet power is highly super-Eddington. Application of the model to the spectral energy distribution of Mrk 501 on MJD 56857.98 shows that the VHE spectrum of the flare is described well by the sum of a synchrotron self-Compton (SSC) component and a distinct π0 bump centered at 3 TeV. Spectral fitting of simulated SSC+π0 spectra for the Cherenkov Telescope Array (CTA) show that a π0 bump could be detected at a 5σ significance level with a 30-min exposure. Conclusions. A harder VHE γ-ray spectrum than the usual SSC prediction or, more occasionally, a distinct narrow bump at VHE energies during hard X-ray flares, can be suggestive of a relativistic hadronic component in blazar jets that otherwise would remain hidden. The production of narrow features or spectral hardenings due to π0 decay in the VHE spectra of blazars is testable with the advent of CTA. [ABSTRACT FROM AUTHOR]

PG 1553 + 113 is one of the few blazars with a convincing quasi-periodic emission in the gamma-ray band. The source is also a very high energy (VHE; >100 GeV) gamma-ray emitter. To better understand its properties and identify the underlying physical processes driving its variability, the MAGIC Collaboration initiated a multiyear, multiwavelength monitoring campaign in 2015 involving the OVRO 40-m and Medicina radio telescopes, REM, KVA, and the MAGIC telescopes, Swift and Fermi satellites, and the WEBT network. The analysis presented in this paper uses data until 2017 and focuses on the characterization of the variability. The gamma-ray data show a (hint of a) periodic signal compatible with literature, but the X-ray and VHE gamma-ray data do not show statistical evidence for a periodic signal. In other bands, the data are compatible with the gamma-ray period, but with a relatively high p -value. The complex connection between the low- and high-energy emission and the non-monochromatic modulation and changes in flux suggests that a simple one-zone model is unable to explain all the variability. Instead, a model including a periodic component along with multiple emission zones is required. [ABSTRACT FROM AUTHOR]

We consider the broad-band spectral energy distribution of the high-energy-peaked blazar Mkn 501 using Swift -XRT/UVOT, NuSTAR and Fermi -LAT observations taken between 2013 and 2022. The spectra were fitted with a one-zone leptonic model using synchrotron and synchrotron self-Compton emission from different particle energy distributions such as a broken power law, log-parabola, as well as distributions expected when the diffusion or the acceleration time-scale are energy-dependent. The jet power estimated for a broken power-law distribution was ∼1047(1044) erg s−1 for a minimum electron energy γmin ∼ 10(103). However, for electron energy distributions with intrinsic curvature (such as the log-parabola form), the jet power is significantly lower at a few times 1042 erg s−1 which is a few per cent of the Eddington luminosity of a 107 M⊙ black hole, suggesting that the jet may be powered by accretion processes. We discuss the implications of these results. [ABSTRACT FROM AUTHOR]

In this work, we provide a detailed analysis of the broad-band temporal and spectral properties of the blazar Ton 599 by using observations from the Fermi Large Area Telescope (LAT) and Swift X-Ray Telescope (XRT)/Ultraviolet–Optical Telescope (UVOT), during its brightest γ-ray flaring. The one-day bin γ-ray light curve exhibits multiple substructures with asymmetric and symmetric profiles. Notably, the γ-ray light curve shows a maximum flux of |$\rm 3.63 \times 10^{-6}\, photon\, cm^{-2}\, s^{-1}$| on MJD 59954.50, which is the highest flux ever observed from this source. The correlation between the γ-ray flux and γ-ray spectral indices suggests a moderate 'harder when brighter' trend. Taking the γ-ray light curve as the reference, a strong correlation is observed with X-ray, optical, and UV energies. Additionally, the γ-rays and optical/UV emission exhibit higher variability compared with X-rays. To understand the parameter variation during the active state of the source, we conducted a statistical broad-band spectral modelling of the source in 10 flux intervals of equal duration. A one-zone leptonic model involving synchrotron, synchrotron-self-Compton, and external Compton processes successfully reproduces the broad-band spectral energy distribution (SED) in each of these flux intervals. We observed that flux variation during the active state is associated mainly with variation in the magnetic field and particle spectral indices. [ABSTRACT FROM AUTHOR]

Blazars, a subclass of active galactic nuclei (AGN), are known to be bright γ-ray sources, frequently exhibiting active (flaring) periods. Blazar PKS 2155-304 is a high synchrotron-peaked BL Lac object located at redshift z  = 0.116. On 2006 July 28, an extremely remarkable outburst of VHE γ-ray emission from this blazar was reported by the H.E.S.S. experiment, with an average flux more than 10 times the low-state level. The variability time-scale of this extraordinary flare was as short as approximately 200 s. In order to guarantee the transparency of the emission region for TeV photons, the fast variability demands an extremely high Doppler factor δD > 50 of the jet within the classical one-zone model, leading to the so-called 'Doppler factor crisis'. Here, we demonstrate that the stochastic dissipation model, which is a multiblob scenario for blazars, can self-consistently explain the giant TeV flares of PKS 2155-304 and the low-state emission before and after the flares, in terms of both multiwavelength spectral and variability characteristics. The required Doppler factor in this model can be as low as 20, which is a reasonable and typical value for blazar jets. The obtained model parameters may shed some light on the physical properties of the relativistic jet. [ABSTRACT FROM AUTHOR]

