Quasi-simultaneous Spectral Energy Distributions Research Articles

Hadronuclear interactions in the jet of low TeV luminosity AGN: Implications for the low-state very-high-energy gamma-ray emission

In the study of radiation mechanisms of AGNs' jets, hadronuclear (pp) interactions are commonly neglected, because the number density of cold protons in the jet is considered insufficient. Very recently, our previous work proves that pp interactions in the low TeV luminosity AGNs, which have potential to generate detectable very-high-energy (VHE) emission, could be important. Based on this, the one-zone pp model is employed to study low-state quasi-simultaneous spectral energy distributions of a sample of low TeV luminosity AGNs in this work. Our modeling results show that the gamma-ray generated in pp interactions can explain the observed TeV spectra and has contribution to higher energy band that could be detected by the Large High Altitude Air Shower Observatory (LHAASO). In the sample, we suggest that M 87, Mrk 421 and Mrk 501 are the most likely objects to be detected by LHAASO in the near future. Other possible origins of VHE emission are also briefly discussed.

The physical properties of γ-ray-quiet flat-spectrum radio quasars: why are they undetected by Fermi-LAT?

ABSTRACT During a decade of operation of the Fermi-Large Area Telescope (LAT), thousands of blazars have been detected in the γ-ray band. However, there are still numbers of blazars that have not been detected in the γ-ray band. In this work, we focus on investigating why some flat-spectrum radio quasars (FSRQs) are undetected by Fermi-LAT. By cross-matching the Candidate Gamma-ray Blazars Survey catalogue with the Fourth Catalog of Active Galactic Nuclei Detected by the Fermi-LAT, we select 11 γ-ray-undetected (γ-ray-quiet) FSRQs as our sample and collect their quasi-simultaneous multiwavelength data. In the framework of the conventional one-zone leptonic model, we investigate their underlying physical properties and study the possibility that they are undetected in the γ-ray by modelling their quasi-simultaneous spectral energy distributions. In contrast to a smaller bulk Lorentz factor suggested by previous works, our results suggest that the dissipation region located relatively far away from the central super-massive black hole is more likely to be the reason why some γ-ray-quiet FSRQs are undetected by Fermi-LAT.

Modelling the broad-band emission of 3C 454.3

ABSTRACT The results of a long-term multiwavelength study of the powerful flat spectrum radio quasar 3C 454.3 using Fermi-LAT and Swift XRT/UVOT data are reported. In the γ-ray band, Fermi-LAT observations show several major flares when the source flux was $\gt 10^{-5}\, {\rm photon\, cm^{-2}\, s^{-1}}$; the peak γ-ray flux above 141.6 MeV, $(9.22\pm 1.96)\times 10^{-5}\, {\rm photon\, cm^{-2}\, s^{-1}}$ observed on MJD 55519.33, corresponds to $2.15\times 10^{50}\, {\rm erg\, s^{-1}}$ isotropic γ-ray luminosity. The analysis of Swift XRT and UVOT data revealed a flux increase, although with smaller amplitudes, also in the X-ray and optical/UV bands. The X-ray emission of 3C 454.3 is with a hard spectral index of ΓX = 1.16–1.75, and the flux in the flaring states increased up to $(1.80\pm 0.18)\times 10^{-10}\, {\rm erg\, cm^{-2}\, s^{-1}}$. Through combining the analysed data, it was possible to assemble 362 high-quality and quasi-simultaneous spectral energy distributions of 3C 454.3 in 2008–2018, which all were modelled within a one-zone leptonic scenario assuming the emission region is within the broad-line region, involving synchrotron, synchrotron self-Compton, and external Compton mechanisms. Such an extensive modelling is the key for constraining the underlying emission mechanisms in the 3C 454.3 jet and allows to derive the physical parameters of the jet and investigate their evolution in time. The modelling suggests that during the flares, along with the variation of emitting electron parameters, the Doppler boosting factor increased substantially, implying that the emission in these periods has most likely originated in a faster moving region.

