Flat Spectrum Radio Quasars Research Articles

Hadronic signatures from magnetically dominated baryon-loaded AGN jets

ABSTRACT 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 (Lγ ≲ 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 (Lγ ≫ 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 $L_{\nu +\bar{\nu }}\ll L_{\gamma }$.

A New Sample of Gamma-Ray Emitting Jetted Active Galactic Nuclei

We considered the fourth catalog of gamma-ray point sources produced by the Fermi Large Area Telescope (LAT) and selected only jetted active galactic nuclei (AGN) or sources with no specific classification, but with a low-frequency counterpart. Our final list is composed of 2980 gamma-ray point sources. We then searched for optical spectra in all the available literature and publicly available databases, to measure redshifts and to confirm or change the original LAT classification. Our final list of gamma-ray emitting jetted AGN is composed of BL Lac Objects (40%), flat-spectrum radio quasars (23%), misaligned AGN (2.8%), narrow-line Seyfert 1, Seyfert, and low-ionization nuclear emission-line region galaxies (1.9%). We also found a significant number of objects changing from one type to another, and vice versa (changing-look AGN, 1.1%). About 30% of gamma-ray sources still have an ambiguous classification or lack one altogether.

Explaining the Multiwavelength Emission of γ-ray Bright Flat-Spectrum Radio Quasar 3C 454.3 in Different Activity States

The origin of gamma-ray flares of blazars is still an open issue in jet physics. In this work, we reproduce the multiwavelength spectral energy distribution (SED) of flat-spectrum radio quasars 3C 454.3 under a one-zone leptonic scenario, investigate the variation of the physical parameters in different activity states, and analyze the possible origin of its γ-ray outburst. Based on the analysis of multiwavelength quasi-simultaneous observations of 3C 454.3 during MJD 55,400–56,000, we consider that the radiation includes synchrotron (Syn), synchrotron self-Compton (SSC), and external Compton (EC) radiations by the simulation, and the seed photons of the external Compton component mainly comes from the broad-line region and dusty molecular torus. The model results show that: (1) We can well reproduce the multiwavelength quasi-simultaneity SED of 3C 454.3 in various activity states by using a one-zone Syn+SSC+EC model. (2) By comparing the physical model parameters of the bright and the quiescent states, we suggest that this γ-ray flaring activity is more likely to be caused by the increase in the doppler factor.

Blazars at Very High Energies: Emission Modelling

Blazars are very broadband cosmic sources with spectra spanning over twenty orders of magnitude in frequency, down to the 100 MHz regime in the radio range, up to VHE at several tens of TeV. The modelling of their spectral energy distribution at high energies currently considers two main classes of models, leptonic and lepto-hadronic, which both succeed fairly well in describing the observed spectra for the two populations of blazars, namely BL Lac objects (BL Lacs) and flat spectrum radio quasars (FSRQs). However they are both confronted with difficulties, in particular to reproduce flaring phenomena monitored with a good multi-spectral and temporal coverage, or to reproduce extreme sources which challenge the basic descriptions. Such a situation has led to a diversity of specific scenarios, the positioning of which in relation to the general context of the sources is generally not clearly fixed. The identification of the dominant particle acceleration mechanism at work and a better understanding of the location of the TeV emitting zone would make it possible to break the degeneracies between models. Multi-wavelength and multi-messenger studies should also help in this regard, with the perspective to elaborate a general reference scenario of blazars and AGNs.