Previous studies on the fitting of spectral energy distributions (SEDs) often apply the external-Compton process to interpret the high-energy peak of low-synchrotron-peaked (LSP) BL Lac objects (LBLs), despite the lack of strong broad emission lines observed for LBLs. In this work, we collect quasi-simultaneous multiwavelength data of 15 LBLs from the Fermi fourth LAT AGN catalogue (4LAC). We propose an analytical method to assess the necessity of external photon fields in the framework of one-zone scenario. Following derived analytical results, we fit the SEDs of these LBLs with the conventional one-zone leptonic model and study their jet physical properties. Our main results can be summarized as follows. (1) We find that most LBLs cannot be fitted by the one-zone synchrotron-self-Compton (SSC) model. This indicates that external photons play a crucial role in the high-energy emission of LBLs, therefore we suggest that LBLs are masquerading BL Lacs. (2) We suggest that the γ-ray emitting regions of LBLs are located outside the broad-line region and within the dusty torus. (3) By extending the analytical method to all types of LSPs in Fermi -4LAC (using historical data), we find that the high-energy peaks of some flat spectrum radio quasars and blazar candidates of unknown types can be attributed to the SSC emission, implying that the importance of external photons could be minor. We suggest that the variability time-scale may help distinguish the origin of the high-energy peak. [ABSTRACT FROM AUTHOR]

The blazar sequence, including negative correlations between radiative luminosity L rad and synchrotron peak frequency ν, and between Compton dominance Y and ν, is widely adopted as a phenomenological description of spectral energy distributions (SEDs) of blazars, although its underlying cause is hotly debated. In particular, these correlations turn positive after correcting Doppler boosting effect. In this work, we revisit the phenomenological and intrinsic blazar sequence with three samples, which are historical sample (SEDs are built with historical data), quasi-simultaneous sample (SEDs are built with quasi-simultaneous data) and Doppler factor corrected sample (a sample with available Doppler factors), selected from literature. We find that phenomenological blazar sequence holds in historical sample, but does not exist in quasi-simultaneous sample, and intrinsic correlation between L rad and ν becomes positive in Doppler factor corrected sample. We also analyse if the blazar sequence still exists in subclasses of blazars, i.e. flat-spectrum radio quasars and BL Lacertae objects, with different values of Y. To interpret these correlations, we apply a simple scaling model, in which physical parameters of the dissipation region are connected to the location of the dissipation region. We find that the model generated results are highly sensitive to the chosen ranges and distributions of physical parameters. Therefore, we suggest that even though the simple scaling model can reproduce the blazar sequence under specific conditions that have been fine-tuned, such results may not have universal applicability. Further consideration of a more realistic emission model is expected. [ABSTRACT FROM AUTHOR]

High redshift blazars are among the most powerful non-explosive sources in the Universe and play a crucial role in understanding the evolution of relativistic jets. To understand these bright objects, we performed a detailed investigation of the multiwavelength properties of 79 γ-ray blazars with redshifts ranging from z = 2.0 to 2.5, using data from Fermi LAT, Swift XRT / UVOT , and NuSTAR observations. In the γ-ray band, the spectral analysis revealed a wide range of flux and photon indices, from 5.32 × 10−10 to 3.40 × 10−7 photon cm−2 s−1 and from 1.66 to 3.15, respectively, highlighting the diverse nature of these sources. The detailed temporal analysis showed that flaring activities were observed in 31 sources. Sources such as 4C+71.07, PKS 1329-049, and 4C + 01.02, demonstrated significant increase in the γ-ray luminosity and flux variations, reaching peak luminosity exceeding 1050 erg s−1. The temporal analysis extended to X-ray and optical/ultraviolet (UV) bands, showed clear flux changes in some sources in different observations. The time-averaged properties of high redshift blazars were derived through modeling the spectral energy distributions with a one-zone leptonic scenario, assuming the emission region is within the broad-line region (BLR) and the X-ray and γ-ray emissions are due to inverse Compton scattering of synchrotron and BLR-reflected photons. This modeling allowed us to constrain the emitting particle distribution, estimate the magnetic field inside the jet, and evaluate the jet luminosity, which is discussed in comparison with the disc luminosity derived from fitting the excess in the UV band. [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]

The recent detection of very high energy (VHE) emissions from flat spectrum radio quasars (FSRQs) at high redshifts has revealed that the universe is more transparent to VHE γ-rays than it was expected. It has also questioned the plausible VHE emission mechanism responsible for these objects. Particularly for FSRQs, the γ-ray emission is attributed to the external Compton (EC) process. We perform a detailed spectral study of Fermi -detected FSRQ 3C 345 using synchrotron, synchrotron self-Compton, and EC emission mechanisms. The simultaneous data available in optical, ultraviolet, X-ray, and γ-ray energy bands is statistically fitted under these emission mechanisms using the χ2-minimization technique. Three high flux states and one low flux state are chosen for spectral fitting. The broad-band spectral energy distribution during these flux states is fitted under different target photon temperatures, and the model VHE flux is compared with the 50 h Cherenkov Telescope Array sensitivity. Our results indicate a significant VHE emission could be attained during the high flux state from MJD 59635−59715 when the target photon temperature is within 900–1200 K. Furthermore, our study shows a clear trend of variation in the bulk Lorentz factor of the emission region as the source transits through different flux states. We also note that during high γ-ray flux states, an increase in external photon temperature demands high bulk Lorentz factors, while this behaviour reverses in case of low γ-ray flux state. [ABSTRACT FROM AUTHOR]