Spectroscopic classification, variability, and SED of the Fermi-detected CSS 3C 286: the radio-loudest NLS1 galaxy?

ABSTRACT 3C 286 is a well-known calibrator source in radio astronomy. It is also one of very few compact steep-spectrum (CSS) sources detected in γ-rays. Here, we perform a detailed spectroscopic and variability analysis and present the first quasi-simultaneous optical to X-ray spectral energy distribution (SED) in order to reveal physical mechanisms that dominate its emission at different wavelengths, and arrive at a reliable optical source classification. The first main result of our study reveals several pitfalls when applying simple broad- or narrow-line Seyfert 1 (BLS1 or NLS1) classification criteria that only look at the [O iii]–Hβ complex. [O iii] and Hβ can be dominated by the same outflow components, in which case full width at half-maximum (Hβ) is no reliable classification criterion, and extinction by intrinsic or intervening material can make the highest velocity Hβ component undetectable. After careful combination of all information from ultraviolet (UV)–optical spectra along with multiwavelength data, we confirm that 3C 286 can be classified as NLS1 galaxy, with line properties and supermassive black hole mass (of order 108 M⊙ and accreting near the Eddington limit) close to the BLS1 regime, making it an important borderline object. The quasi-simultaneous SED taken with Swift shows a sharp rise in the optical–UV, implying the presence of a strong accretion-disc component with extreme UV excess, consistent with emission-line diagnostics. Finally, we report the discovery of X-ray variability of 3C 286, plausibly dominated by jet emission, and variable by at least a factor of ∼4. This result suggests to exercise caution when using 3C 286 as radio calibrator in high-resolution radio very long baseline interferometry observations.

Powerful extragalactic jets dissipate their kinetic energy far from the central black hole

Accretion onto the supermassive black hole in some active galactic nuclei (AGN) drives relativistic jets of plasma, which dissipate a significant fraction of their kinetic energy into gamma-ray radiation. The location of energy dissipation in powerful extragalactic jets is currently unknown, with implications for particle acceleration, jet formation, jet collimation, and energy dissipation. Previous studies have been unable to constrain the location between possibilities ranging from the sub-parsec-scale broad-line region to the parsec-scale molecular torus, and beyond. Here we show using a simple diagnostic that the more distant molecular torus is the dominant location for powerful jets. This diagnostic, called the seed factor, is dependent only on observable quantities, and is unique to the seed photon population at the location of gamma-ray emission. Using 62 multiwavelength, quasi-simultaneous spectral energy distributions of gamma-ray quasars, we find a seed factor distribution which peaks at a value corresponding to the molecular torus, demonstrating that energy dissipation occurs ~1 parsec from the black hole (or ~104 Schwarzchild radii for a 109M⊙ black hole).

The Physical Properties of Fermi-4LAC Flat Spectrum Radio Quasars

Abstract In this work, we collect quasi-simultaneous infrared, optical, X-ray and γ-ray data of 60 Fermi fourth LAT AGN catalog (4LAC) flat spectrum radio quasars (FSRQs). In the framework of the conventional one-zone leptonic model, we investigate the physical properties of Fermi-4LAC FSRQs’ jets by modeling their quasi-simultaneous spectral energy distributions (SEDs). Our main results are summarized as follows. (1) There is a linear correlation between synchrotron peak frequency and curvature of the electron energy distribution. As suggested by previous works, the slope of the best linear fitting equation of this correlation is consistent with statistic acceleration that needs a fluctuation of fractional acceleration gain. (2) The γ-ray dissipation regions are located at the range from 0.1 to 10 pc away from the supermassive black hole, and located outside the broad-line region and within the dusty torus. (3) A size relation P e (the kinetic power carried in relativistic electrons) ∼P B (Poynting flux) ≤P r (the radiative power) <P p (the kinetic power in cold protons) is found in our modeling. Among them, P e ∼ P B suggests that SEDs of almost all FSRQs with parameters are close to equipartition between the magnetic field and the relativistic electrons. The P e < P r suggest that the most energy of the relativistic electrons are dissipated by EC radiation for FSRQs. (4) There is an anticorrelation between the peak energy of SEDs (γ peak) and the jet power (P jet), which is consistent with the blazar sequence.