Long-term Optical and γ-Ray Variability of the Blazar PKS 1222+216

The γ-ray emission from flat-spectrum radio quasars (FSRQs) is thought to be dominated by the inverse Compton scattering of the external sources of photon fields, e.g., accretion disk, broad-line region (BLR), and torus. FSRQs show strong optical emission lines and hence can be a useful probe of the variability in BLR output, which is the reprocessed disk emission. We study the connection between the optical continuum, Hγ line, and γ-ray emissions from the FSRQ PKS 1222+216, using long-term (∼2011–2018) optical spectroscopic data from Steward Observatory and γ-ray observations from Fermi Large Area Telescope (LAT). We measured the continuum (F C,opt) and Hγ (F Hγ ) fluxes by performing a systematic analysis of the 6029–6452 Å optical spectra. We observed stronger variability in F C,opt than F Hγ , an inverse correlation between the Hγ equivalent width and F C,opt, and a redder-when-brighter trend. Using discrete cross-correlation analysis, we found a positive correlation (DCF ∼ 0.5) between the F γ‐ray>100 MeV and F C,opt (6024–6092 Å) light curves with a time lag consistent with zero at the 2σ level. We found no correlation between the F γ‐ray>100 MeV and F Hγ light curves, probably dismissing the disk contribution to the optical and γ-ray variability. The observed strong variability in the Fermi-LAT flux and F γ‐ray>100 MeV − F C,opt correlation could be due to the changes in the particle acceleration at various epochs. We derived the optical-to-γ-ray spectral energy distributions during the γ-ray flaring and quiescent epochs that show a dominant disk component with no variability. Our study suggests that the γ-ray emission zone is likely located at the edge of the BLR or in the radiation field of the torus.

New Identifications and Multiwavelength Properties of Extragalactic Fermi Gamma-Ray Sources in the SPT-SZ Survey Field

The fourth Fermi Large Area Telescope catalog (4FGL) contains 5064 γ-ray sources detected at high significance, but 26% of them still lack associations at other wavelengths. The SPT-SZ survey, conducted between 2008 and 2011 with the South Pole Telescope (SPT), covers 2500 deg2 of the southern sky in three millimeter-wavelength (mm) bands and was used to construct a catalog of nearly 5000 emissive sources. In this study, we introduce a new cross-matching scheme to search for multiwavelength counterparts of extragalactic γ-ray sources using a mm catalog. We apply a Poissonian probability to evaluate the rate of spurious false associations and compare the multiwavelength associations from the radio, mm, near-infrared, and X-ray with 4FGL γ-ray sources. In the SPT-SZ survey field, 85% of 4FGL sources are associated with mm counterparts. These mm sources include 94% of previously associated 4FGL sources and 56% of previously unassociated 4FGL sources. The latter group contains 40 4FGL sources for which SPT has provided the first identified counterparts. Nearly all of the SPT-associated 4FGL sources can be described as flat-spectrum radio quasars or blazars. We find that the mm band is the most efficient wavelength for detecting γ-ray blazars when considering both completeness and purity. We also demonstrate that the mm band correlates better to the γ-ray band than the radio or X-ray bands. With the next generation of CMB experiments, this technique can be extended to greater sensitivities and more sky area to further complete the identifications of the remaining unknown γ-ray blazars.

BASS. XXXIII. Swift-BAT Blazars and Their Jets through Cosmic Time

We derive the most up-to-date Swift-Burst Alert Telescope (BAT) blazar luminosity function in the 14–195 keV range, making use of a clean sample of 118 blazars detected in the BAT 105 month survey catalog, with newly obtained redshifts from the BAT Active Galatic Nucleus Spectroscopic Survey. We determine the best-fit X-ray luminosity function for the whole blazar population, as well as for flat-spectrum radio quasars (FSRQs) alone. The main results are: (1) at any redshift, BAT detects the most luminous blazars, above any possible break in their luminosity distribution, which means we cannot differentiate between density and luminosity evolution; (2) the whole blazar population, dominated by FSRQs, evolves positively up to redshift z ∼ 4.3, confirming earlier results and implying lower number densities of blazars at higher redshifts than previously estimated. The contribution of this source class to the cosmic X-ray background at 14–195 keV can range from 5%–18%, while possibly accounting for 100% of the MeV background. We also derived the average 14 keV–10 GeV spectral energy distribution for BAT blazars, which allows us to predict the number counts of sources in the MeV range, as well as the expected number of high-energy (>100 TeV) neutrinos. A mission like COSI will detect 40 MeV blazars, of which two may have coincident neutrino detections. Finally, taking into account beaming selection effects, the distribution and properties of the parent population of these extragalactic jets are derived. We find that the distribution of viewing angles is quite narrow, with most sources aligned within <5° of the line of sight. Moreover, the average Lorentz factor, 〈Γ〉 = 8–12, is lower than previously suggested for these powerful sources.