The origin of the diffuse flux of TeV–PeV astrophysical neutrinos is still unknown. The γ-ray blazar PKS 0735+178, located outside the 90 percent localization region at 2.2° from the best-fitting IC-211208A event, was found to be flaring across all wavebands. In addition to leptonic synchrotron (SYN) and SYN self-Compton (SSC) emission, we invoke photohadronic (p γ) interactions inside the jet to model the spectral energy distribution (SED) and neutrino emission. We analyse the 100 d γ-ray and X-ray data and 10 d around the neutrino event is chosen to generate the broad-band SED. The temporal light curve indicates that the source was in a high state in optical, UV, γ-ray, and X-ray frequencies during the neutrino detection epoch. In the one-zone lepto-hadronic model, the SSC photons do not provide enough seed photons for p γ interactions to explain the neutrino event. However, including an external photon field yields a neutrino event rate of 0.12 in 100 d, for the IceCube detector, using physically motivated values of the magnetic field, an external photon field peaking at optical wavelength, and other jet parameters. The radiation from secondary electrons at X-ray energies severely constrains the neutrino flux to a lower value than found in previous studies. Moreover, the flux of high-energy γ-rays at GeV energies from the decay of neutral pions is sub-dominant at the high-energy peak of the SED, suggesting a higher correlation of neutrinos flux with X-ray flux is plausible. [ABSTRACT FROM AUTHOR]

We present the results of our study of cross-correlations between long-term multiband observations of the radio variability of the blazar 3C 279. More than a decade (2008–2022) of radio data were collected at seven different frequencies ranging from 2 to 230 GHz. The multiband radio light curves show variations in flux, with the prominent flare features appearing first at higher-frequency and later in lower-frequency bands. This behaviour is quantified by cross-correlation analysis, which finds that the emission at lower-frequency bands lags that at higher-frequency bands. Lag versus frequency plots are well fit by straight lines with negative slope, typically ∼−30 day GHz−1. We discuss these flux variations in conjunction with the evolution of bright moving knots seen in multiepoch Very Long Baseline Array maps to suggest possible physical changes in the jet that can explain the observational results. Some of the variations are consistent with the predictions of shock models, while others are better explained by a changing Doppler beaming factor as the knot trajectory bends slightly, given a small viewing angle to the jet. [ABSTRACT FROM AUTHOR]

We carried out a detailed temporal and spectral study of the BL Lacertae (BL Lac) by using the long-term Fermi -Large Area Telescope (LAT) and Swift -X-ray Telescope (XRT)/Ultraviolet Optical Telescope (UVOT) observations, during the period MJD 59000–59943. The daily-binned γ-ray light curve displays a maximum flux of |$1.74\pm 0.09\times 10^{-5} \,\rm photons\, cm^{-2}\, s^{-1}$| on MJD 59868, which is the highest daily γ-ray flux observed from BL Lac. The γ-ray variability is characterized by power spectral density (PSD), rms–flux relation, and flux distribution study. We find that the power-law model fits the PSD with index ∼1, which suggests a long-memory process at work. The observed rms–flux relation exhibits a linear trend, which indicates that the γ-ray flux distribution follows a lognormal distribution. The skewness/Anderson–Darling test and histogram fit reject the normality of flux distribution, and instead suggest that the flux distribution is a lognormal distribution. The fractional variability amplitude shows that the source is more variable in the X-ray band than in optical/ultraviolet/γ-ray bands. In order to obtain an insight into the underlying physical process, we extracted broad-band spectra from different time periods of the light curve. The broad-band spectra are statistically fitted with the convolved one-zone leptonic model with different forms of the particle energy distribution. We found that spectral energy distribution during different flux states can be reproduced well with the synchrotron, synchrotron self-Compton, and external Compton emissions from a broken power-law electron distribution, ensuring equipartition condition. A comparison between the best-fitting physical parameters shows that the variation in different flux states is mostly related to an increase in the bulk Lorentz factor and spectral hardening of the particle distribution. [ABSTRACT FROM AUTHOR]

Aims. The BL Lac 1ES 2344+514 is known for temporary extreme properties characterised by a shift of the synchrotron spectral energy distribution (SED) peak energy νsynch, p above 1 keV. While those extreme states have only been observed during high flux levels thus far, additional multi-year observing campaigns are required to achieve a coherent picture. Here, we report the longest investigation of the source from radio to very high energy (VHE) performed so far, focussing on a systematic characterisation of the intermittent extreme states. Methods. We organised a monitoring campaign covering a 3-year period from 2019 to 2021. More than ten instruments participated in the observations in order to cover the emission from radio to VHE. In particular, sensitive X-ray measurements by XMM-Newton, NuSTAR, and AstroSat took place simultaneously with multi-hour MAGIC observations, providing an unprecedented constraint of the two SED components for this blazar. Results. While our results confirm that 1ES 2344+514 typically exhibits νsynch, p > 1 keV during elevated flux periods, we also find periods where the extreme state coincides with low flux activity. A strong spectral variability thus happens in the quiescent state, and is likely caused by an increase in the electron acceleration efficiency without a change in the electron injection luminosity. On the other hand, we also report a strong X-ray flare (among the brightest for 1ES 2344+514) without a significant shift of νsynch, p. During this particular flare, the X-ray spectrum is among the softest of the campaign. It unveils complexity in the spectral evolution, where the common harder-when-brighter trend observed in BL Lacs is violated. By combining Swift-XRT and Swift-UVOT measurements during a low and hard X-ray state, we find an excess of the UV flux with respect to an extrapolation of the X-ray spectrum to lower energies. This UV excess implies that at least two regions significantly contribute to the infrared/optical/ultraviolet/X-ray emission. Using the simultaneous MAGIC, XMM-Newton, NuSTAR, and AstroSat observations, we argue that a region possibly associated with the 10 GHz radio core may explain such an excess. Finally, we investigate a VHE flare, showing an absence of simultaneous variability in the 0.3−2 keV band. Using time-dependent leptonic modelling, we show that this behaviour, in contradiction to single-zone scenarios, can instead be explained by a two-component model. [ABSTRACT FROM AUTHOR]