On the injection of relativistic electrons in the jet of 3C 279

ABSTRACTThe acceleration of electrons in 3C 279 is investigated through analysing the injected electron energy distribution (EED) in a time-dependent synchrotron self-Compton+external Compton emission model. In this model, it is assumed that relativistic electrons are continuously injected into the emission region, and the injected EED [$Q_{\rm e}^\prime (\gamma ^\prime)$] follows a single power-law form with low- and high-energy cut-offs $\rm \gamma _{min}^{\prime }$ and $\rm \gamma _{max}^{\prime }$, respectively, and the spectral index n, i.e. $Q_{\rm e}^\prime (\gamma ^\prime)\propto \gamma ^{\prime -n}$. This model is applied to 14 quasi-simultaneous spectral energy distributions of 3C 279. The Markov Chain Monte Carlo (MCMC)-fitting technique is performed to obtain the best-fitting parameters and the uncertainties on the parameters. The results show that the injected EED is well constrained in each state. The value of n is in the range of 2.5–3.8, which is larger than that expected by the classic non-relativistic shock acceleration. However, the large value of n can be explained by the relativistic oblique shock acceleration. The flaring activity seems to be related to an increased acceleration efficiency, reflected in an increased $\gamma ^{\prime }_{\rm min}$ and electron injection power.

The physical properties ofFermiTeV BL Lac objects’ jets

We investigate the physical properties of \textit{Fermi} TeV BL Lac objects jets by modeling the quasi-simultaneous spectral energy distribution of 29 \textit{Fermi} TeV BL Lacs in the frame of a one-zone leptonic synchrotron self-Compton model. Our main results are the following: (i) There is a negative correlation between $B$ and $\delta$ in our sample, which suggests that $B$ and $\delta$ are dependent on each other mainly in Thomson regime. (ii) There are negative correlations between $\nu_{\text{sy}}$ and $r$, the $\nu_{\text{IC}}$ and $r$, which is a signature of the energy-dependence statistical acceleration or the stochastic acceleration. There is a significant correlation between $r$ and $s$, which suggests that the curvature of the electron energy distribution is attributed to the energy-dependence statistical acceleration mechanism. (iii) By assuming one proton per relativistic electron, we estimate the jet power and radiative power. A size relation $P_{\text{e}} \sim P_{\text{p}} > P_{\text{r}} \gtrsim P_{\text{B}}$ is found in our sample. The $P_{\text{e}}>P_{\text{B}}$ suggests that the jets are particle dominated, and the $P_{\text{e}}\sim P_{\text{p}}$ means that the mean energy of relativistic electrons approaches $m_{\text{p}}/m_{\text{e}}$. There are not significant correlations between $P_{\text{jet}}$ and black hole mass in high or low state with a sub-sample of 18 sources, which suggests that the jet power weakly depends on the black hole mass. (iv) There is a correlation between the changes in the flux density at 1 TeV and the changes in the $\gamma_{\text{peak}}$, which suggests the change/evolution of electron energy distribution may be mainly responsible for the flux variation.

Radiation Mechanisms and Physical Properties of GeV -Ray Source GB 1310+487

With the adoption of the synchrotron self-Compton (SSC) + external Compton (EC) model, investigation has been conducted on the radiation mechanisms of the GeV-bright active galaxy GB 1310+487 through modeling its quasi-simultaneous spectral energy distributions in active and quiescent states. We find that from the flare states to the post-flare state, the magnetic field (B) has experienced a gradual increase, whereas the electron spectrum’s prefactor (N0) and break energy () decrease accordingly. The model indicates that the energy of the Poynting flux may be used to accelerate electrons. As for the GeV emission in the source, it is dominated by the EC process while the SSC contribution is negligible. Varying energy distribution of the emitting electrons, rather than location of the radiation blob, appears to be the driving factor for the observed changes in EC emission. Despite the similar total jet power and the same likely energy carrier (cold protons), the radiation efficiency of GB 1310+487 is lower than what is found in bright Fermi blazars.