Exploring short-term optical variability of blazars using TESS

ABSTRACT 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.

The jet apparent motion and central engine study of Fermi blazars

ABSTRACT The study of blazar jet has been performed for several decades via VLBI technique, while its generation and propagation stay unclear. In the present work, we compiled a sample of 407 VLBI detected Fermi blazars (VFBs) and studied the correlations between apparent velocity (${\rm log}\, \beta _{\rm app}$) and jet/accretion disc properties. We found a positive correlation between γ-ray luminosity (${\rm log}\, L_{\rm \gamma }$) and ${\rm log}\, \beta _{\rm app}$; the correlation suggests that the apparent motion of jet knot is related to the jet power. The correlations between ${\rm log}\, \beta ^{\rm max}_{\rm app}$ and the jet radiation power (${\rm log}\, L_{\rm rad}$), and between ${\rm log}\, \beta ^{\rm max}_{\rm app}$ and the jet extended region luminosity at 5 GHz (${\rm log}\, L_{\rm 5GHz}^{\rm ext}$), which is an indicator of jet kinetic power, reveal that the knots apparent motion is correlated with both jet radiation power and the kinetic power. But this indication is not held for flat-spectrum radio quasar (FSRQs) in terms of the correlation ${\rm log}\, L_{\rm 5GHz}^{\rm ext}\, versus \, {\rm log}\, \beta ^{\rm max}_{\rm app}$. Besides, ${\rm log}\, \beta ^{\rm max}_{\rm app}$ has a moderate correlation with accretion disc luminosity (${\rm log}\, L_{\rm Disk}$) and the normalized accretion disc luminosity ${\rm log}\, (L_{\rm Disk}/L_{\rm Edd})$, which may suggest both the power of accretion disc and the accretion rate are critical to generate knots and to accelerate them. In addition, we found the VFBs have larger average values of γ-ray luminosity (${\rm log}\, L_{\rm \gamma }$), γ-ray photon index (αph), and variability index (${\rm log}\, VI$) than the rest of Fermi blazars. Through Gaussian mixture models method, we generated a criteria, ${\rm log}\, L_{\rm \gamma } \gt 45.40$, αph &amp;gt; 2.24, and ${\rm log}\, VI \gt 1.71$ to find VFB candidates, selected 228 VFB candidates from the rest of Fermi blazar.

An Extensive Study of Blazar Broad Emission Line: Changing-look Blazars and the Baldwin Effect

It is known that the blazar jet emissions are dominated by nonthermal radiation, while the accretion disk jets are normally dominated by thermal emission. In this work, our aim is to study the connection between the two types of emission by investigating the correlation between the blazar emission-line intensity property, which embodies the nature of an accretion disk, and the γ-ray flux property, which is the representative of jet emission. We compiled a sample of 656 blazars with available emission-line equivalent widths (EWs), the GeV γ-ray flux, and the spectral energy distribution (SED) information from the literature. In this work, we found 55 previous blazar candidates of uncertain types (BCUs) that are now identified as flat-spectrum radio quasars (FSRQs), and found 52 “changing-look” blazars based on their EWs, 45 of which are newly confirmed. These changing-look blazars have a larger accretion ratio () than BL Lacertae (BL Lac) objects. In addition, we suggest that the lower synchrotron peak blazars (LSPs) could be the source of changing-look blazars because 90.7% of the changing-look blazars in this work are confirmed as LSPs. An anticorrelation between EW and continuum intensity, the so-called global “Baldwin effect” (BEff), has been confirmed. We suggest that the steeper global BEff observed for the blazar than for radio-quiet active galactic nuclei (RQ-AGNs) is caused by the inverse Compton scattering of broad-emission-line photons. This interpretation is further supported by the positive correlation between the emission-line EW and intrinsic inverse Compton luminosity.