Context. The origin of the diffuse astrophysical neutrino flux observed by the IceCube experiment is still under debate. Multiple associations have been reported between high-energy neutrino events and individual bla/ars, such as the source TXS 0506+056, which are active galaxies with relativistic jets pointing toward Earth. From a theoretical perspective, the properties of these sources as neutrino emitters are not yet well understood. Aims. By systematically modeling the effect of cosmic-ray protons on the multiwavelength data from the largest sample of bright gamma-ray bla/ars to date, we expect to learn about the multi-messenger nature of the active galaxy population as a whole, as well as the relationship between neutrino production and the multiwavelength spectrum of these sources. Methods. We predict the emitted multiwavelength and neutrino spectrum using a self-consistent numerical radiation model applied individually to each source in the sample. We then study the properties of the full population and identify empirical relations. We focus on public multiwavelength data from the radio to the gamma-ray bands from a sample of 324 bla/ars detected by the Fermi Large Area Telescope (LAT), most of which are flat-spectrum radio quasars (FSRQs). This amounts to 34% of all FSRQs in the latest Fermi catalog. Results. We demonstrate that the optical and gigaelectronvolt gamma-ray broadband features are generally well described by electron emission, which helps for the location of the emission region relative to the central black hole to be constrained. For 33% of the bla/ars in our sample, a description of the observed X-ray spectrum benefits from an additional component from proton interactions, in agreement with recent studies of individual IceCube candidate bla/ars. We show that, on average, bla/ars that are brighter in gigaelectronvolt gamma rays have a higher neutrino production efficiency but a lower best-fit baryonic loading. The predicted neutrino luminosity shows a positive correlation both with the observed flux of gigaelectronvolt gamma rays and with the predicted flux of megaelectronvolt gamma rays. We also estimate the diffuse neutrino flux from gamma-ray bla/ars by extrapolating the result to the Fermi population, and we show that it may be at the level of ~20% of the diffuse neutrino flux observed by IceCube, in agreement with current limits from stacking analyses. We discuss the implications of our results for future neutrino searches and suggest promising sources for potential detections. [ABSTRACT FROM AUTHOR]

The long term broad-band spectral study of Flat Spectrum Radio Quasars during different flux states has the potential to infer the emission mechanisms and the cause of spectral variations. To scrutinize this, we performed a detailed broad-band spectral analysis of 3C 279 using simultaneous Swift - XRT / UVOT and Fermi - LAT observations spanning from 2008 August to 2022 June. We also supplement this with the simultaneous Nu STAR observations of the source. The optical/UV, X-ray, and γ-ray spectra were individually fitted by a power law to study the long term variation in the flux and the spectral indices. A combined spectral fit of simultaneous optical/UV and X-ray spectra was also performed to obtain the transition energy at which the spectral energy distribution is minimum. The correlation analysis suggests that the long term spectral variations of the source are mainly associated with the variations in the low energy index and the break energy of the broken power-law electron distribution which is responsible for the broad-band emission. The flux distribution of the source represents a lognormal variability while the γ-ray flux distribution showed a clear double lognormal behaviour. The spectral index distributions were again normal except for γ-ray which showed a double-Gaussian behaviour. This indicates that the lognormal variability of the source may be associated with the normal variations in the spectral index. The broad-band spectral fit of the source using synchrotron and inverse Compton processes indicates different emission processes are active at optical/UV, X-ray, and γ-ray energies. [ABSTRACT FROM AUTHOR]

In this work, a time-dependent modelling is developed to study the emission properties of blazars in the low state. Motivated by various observations, we speculate and assume that numerous discrete radiation zones throughout the jet of a blazar contribute to the broad-band emission. We model the temporal evolution of the electron spectrum in each emission zone taking into account the injection, cooling, and escape of relativistic electrons. By doing so, we are able to calculate the multiwavelength emission of each radiation zone. The observed emission of a blazar is then the superposition of the emission from all discrete radiation zones. We revisit the multiwavelength spectral energy distributions, light curves, and polarization under the model, and discuss its potential to reproduce the flat radio spectra, the core-shift phenomena, the minute-scale gamma-ray variability, and the large polarization-angle swings, which are difficult to explain under the conventional one-zone models simultaneously. [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]

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]

The detection of hard X-ray spectra (spectral index <2) from the kiloparsec-scale jet of active galactic nuclei cannot be accounted for by the synchrotron emission mechanism from the electron distribution responsible for the radio/optical emission. Alternate explanations are the inverse Compton scattering of cosmic microwave background photons (IC/CMB) or synchrotron emission from a second electron population. When the X-ray emission is interpreted as an IC/CMB process, the Compton spectrum often peaks at GeV energy and many sources are predicted to be Fermi candidate sources. The absence of significant gamma-ray flux from some of these galaxies by Fermi disfavours the IC/CMB interpretation of the high-energy emission. We extend this study to predict the very-high-energy (VHE) gamma-ray emission due to the IC/CMB model, which can be investigated by the Cherenkov Telescope Array Observatory (CTAO). The model parameters deciding the broad-band spectral energy distribution are estimated using an analytical approximation of the emissivity functions. The emission model is extrapolated to VHE and then compared with the CTAO sensitivity. For this particular study, we have selected 18 knots with harder X-ray spectra and for which the IC/CMB model for X-ray emission has been suggested. [ABSTRACT FROM AUTHOR]