Dynamic changes of emitting electron distribution in the jet of 3C 279: signatures of acceleration and cooling

We study the dynamic changes of electron energy distribution (EED) through systematically analysing the quasi-simultaneous spectral energy distributions (SEDs) of the flat spectrum radio quasar 3C 279 in different states. With Markov chain Monte Carlo (MCMC) technique we model fourteen SEDs of 3C 279 using a leptonic model with a three-parameter log-parabola electron energy distribution (EED). The 14 SEDs can be satisfactorily fitted with the one-zone leptonic model. The observed $\gamma$ rays in 13 states are attributed to Compton scattering of external infrared photons from a surrounding dusty torus. The curved $\gamma$-ray spectrum observed during 2-8 April 2014 is well explained by the external Compton of dust radiation. It is found that there is a clear positive correlation between the curvature parameter $b$ of the EED and the electron peak energy $\gamma'_{\rm pk}$. No clear correlation between $b$ and the synchrotron peak frequency $\nu_{\rm s}$ is found, due to the varied product of Doppler factor and fluid magnetic field from state to state. We interpret the correlation of $b-\gamma'_{\rm pk}$ in a stochastic acceleration scenario. This positive correlation is in agreement with the prediction in the stage when the balance between acceleration and radiative cooling of the electrons is nearly established in the case of the turbulence spectral index $q=2$.

Location of gamma-ray flaring region in quasar 4C +21.35

Abstract 4C +21.35 is a flat-spectrum-radio-quasar-type blazar, in which the rapid variability of very high energy (VHE, Eγ ≳ 100 GeV) emission as short as ∼ 10 min was observed by MAGIC Cherenkov telescopes, and the VHE spectrum extends up to at least 400 GeV. In this paper, by using a flat broad-line region (BLR) structure, we study the location and properties of the γ-ray-emitting region of 4C +21.35 under the constraints of multiwavelength data. We fit three quasi-simultaneous spectral energy distributions using a homogeneous one-zone leptonic model, in which a flat BLR with aperture angle α = 25° and a spherically symmetric hot dusty torus with a temperature of Tsub = 1200 K are assumed. The results show that the jet structure of 4C +21.35 is nearly conical with a half-opening angle of θj ≃ 0 $_{.}^{\circ}$29–0 $_{.}^{\circ}$6. Furthermore, the emitting region is located within the BLR clouds and approaches the outer radius of the BLR during the flaring states, while it is well beyond the dusty torus in quiescent state. The quiescent high-energy emission is dominated by the synchrotron self-Compton process, the high-energy emission during the flaring periods is dominated by Compton scattering of BLR and dusty torus photons with a value of the Compton-dominance parameter of about 30. Moreover, the fit to optical/ultraviolet data provides further support that the central black hole mass of 4C +21.35 is 6 × 108 M⊙.

Modeling the spectral energy distribution of 3C 454.3 in a “flat” broad-line region scenario

Abstract The broad-line region (BLR) of flat-spectrum radio quasars (FSRQs) could have a “flat” geometrical structure to allow GeV gamma-ray photons to escape, to produce the observed gamma-ray flares with short timescales. In this paper, we collect the quasi-simultaneous spectral energy distributions (SEDs) of the FSRQ 3C 454.3 obtained by the multi-wavelength campaigns spanning from 2007 July to 2011 January, and use a model with a “flat” structure BLR, an accretion disc and a dust torus to explain the SEDs of gamma-ray outbursts. We obtain the following results: (i) the jet is almost in equipartition between magnetic and particle energy densities during the outbursts; (ii) when the emitting region is located inside the cavity of the BLR, the covering factor fBLR of the BLR is very small—as the emitting region goes into the BLR structure, fBLR increases; (iii) the aperture angle α describing the BLR structure is about 45°; (iv) the central black hole mass is about 5 × 108 M⊙ rather than 4.4 × 109 M⊙.