Variability Signatures of a Burst Process in Flaring Gamma-Ray Blazars

Blazars exhibit stochastic flux variability across the electromagnetic spectrum, often exhibiting heavy-tailed flux distributions, commonly modeled as lognormal. However, Tavecchio et al. and Adams et al. found that the high-energy gamma-ray flux distributions of several of the brightest flaring Fermi-LAT flat-spectrum radio quasars (FSRQs) are well modeled by an even heavier-tailed distribution, which we show is the inverse gamma distribution. We propose an autoregressive inverse gamma variability model in which an inverse gamma flux distribution arises as a consequence of a shot-noise process. In this model, discrete bursts are individually unresolved and averaged over within time bins, as in the analysis of Fermi-LAT data. Stochastic variability on timescales longer than the time-bin duration is modeled using first-order autoregressive structure. The flux distribution becomes approximately lognormal in the limiting case of many weak bursts. The fractional variability is predicted to decrease as the time-bin duration increases. Using simulated light curves, we show that the proposed model is consistent with the typical gamma-ray variability properties of FSRQs and BL Lac objects. The model parameters can be physically interpreted as the average burst rate, the burst fluence, and the timescale of long-term stochastic fluctuations.

The Classification of Blazar Candidates of Uncertain Types

In this work, the support vector machine (SVM) method is adopted to separate BL Lacertae objects (BL Lacs) and flat spectrum radio quasars (FSRQs) in the plots of the photon spectrum index against the photon flux, αph∼logF, those of the photon spectrum index against the variability index, αph∼logVI and those of the variability index against the photon flux, logVI∼logF. Then, we used the dividing lines to distinguish BL Lacs from FSRQs in the blazar candidates of uncertain types from the Fermi/LAT catalogue. Our main conclusions are: 1. We separate BL Lacs and FSRQs by αph=−0.123logF+1.170 in the αph∼logF plot, αph=−0.161logVI+2.594 in the αph∼logVI plot and logVI=0.792logF+9.203 in the logVI∼logF plot. 2. We obtain 932 BL Lac candidates and possible BL Lac candidates, and 585 FSRQ candidates and possible FSRQ candidates. 3. Discussion is given regarding comparisons with the literature.

Modelling the persistent low-state γ-ray emission of the PKS 1510−089 blazar with electromagnetic cascades initiated in hadronuclear interactions

ABSTRACT 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.

Very-high-energy flat spectral radio quasar candidates

ABSTRACT The attenuation of very-high-energy (VHE) photons by the extragalactic background light (EBL) prevents the observation of high-redshift flat spectrum radio quasars (FSRQs). However, the correlation of the VHE spectral index with source redshift suggests that the EBL intensity may be less than what is predicted. This deviation can draw new constraints on the opacity of the Universe to VHE gamma-rays. Therefore, more FSRQs may fall above the sensitivity of the forthcoming VHE telescopes than the ones predicted by the existing EBL models. In order to account for the lower EBL intensity predicted by the index–redshift correlation, we introduce a redshift-dependent correction factor to the opacity, estimated from a commonly used cosmological EBL model. Considering this modified opacity, we identify the plausible VHE FSRQ candidates by linearly extrapolating the Fermi gamma-ray spectrum at 10 GeV to the VHE regime. Our study suggests that among 744 FSRQs reported in the Fermi Fourth Catalogue Data Release 2, 32 FSRQs will be detectable by the Cherenkov Telescope Array Observatory (CTAO). Because FSRQs are proven to be highly variable, we assume a scenario where the average Fermi gamma-ray flux increases by a factor of 10, and this predicts an additional 90 FSRQs that can be detected by the CTAO.