Radio galaxies are a subclass of active galactic nuclei (AGN) in which accretion onto the supermassive black hole releases energy into the environment via relativistic jets. The jets are not constantly active throughout the life of the host galaxy and alternate between active and quiescent phases. Remnant radio galaxies are detected during a quiescent phase and define a class of unique sources that can be used to constrain the AGN duty cycle. We present, for the first time, a spatially resolved radio analysis of the radio galaxy associated with the galaxy NGC 6086 down to 144 MHz and constraints on the spectral age of the diffuse emission to investigate the duty cycle and evolution of the source. We used three new low-frequency, high-sensitivity observations; the first was performed with the Low Frequency Array at 144 MHz and the other two with the upgraded Giant Metrewave Radio Telescope at 400 MHz and 675 MHz, respectively. To these, we add two Very Large Array archival observations at higher frequencies (1400 and 4700 MHz). In the new observations in the frequency range 144–675 MHz, we detect a second pair of larger lobes and three regions within the remnant emission with a filamentary morphology. We analysed the spectral index trend in the inner remnant lobes and see systematically steeper values (αlow∼1.1–1.3) at the lower frequencies compared to the gigahertz frequencies (αhigh∼0.8–0.9). Steeper spectral indices are found in the newly detected outer lobes (up to αouter∼2.1), as expected if they trace a previous phase of activity of the AGN. However, the differences between the spectra of the two outer lobes suggest different dynamical evolutions within the intra-group medium during their expansion and/or different magnetic field values. Using a single-injection radiative model and assuming equipartition conditions, we place constraints on the age of the inner and outer lobes and derive the duty cycle of the source. We estimate that the duration of the two active phases was 45 Myr and 18 Myr and the duration of the two inactive phases was 66 Myr and 33 Myr. This results in a total active time of ton ∼ 39%. The filamentary structures have a steep spectral index (∼1) without any spectral index trend, and only one of them shows a steepening in the spectrum. Their origin is not yet clear, but they may have formed due to the compression of the plasma or due to magnetic field substructures. [ABSTRACT FROM AUTHOR]

Galaxies exhibiting a specific large-scale extended radio emission, such as X-shaped radio galaxies, belong to a rare class of winged radio galaxies. The morphological evolution of these radio sources is explained using several theoretical models, including galaxy mergers. However, such a direct link between a perturbed radio morphology and a galaxy merger remains observationally sparse. Here, we investigate a unique radio galaxy J1159+5820, whose host CGCG 292-057 displays the optical signature of a post-merger system with a distinct tidal tail feature, and an X-shaped radio morphology accompanied by an additional pair of inner lobes. We observed the target on a wide range of radio frequencies ranging from 147 to 4959 MHz, using dedicated GMRT and VLA observations, and supplemented it with publicly available survey data for broad-band radio analysis. Particle injection models were fitted to radio spectra of lobes and different parts of the wings. Spectral ageing analysis performed on the lobes and the wings favours a fast jet realignment model with a reorientation time-scale of a few million years. We present our results and discuss the possible mechanisms for the formation of the radio morphology. [ABSTRACT FROM AUTHOR]

The incidence of young but fading radio sources provides important information on the life cycle of radio emission in radio-loud active galactic nuclei. Despite its importance for constraining the models of radio source evolution, there are no systematic studies of remnants in complete samples of young radio sources. We report results of the study of 18 compact steep-spectrum (CSS) radio sources, selected from the statistically complete B3-VLA (Very Large Array) CSS sample, characterized by a steep optically thin spectrum (α ≥ 1.0) and no core detection in earlier studies. Our deep multifrequency VLA, pc-scale Very Long Baseline Array, and enhanced Multi Element Remotely Linked Interferometer Network (e-MERLIN) observations allowed us to locate the core component in 10 objects. In three CSS sources, there is no clear evidence of present-time active regions, suggesting that they are likely in a remnant phase. Among sources with core detection, we find three objects that have no clear active regions (hotspots) at the edges of the radio structure, suggesting that the radio emission may have just restarted. Our results support a power-law distribution of the source ages, although the poor statistics prevents us from setting solid constraints on the percentage of remnants and restarted sources in subpopulations of radio sources. [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]

Flaring episodes in blazars represent one of the most violent processes observed in extra-galactic objects. Studies of such events shed light on the energetics of the physical processes occurring in the innermost regions of blazars, which cannot otherwise be resolved by any current instruments. In this work, we present some of the largest and most rapid flares captured in the optical band in the blazars 3C 279, OJ 49, S4 0954+658, TXS 1156+295, and PG 1553+113. The source flux was observed to increase by nearly ten times within a time-scale of a few weeks. We applied several methods of time series analysis and symmetry analysis. Moreover, we also performed searches for periodicity in the light curves of 3C 279, OJ 49 and PG 1553+113 using the Lomb–Scargle method and found plausible indications of quasi-periodic oscillations (QPOs). In particular, the 33- and 22-day periods found in 3C 279, i.e. a 3:2 ratio, are intriguing. These violent events might originate from magnetohydrodynamical instabilities near the base of the jets, triggered by processes modulated by the magnetic field of the accretion disc. We present a qualitative treatment as the possible explanation for the observed large amplitude flux changes in both the source-intrinsic and source-extrinsic scenarios. [ABSTRACT FROM AUTHOR]

The cool-core galaxy cluster RXJ1720.1+2638 hosts extended radio emission near the cluster core, known as a minihalo. The origin of this emission is still debated and one piece of the puzzle has been the question of whether the supermassive black hole in the brightest central galaxy is actively powering jets. Here, we present high-resolution e-MERLIN observations clearly indicating the presence of sub-kpc jets; this may have implications for the proposed origin of the minihalo emission, providing an ongoing source of relativistic electrons rather than a single burst sometime in the past, as previously assumed in simulations attempting to reproduce observational characteristics of minihalo-hosting systems. [ABSTRACT FROM AUTHOR]