The physical properties of Fermi BL Lac objects jets

We investigate the physical properties of BL Lac objects (BL Lacs) jets by modeling the quasi-simultaneous spectral energy distributions (SEDs) of 22 {\it Fermi} BL Lac objects in the frame of a simple one-zone synchrotron self-Compton (SSC) model. We obtained the best-fit model parameters and their uncertainties for each BL Lac object through the $\chi^2$-minimization procedure and discussed their implications on the physical processes. The modeling results show that the one-zone SSC model can successfully fit the SEDs of high-synchrotron-peaked BL Lacs (HBLs) and intermediate-synchrotron-peaked BL Lacs (IBLs), but fails to explain the SEDs of low-synchrotron-peaked BL Lacs (LBLs). The statistical analysis results for model parameters are summarized as follows. (1) No correlation is found between magnetic field ($B$) and the broken energy of relativistic electrons distribution ($\gamma^{\prime}_{\rm b}$) for HBLs and IBLs, but there are inverse correlations between $\gamma^{\prime}_{\rm b}$ and the radius of emitting blob ($R^{\prime}_{\rm b}$) as well as the electrons number $K^{\prime}_{\rm e}$ for HBLs and IBLs. It's therefore concluded that the variation of $\gamma^{\prime}_{\rm b}$ is mainly caused by that of $R^{\prime}_{\rm b}$ rather than $B$ for HBLs and IBLs. (2) The Poynting flux in jets can not account for the observed radiations since the power in Poynting flux is smaller than the radiative power, and the cold protons could be the primary energy carrier in the jets.

On the location of the γ-ray emission region for 21 flat spectrum radio quasars with quasi-simultaneous observations

We try to infer the location of the GeV emission region for 21 flat spectrum radio quasars (FSRQs) with quasi-simultaneous spectral energy distributions (SEDs), in which the SEDs of 21 FSRQs are reproduced by the one-zone leptonic model including the synchrotron-self Compton (SSC) and external Compton (EC) processes. We suggest that the X-ray emission could be produced by the SSC process and the GeV emission could come from the EC process. The EC emission could originate from the inverse Compton (IC) scattering of photons from the broad line region (BLR) and accretion disk or dust torus by the same electron population, which mainly depend on the location of the $\gamma$-ray emission region. We propose a method to constrain the location of the GeV emission region based on the spectral shapes. When the GeV emission is located within the BLR, the IC scattering could occur at the Klein-Nishina regime and produce a broken/steep spectrum in the GeV energy band. When the GeV emission is produced outside the BLR, the IC scattering could take place at the Thomson regime and the GeV spectrum would have the same spectral index as the optical-infrared spectrum. We infer that the location of the GeV emission region is inside the BLR for 5 FSRQs and beyond the BLR for 16 FSRQs. Our results show that the ratio of the magnetic field and electron energy density is close to equipartition condition for 21 FSRQs.

Multi-wavelength emission from 3C 66A: clues to its redshift and gamma-ray emission location

The quasi-simultaneous multi-wavelength emission of TeV blazar 3C 66A is studied by using a one-zone multi-component leptonic jet model. It is found that the quasi-simultaneous spectral energy distribution of 3C 66A can be well reproduced; in particular, the first three months of its average Fermi-LAT spectrum can be well reproduced by the synchrotron self-Compton component plus external Compton component of the broad line region (BLR). Clues to its redshift and gamma-ray emission location are obtained. The results indicate the following. (i) On the redshift: The theoretical intrinsic TeV spectra can be predicted by extrapolating the reproduced GeV spectra. Through comparing these extrapolated TeV spectra with the corrected observed TeV spectra from extragalactic background light, it is suggested that the redshift of 3C 66A could be between 0.1 and 0.3, with the most likely value being ∼ 0.2. (ii) On the gamma-ray emission location: To well reproduce the GeV emission of 3C 66A under different assumptions on the BLR, the gamma-ray emission region is always required to be beyond the inner zone of the BLR. The BLR absorption effect on gamma-ray emission confirms this point.