Revisiting the evidences for spectral anomalies in distant blazars: New data on the photon-ALP mixing

ABSTRACT 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).

Intra-night optical variability monitoring of $$\gamma $$-ray emitting blazars

We present the results obtained from our campaign to characterize the intra-night optical variability properties of blazars detected by the Fermi large area telescope. This involves R-band monitoring observations of a sample of 18 blazars, which includes five flat spectrum radio quasars (FSRQs) and thirteen BL Lac objects (BL Lacs) covering the redshift range $$z = 0.085-1.184$$ . Our observations carried out using the 1.3m J. C. Bhattacharya telescope cover a total of 40 nights ( $$\sim $$ 200 h) between the period December 2016 and March 2020. We characterized variability using the power enhanced F-test. We found duty cycle (DC) variability of about 11% for FSRQs and 12% for BL Lacs. Dividing the sample into different sub-classes based on the position of the synchrotron peak in their broadband spectral energy distribution (SED), we found DC of $$\sim $$ 16%, $$\sim $$ 10% and $$\sim $$ 7% for low synchrotron peaked (LSP), intermediate synchrotron peaked (ISP) and high synchrotron peaked (HSP) blazars. Such high DC of variability in LSP blazars could be understood in the context of the R-band tracing the falling part (contributed by high energy electrons) of the synchrotron component of the broadband SED. Also, the R-band tracing the rising synchrotron part (produced by low energy electrons) in the case of ISP and HSP blazars, could cause lesser variability in them. Thus, the observed high DC of variability in LSP blazars relative to ISP and HSP blazars is in accordance with the leptonic model of emission from blazar jets.

Classification and Jet Power of Fermi Blazars

In this work, we compile a sample of 449 Fermi blazars with the luminosity of the broadline region, the black hole mass, the beam radio luminosity, and the jet power; obtain the beam power and the black hole spin; investigate the dividing line between BL Lacertae objects (BL Lacs) and flat-spectrum radio quasars (FSRQs) to identify the discrepancies in their dominant mechanisms; and revisit the dependence of the jet power on the disk accretion luminosity, the black hole mass, and the black hole spin. We come to the following conclusions. (1) A boundary of log (L BLR/L Edd) = −3.14, separating the BL Lacs and the FSRQs, is obtained from the Bayesian analysis, which is consistent with the results from the literature. We employ the boundary to divide the blazar candidates of uncertain types into candidates for BL Lacs or FSRQs, and we find five changing-look blazars at the same time. (2) A strong correlation is found between black hole mass and intrinsic γ-ray luminosity, but a weaker correlation is found between black hole mass and observed γ-ray luminosity. The latter is weakened by jet effects: it is apparently weak for BL Lacs that have disordered amplification of the Doppler factor, since their mechanism is dominated by jets, while it is moderate for FSRQs, since their mechanism is dominated by accretion processes. (3) The jets of both FSRQs and BL Lacs are likely governed by the Blandford–Znajek mechanism.

A multiwavelength study of the flat-spectrum radio quasar NVSS J141922−083830 covering four flaring episodes

ABSTRACT We present multiwavelength observations and a model for flat-spectrum radio quasar (FSRQ) NVSS J141922−083830, originally classified as a blazar candidate of unknown type (BCU II object) in the Third Fermi-LAT AGN Catalog. Relatively bright flares (&amp;gt;3 magnitudes) were observed on 2015 February 21 (MJD 57074) and 2018 September 8 (MJD 58369) in the optical band with the MASTER Global Robotic Nettelescopes. Optical spectra obtained with the Southern African Large Telescopeon 2015 March 1 (MJD 57082), during outburst, and on 2017 May 30 (MJD 57903), during quiescence, showed emission lines at 5325 Å and at ≈3630 Å that we identified as the Mg ii 2798 Å and C iii] 1909 Å lines, respectively, and hence derived a redshift $z$ = 0.903. Analysis of Fermi-Large Area Telescope (LAT) data was performed in the quiescent regime (5 yr of data) and during four prominent flaring states in 2014 February–April, 2014 October–November, 2015 February–March, and 2018 September. We present spectral and timing analysis with Fermi-LAT. We report a hardening of the gamma-ray spectrum during the last three flaring periods, with a power-law spectral index Γ = 2.0–2.1. The maximum gamma-ray flux level was observed on 2014 October 24 (MJD 56954) at (7.57 ± 1.83) × 10−7 ph cm−2 s−1. The multiwavelength spectral energy distribution (SED) during the 2015 February–March flare supports the earlier evidence of this blazar to belong to the FSRQ class. The SED can be well represented with a single-zone leptonic model with parameters typical of FSRQs, but also a hadronic origin of the high-energy emission cannot be ruled out.