Tidal disruption events (TDEs) may occur in supermassive black holes (SMBHs) surrounded by clouds. TDEs can generate ultrafast and large opening-angle outflow with a velocity of ∼0.01–0.2  c , which will collide with clouds with time lags depending on outflow velocity and cloud distances. Since the fraction of the outflow energy transferred into cloud's radiation is anticorrelated with the cloud density, high-density clouds was thought to be inefficient in generating radiation. In this work, we studied the radiation from the outflow-cloud interactions for high-density clouds, and found that thermal conduction plays crucial roles in increasing the cloud's radiation. Up to 10 per cent of the bow shock energy can be transferred into clouds and gives rise to X-ray emission with equivalent temperature of 105–6 K due to the cooling catastrophe. The inverse Compton scattering of TDE UV/optical photons by relativistic electrons at bow shock generates power-law X-ray spectra with photon indices Γ ∼ 2–3. This mechanism may account for some TDE candidates with delayed X-ray emission, and can be examined by delayed radio and gamma-ray emissions. [ABSTRACT FROM AUTHOR]

Blazars are a rare class of active galactic nuclei (AGNs) with relativistic jets pointing towards the observer. Jets are thought to be launched as Poynting-flux dominated outflows that accelerate to relativistic speeds at the expense of the available magnetic energy. In this work, we consider electron–proton jets and assume that particles are energized via magnetic reconnection in parts of the jet where the magnetization is still high (σ ≥ 1). The magnetization and bulk Lorentz factor Γ are related to the available jet energy per baryon as μ = Γ(1 + σ). We adopt an observationally motivated relation between Γ and the mass accretion rate into the black hole |$\dot{m}$|⁠ , which also controls the luminosity of external radiation fields. We numerically compute the photon and neutrino jet emission as a function of μ and σ. We find that the blazar SED is produced by synchrotron and inverse Compton radiation of accelerated electrons, while the emission of hadronic-related processes is subdominant except for the highest magnetization considered. We show that low-luminosity blazars (L γ ≲ 1045 erg s−1) are associated with less powerful, slower jets with higher magnetizations in the jet dissipation region. Their broad-band photon spectra resemble those of BL Lac objects, and the expected neutrino luminosity is |$L_{\nu +\bar{\nu }}\sim (0.3-1)\, L_{\gamma }$|⁠. High-luminosity blazars (L γ ≫ 1045 erg s−1) are associated with more powerful, faster jets with lower magnetizations. Their broad-band photon spectra resemble those of flat spectrum radio quasars, and they are expected to be dim neutrino sources with |$L_{\nu +\bar{\nu }}\ll L_{\gamma }$|⁠. [ABSTRACT FROM AUTHOR]

The goal of this paper is to investigate the flux and spectral index distribution of FR II radio galaxy 4C 14.11. We focused on the distribution of flux and spectral indices over the lobes, as well as in their hot spots. For that purpose, we used publicly available observations of this radio galaxy given at 1450 and 8440 MHz. Particularly, we used Leahy's Atlas of radio-emitting double radio sources, Jodrell Bank Centre for Astrophysics in Manchester, as well as NASA/IPAC Extragalactic Database. We found that the non-thermal (synchrotron) radiation dominates over the areas of the lobes. Distinction between hot spots and rest of the lobes are much smaller in the spectral index than in the flux. We also found that over the inner parts of both lobes, spectral index α is flat in average and significantly higher than 1.2, indicating that 4C 14.11 is old AGN. [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]

BL Lacertae (BL Lac) object OJ 287 is one of the most dynamic blazars across the directly accessible observational windows: spectral, timing, polarization and imaging. Apart from behaviors, considered the characteristics of blazars, it exhibits peculiar timing features like quasi-periodicity in optical flux as well as radio-detected knots position and has shown diverse transient spectral features like a new broadband continuum dominated activity phase, Seyfert-like soft-X-ray excess, highly transient iron line absorption feature, a thermal-like continuum-dominated optical phase, large optical polarization swings associated with one of the timing features, etc., that are rare in blazars and contrary to currently prevailing view of BL Lacs. Theoretical considerations, supported by existing observations invoke scenarios involving a dynamical interplay of accretion and/or strong-gravity-induced events (tidal forces) in a binary supermassive black hole (SMBH) scenario to impact-induced jet and only jet activities. Many of these scenarios have some definite and quite distinctive observationally testable predictions/claims. These considerations make OJ 287 the only BL Lac to have an activity phase with dominance related to accretion and/or accretion-perturbation-induced jet activities. We present a brief overview of the unique spectral features and discuss the potential of these features in exploring not only relativistic jet physics, but also issues pertaining to accretion and accretion-regulated jet activities, i.e., the whole spectrum of issues related to the jet-accretion paradigm. [ABSTRACT FROM AUTHOR]

Identifying active galactic nuclei (AGNs) and isolating their contribution to a galaxy's energy budget is crucial for studying the co-evolution of AGNs and their host galaxies. Brightness temperature (Tb) measurements from high-resolution radio observations at GHz frequencies are widely used to identify AGNs. Here, we investigate using new sub-arcsecond imaging at 144 MHz with the International LOFAR Telescope to identify AGNs using Tb in the Lockman Hole field. We use ancillary data to validate the 940 AGN identifications, finding 83 percent of sources have AGN classifications from SED fitting and/or photometric identifications, yielding 160 new AGN identifications. Considering the multiwavelength classifications, brightness temperature criteria select over half of radio-excess sources, 32 percent of sources classified as radio-quiet AGNs, and 20 percent of sources classified as star-forming galaxies. Infrared colour–colour plots and comparison with what we would expect to detect based on peak brightness in 6 arcsec LOFAR maps imply that the star-forming galaxies and sources at low flux densities have a mixture of star-formation and AGN activity. We separate the radio emission from star-formation and AGN in unresolved, Tb -identified AGNs with no significant radio excess and find the AGN comprises 0.49 ± 0.16 of the radio luminosity. Overall, the non-radio excess AGNs show evidence for having a variety of different radio emission mechanisms, which can provide different pathways for AGNs and galaxy co-evolution. This validation of AGN identification using brightness temperature at low frequencies opens the possibility for securely selecting AGN samples where ancillary data are inadequate. [ABSTRACT FROM AUTHOR]