Multiwavelength intraday variability of the BL Lacertae S5 0716+714

We report results from a 1 week multi-wavelength campaign to monitor the BL Lac object S5 0716+714 (on December 9-16, 2009). In the radio bands the source shows rapid (~ (0.5-1.5) day) intra-day variability with peak amplitudes of up to ~ 10 %. The variability at 2.8 cm leads by about 1 day the variability at 6 cm and 11 cm. This time lag and more rapid variations suggests an intrinsic contribution to the source's intraday variability at 2.8 cm, while at 6 cm and 11 cm interstellar scintillation (ISS) seems to predominate. Large and quasi-sinusoidal variations of ~ 0.8 mag were detected in the V, R and I-bands. The X-ray data (0.2-10 keV) do not reveal significant variability on a 4 day time scale, favoring reprocessed inverse-Compton over synchrotron radiation in this band. The characteristic variability time scales in radio and optical bands are similar. A quasi-periodic variation (QPO) of 0.9 - 1.1 days in the optical data may be present, but if so it is marginal and limited to 2.2 cycles. Cross-correlations between radio and optical are discussed. The lack of a strong radio-optical correlation indicates different physical causes of variability (ISS at long radio wavelengths, source intrinsic origin in the optical), and is consistent with a high jet opacity and a compact synchrotron component peaking at ~= 100 GHz in an ongoing very prominent flux density outburst. For the campaign period, we construct a quasi-simultaneous spectral energy distribution (SED), including gamma-ray data from the FERMI satellite. We obtain lower limits for the relativistic Doppler-boosting of delta >= 12-26, which for a BL\,Lac type object, is remarkably high.

THE SPECTRAL ENERGY DISTRIBUTION OFFERMIBRIGHT BLAZARS

(Abridged) We have conducted a detailed investigation of the broad-band spectral properties of the \gamma-ray selected blazars of the Fermi LAT Bright AGN Sample (LBAS). By combining our accurately estimated Fermi gamma-ray spectra with Swift, radio, infra-red, optical and other hard X-ray/gamma-ray data, collected within three months of the LBAS data taking period, we were able to assemble high-quality and quasi-simultaneous Spectral Energy Distributions (SED) for 48 LBAS blazars.The SED of these gamma-ray sources is similar to that of blazars discovered at other wavelengths, clearly showing, in the usual Log $\nu $ - Log $\nu$ F$_\nu$ representation, the typical broad-band spectral signatures normally attributed to a combination of low-energy synchrotron radiation followed by inverse Compton emission of one or more components. We have used these SEDs to characterize the peak intensity of both the low and the high-energy components. The results have been used to derive empirical relationships that estimate the position of the two peaks from the broad-band colors (i.e. the radio to optical and optical to X-ray spectral slopes) and from the gamma-ray spectral index. Our data show that the synchrotron peak frequency $\nu_p^S$ is positioned between 10$^{12.5}$ and 10$^{14.5}$ Hz in broad-lined FSRQs and between $10^{13}$ and $10^{17}$ Hz in featureless BL Lacertae objects.We find that the gamma-ray spectral slope is strongly correlated with the synchrotron peak energy and with the X-ray spectral index, as expected at first order in synchrotron - inverse Compton scenarios. However, simple homogeneous, one-zone, Synchrotron Self Compton (SSC) models cannot explain most of our SEDs, especially in the case of FSRQs and low energy peaked (LBL) BL Lacs. (...)

Broad–band spectral evolution of GRB afterglows

Quasi-simultaneous Spectral Energy Distributions (SEDs) at dierent epochs of four among the most intensively observed Gamma-Ray Burst (GRB) afterglows have been constructed in order to investigate and compare the temporal evolution of their multiwave- length emission. The data suggest that generally the peak frequency moves toward lower frequencies with time. SED local maxima in the optical/IR range are present in some cases, possibly due to dust absorption within the host galaxy.