Constraining the γ-Ray Emission Region for Fermi-detected FSRQs by the Seed Photon Approach

The location of γ-ray-emitting region in blazars has been an open issue for several decades and is still being debated. We use the Paliya et al. sample of 619 γ-ray-loud flat-spectrum radio quasars with available spectral energy distributions and employ a seed photon factor approach to locate the γ-ray production region. This method efficiently sets up a relation between the peak frequencies and luminosities for the synchrotron emission and inverse Compton scattering, together with a combination of the energy density and characteristic energy for the external seed photon field, namely, , an indicative factor of seed photons in units of Gauss. By comparing it with canonical values of the broad-line region (BLR) and molecular dusty torus (DT), we principally ascertain that the GeV emission originated far beyond the BLR and close to the DT—farther out at parsec scales from the central black hole, which supports a far-site scenario for γ-ray blazars. We probe the idea that inverse Compton scattering of infrared seed photons is happening in the Thomson regime. This approach and our findings are based on the validity of the external Compton model, which is applicable in understanding the GeV emission mechanism in FSRQs. However, the completeness of this framework has been challenged by reports of neutrino emission from blazars. Thus, we also shed new light on the neutrino production region by using our derived results because blazars are promising neutrino emitters.

The dichotomy between low-frequency-peaked BL Lacs and flat-spectrum radio quasars

ABSTRACT By comparing the Fourth Fermi Large Area Telescope active galactic nuclei (AGNs) Catalog Data Release 2 (4LAC-DR2) and the Radio Fundamental Catalog (RFC-2021b), we obtain an AGN sample containing 2816 AGNs. We calculate the γ-ray loudness (Gγ) of the sample sources, and establish the relevant relationship between several gamma/radio properties. The results show that: 1) the synchrotron peak frequency ($\nu _\mathrm{peak}^S$) strongly correlates with the γ-ray photon spectral index (Γγ), γ-ray luminosity (Lγ), and radio luminosity (LR); 2) the low-synchrotron-peaked BL Lac objects (LBLs) and the flat-spectrum radio quasars (FSRQs) partially overlap in log $\nu _\mathrm{peak}^S\!-\!\Gamma _\gamma$, log Lγ–Γγ, log LR–Γγ, log Gγ–Γγ, log $\nu _\mathrm{peak}^S\!-\!$log Lγ, log $\nu _\mathrm{peak}^S\!-\!$log LR, and log $\nu _\mathrm{peak}^S\!-\!$log Gγ space. We plotted the distribution of redshift (z), $\nu _\mathrm{peak}^S$, Γγ, Lγ, VLBI 8.4 GHz flux density ($S_{\rm {8.4\, GHz}}$), and Gγ to distinguish FSRQs and LBLs. We note that $\nu _\mathrm{peak}^S$, Γγ, and $S_{8.4\, {\rm GHz}}$ show similar bimodal distributions. To further determine whether FSRQs are misjudged as LBLs, we examined the Compton dominance (CD), and found that CD &amp;lt; 1 for only two out of 39 sources. This measurement provides further evidence that FSRQs may be mistaken for LBLs.