Blazars may accelerate protons and/or nuclei as well as electrons. The hadronic component of accelerated particles in blazars may constitute the bulk of their high-energy budget; nevertheless, this component is elusive because of the high value of the energy threshold of proton interactions with photon fields inside the source. However, the broad line regions (BLRs) of some flat spectrum radio quasars (FSRQs) may contain a sufficient amount of matter to render primary protons 'visible' in γ-rays via hadronuclear interactions. In this paper, we study the persistent γ-ray emission of the FSRQ PKS 1510−089 in its low state, utilizing the publicly available Fermi -LAT data, as well as using the spectrum measured with the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes. We find an indication that there is an excess of γ-rays at the energy range ≳ 20 GeV with respect to a simple baseline log-parabolic intrinsic spectral model. This excess could be explained in a scenario invoking hadronuclear interactions of primary protons on the BLR material with the subsequent development of electromagnetic cascades in photon fields. We present a Monte Carlo calculation of the spectrum of this cascade component, taking as input the BLR photon field spectrum calculated with the cloudy code. To our knowledge, this is the first calculation of an electromagnetic cascade spectrum inside a blazar based on a direct calculation of the photon field spectrum with a spectral synthesis code. [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]

The magnetic field is believed to play a critical role in the bulk acceleration and propagation of jets produced in active galactic nuclei (AGN). Polarization observations of AGN jets provide valuable information about their magnetic fields. As a result of radiative transfer, jet structure, and stratification, among other factors, it is not always straightforward to determine the magnetic field structures from observed polarization. We review these effects and their impact on polarization emission at a variety of wavelengths, including radio, optical, and ultraviolet wavelengths in this paper. It is also possible to study the magnetic field in the launching and acceleration regions of AGN jets by using very long baseline interferometry (VLBI), which occurs on a small physical scale. Due to the weak polarization of the jets in these regions, probing the magnetic field is generally difficult. However, recent VLBI observations have detected significant polarization and Faraday rotation in some nearby sources. We present the results of these observations as well as prospects for future observations. Additionally, we briefly discuss recently developed polarization calibration and imaging techniques for VLBI data, which enable more in-depth analysis of the magnetic field structure around supermassive black holes and in AGN jets. [ABSTRACT FROM AUTHOR]

One of the most important questions in blazar physics is the origin of broad-band emission and fast-flux variation. In this work, we studied the broad-band temporal and spectral properties of a TeV blazar 1ES 1727+502 and explore the one-zone synchrotron self-Compton (SSC) model to fit the broad-band spectral energy distribution (SED). We collected the long-term (2014–2021) multiband data that include both the low- and high-flux states of the source. The entire light curve is divided into three segments of different flux states and the best-fitting parameters obtained by broad-band SED modelling corresponding to three flux states were then compared. The TeV blazar 1ES 1727+502 has been observed to show the brightest flaring episode in the X-ray followed by the optical–UV and γ-rays. The fractional variability estimated during various segments behaves differently in multiple wavebands, suggesting a complex nature of emission in this source. This source has shown a range of variability time from days scale to month scale during this long period of observations between 2014 and 2021. A 'harder-when-brighter' trend is not prominent in the X-ray but seen in the optical–UV and an opposite trend is observed in the γ-ray. The complex nature of correlation among various bands is observed. The SED modelling suggests that the one-zone SSC emission model can reproduce the broad-band spectrum in the energy range from optical–UV to very high energy γ-ray. [ABSTRACT FROM AUTHOR]

It is well known that the γ-ray emission in blazars originates in the relativistic jet pointed at the observers. However, it is not clear whether the exact location of the GeV emission is less than a parsec (pc) from the central engine, such that it may receive sufficient amount of photons from the broad-line region (BLR) or farther out at 1–100 pc range. The former assumption has been successfully used to model the spectral energy distribution of many blazars. However, simultaneous detection of TeV γ-rays along with GeV outbursts in some cases indicate that the emission region must be outside the BLR. In addition, GeV outbursts have sometimes been observed to be simultaneous with the passing of a disturbance through the so-called 'very long baseline interferometry (VLBI) core', which is located tens of pc away from the central engine. Hence, the exact location of γ-ray emission remains ambiguous. Here we present a method that we have developed to constrain the location of the emission region. We identify simultaneous months time-scale GeV and optical outbursts in the light curves spanning over 8 yr of a sample of 11 blazars. Using theoretical jet emission models we show that the energy ratio of simultaneous optical and GeV outbursts is strongly dependent on the location of the emission region. Comparing the energy dissipation of the observed multiwavelength outbursts and that of the simulated flares in our theoretical model, we find that most of the above outbursts originate beyond the BLR at approximately a few pc from the central engine. [ABSTRACT FROM AUTHOR]

We propose a simple analytical jet model of magnetic jets, in which radially averaged profiles of main physical quantities are obtained based on conservation laws and some results of published general relativistic magnetohydrodynamic jet simulations. We take into account conversion of the magnetic energy flux to bulk acceleration in jets formed around rotating black holes assuming the mass continuity equation and constant jet power, which leads to the Bernoulli equation. For assumed profiles of the bulk Lorentz factor and the radius, this gives us the profile of the toroidal magnetic field component along the jet. We then consider the case where the poloidal field component is connected to a rotating black hole surrounded by an accretion disc. Our formalism then recovers the standard formula for the power extracted from a rotating black hole. We find that the poloidal field strength dominates over the toroidal one in the comoving frame up to large distances, which means that jets should be more stable to current-driven kink modes. The resulting magnetic field profiles can then be used to calculate the jet synchrotron emission. [ABSTRACT FROM AUTHOR]

A tidal disruption event (TDE), AT2019dsg, was observed to be associated with a PeV neutrino event, IceCube-191001A, lagging the optical outburst by a half year. It is known that TDEs may generate ultra fast outflows. If the TDE occurs in a cloudy environment, the outflow-cloud interactions may form shock waves which generate accelerated protons and hence, delayed neutrinos from hadronic interactions in clouds. Here, we investigate the neutrino production in AT2019dsg by examining the TDE outflow-cloud interaction model. We find that, for an outflow with a velocity of 0.07 c and a kinetic luminosity of |$10^{45}\rm \,erg\ \,s^{-1}$|⁠ , protons may be accelerated up to ∼60 PeV by the bow shocks, and generate PeV neutrinos by interactions with clouds. The predicted neutrino number in this model depends on the uncertainties of model parameters and in order to match the observations, some challenging values of parameters have been involved. The PeV neutrino event number can be ∼4 × 10−3 for a hard proton index Γ = 1.5. [ABSTRACT FROM AUTHOR]

We conducted a detailed long-term spectral and temporal study of flat spectrum radio quasar 4C  + 01.02, by using the multiwavelength observations from Fermi -LAT, Swift -XRT, and Swift -UVOT. The 2-d bin γ-ray light curve in the 2014–2017 active state displays 14 peak structures with a maximum integral flux |$(\rm \mathit{ E} \gt 100 \ MeV)$| of |$\rm (2.5 \pm 0.2) \times 10^{-6}\ ph\ cm^{-2}\ s^{-1}$| at MJD 57579.1, which is approximately 61 times higher than the base flux of |$\rm (4.1 \pm 0.3) \times 10^{-8}\ ph\ cm^{-2}\ s^{-1}$|⁠ , calculated by averaging the flux points when the source was in quiescent state. The shortest γ-ray variability of 0.66 ± 0.08 d is observed for the source. The correlation study between γ-ray spectral index and flux suggests that the source deviates from the usual trend of harder when brighter feature shown by blazars. To understand the likely physical scenario responsible for the flux variation, we performed a detailed broad-band spectral analysis of the source by selecting different flux states from the multiwavelength light curve. A single zone leptonic model was able to reproduce the broad-band spectral energy distribution (SED) of each state. The parameters of the model in each flux state are determined using a χ2 fit. We observed that the synchrotron, synchrotron-self-Compton (SSC), and External-Compton (EC) processes produce the broad-band SED under varied flux states. The adjoining contribution of the seed photons from the broad-line region (BLR) and the IR torus for the EC process are required to provide adequate fits to the GeV spectrum in all the chosen states. [ABSTRACT FROM AUTHOR]

The radiative loss interpretation for the broken power-law spectra of blazars is often questioned since the difference between the indices does not support this inference. Using the blazar Mkn 421 as a case study, we performed a detailed analysis of its characteristic photon energy where the spectral index changes significantly. We used the observations of the source by Swift –XRT from 2008 to 2019 to identify the characteristic photon energy and the corresponding spectral indices. The spectra in the energy range 0.3–10.0 keV can be well fitted by a log parabola as well as a smooth broken power law. From the smooth broken power-law spectral fit, we show that the spectral indices before and after the characteristic photon energy are strongly anticorrelated. Further, the spectral curvature measured at the characteristic photon energy indicates an anticorrelation with the low-energy spectral index while the high-energy spectral index shows a positive correlation. These findings are at variance with a simple radiative loss interpretation for the characteristic photon energy, and alternative scenarios are thus discussed. Though these scenarios are, in principle, capable of reproducing the correlation results, they deviate significantly from the observed properties. [ABSTRACT FROM AUTHOR]

We present a temporal and spectral study of the blazar PKS 0208−512, using recent flaring activity from 2019 November to 2020 May, as detected by the Fermi Large Area Telescope. The contemporaneous X-ray and optical/ultraviolet observations from the Swift X-Ray Telescope and UltraViolet and Optical Telescope are also used. During the activity state, the 2-d binned γ-ray light curve shows multiple peaks indicating subflares. To understand the possible physical mechanisms behind flux enhancement, we divided the activity state of the source into several flux states and carried out detailed temporal and spectral studies. The timing analysis of light curves suggests that peaks of subflares have rise and decay times of the order of days, with the flux doubling time ∼ 2 d. The 2-d binned γ-ray light curve shows double-lognormal flux distribution. The broad-band spectral energy distribution (SED) for three selected flux states can be well fitted under synchrotron, synchrotron self-Compton and external Compton emission mechanisms. We obtained the physical parameters of the jet by SED modelling and their confidence intervals through χ2-statistics. Our SED modelling results suggest that during the quiescent state, the γ-ray spectrum can be explained by considering the external Compton scattering of infrared photons from the dusty torus. However, γ-ray spectra corresponding to flares demand additional target photons from the broad-line region (BLR) along with infrared. These suggest that, during flares, the emission region is close to the edge of the BLR, while for the quiescent state, the emission region is away from the BLR. The best-fitting results suggest that a marginal increase in the magnetic field during the flaring episode can result in flux enhancement. This is possibly associated with the efficiency of particle acceleration during flaring states compared with the quiescent state. [ABSTRACT FROM AUTHOR]