Keywords

The Circinus galaxy is a nearby composite starburst/active galactic nucleus (AGN) system. In this work we re-analyze the GeV emission from Circinus with 10 yr of Fermi-LAT Pass 8 data. In the energy range of 1–500 GeV, the spectrum can be well fitted by a power-law model with a photon index of Γ = 2.20 ± 0.14, and its photon flux is (5.90 ± 1.04) × 10⁻¹⁰ photons cm⁻² s⁻¹. Our 0.1–500 GeV flux is several times lower than that reported in previous literature, which is roughly in compliance with the empirical relation for star-forming and Local Group galaxies and might be reproduced by the interaction between cosmic rays and the interstellar medium. The ratio between the γ-ray luminosity and the total infrared luminosity is near the proton calorimetric limit, indicating that Circinus may be a proton calorimeter. However, marginal evidence for variability of the γ-ray emission is found in the timing analysis, which may indicate the activity of an AGN jet. More Fermi-LAT data and future observation of CTA are required to fully reveal the origin of its γ-ray emission.

Characteristic variability timescales in blazar γ-ray light curves can provide insights into the physical processes responsible for γ-ray variability. The power spectral density (PSD) is capable of revealing such timescales, which may appear as breaks or periodicities. Continuous-time autoregressive moving-average (CARMA) models can be used to accurately estimate a light curve's PSD. Through a light-curve simulation study, we develop a methodology to identify PSD breaks using CARMA models. Using this methodology, we study the γ-ray light curves of 13 bright blazars observed with the Fermi Large Area Telescope in the 0.1–300 GeV band over 9.5 yr. We present the blazar γ-ray PSDs, which provide evidence for low-frequency breaks on timescales ∼1 yr in four sources, and an additional high-frequency break on a timescale ∼9 days in one source.

PKS 1510-089 is one of the most variable blazars in the third Fermi-LAT source catalog. During 2015, this source has shown four flares identified as flares A, B, C, and D in between three quiescent states: Q1, Q2, and Q3. The multiwavelength data from Fermi-LAT, Swift-XRT/Ultraviolet/Optical Telescope, Owens Valley Radio Observatory, and Sub-millimeter array Observatory are used in our work to model these states. Different flux doubling times have been observed in different energy bands, which indicate that there could be multiple emission zones. The flux doubling time from the gamma-ray and X-ray light curves are found to be 10.6 hr, 2.5 days, and the average flux doubling time in the optical/UV band is 1 day. It is possible that the gamma-ray and optical/UV emission are produced in the same region whereas X-ray emission is coming from a different region along the jet axis. We have also estimated the discrete correlation functions (DCFs) among the light curves of different energy bands to infer about their emission regions. However, our DCF analysis does not show significant correlation in different energy bands though it shows peaks in some cases at small time lags. We perform a two-zone multiwavelength time-dependent modeling with one emission zone located near the outer edge of the broad line region and another further away in the dusty/molecular torus (DT/MT) region to study this high state.

The attenuation produced by extragalactic background light (EBL) in γ-ray spectra of blazars has been used to constrain the Hubble constant (H₀) and matter density (Ω_m) of the universe. We propose to estimate H₀ and Ω_m using the well-measured >10 GeV extragalactic γ-ray background (EGB). This suggestion is based on the fact that the >10 GeV EGB is totally explained by the emissions from blazars, and an EBL-absorption cutoff occurs at ∼50 GeV in the EGB spectrum. We fit the >10 GeV EGB data with modeled EGB spectrum. This results in ${H}_{0}={64.9}_{-4.3}^{+4.6}\,\mathrm{km}\,{{\rm{s}}}^{-1}\,{\mathrm{Mpc}}^{-1}$ and ${{\rm{\Omega }}}_{{\rm{m}}}={0.31}_{-0.14}^{+0.13}$. Note that the uncertainties may be underestimated due to the limit of our realization for EBL model. H₀ and Ω_m are degenerate in our method. Independent determination of Ω_m by other methods would improve the constraint on H₀.

We present a procedure to generally constrain the environments of neutrino-producing sites in photomeson production models of jetted active galactic nuclei (AGNs) where any origin of the dominant target photon field can be accommodated. For this purpose we reconstruct the minimum target photon spectrum required to produce the (observed) neutrino spectrum, and derive the distributions of all corresponding secondary particles. These initiate electromagnetic cascades with an efficiency that is linked to the neutrino production rate. The derived photon spectra represent the minimum radiation emerging from the source that is strictly associated with the photohadronically produced neutrinos. Using the 2014/15 neutrino spectrum observed by IceCube from TXS 0506+056, we conduct a comprehensive study of these cascade spectra and compare them to the simultaneous multiwavelength emission. For this set of observations, photopion production from a cospatially produced (comoving) photon target can be ruled out as well as a setup where synchrotron- or synchrotron–Compton-supported cascades on a stationary (AGN rest frame) target photon field operate in this source. However, a scenario where Compton-driven cascades develop in the stationary soft X-ray photon target, which photohadronically produced the observed neutrinos, appears feasible with required proton kinetic jet powers near the Eddington limit. The source is then found to produce neutrinos inefficiently, and emits GeV photons significantly below the observed Fermi-Large Area Telescope flux. Hence, the neutrinos and the bulk of the gamma-rays observed in 2014/15 from TXS 0506+056 cannot have been initiated by the same process.

It was previously suggested that e^± pair halos could be observed in γ-rays. Searching for pair halos is a challenging topic in γ-ray astronomy that has not yet yielded a convincing detection for an individual object. Here we propose that X-ray observations could reveal the existence of e^± pair halos. To support our suggestion, we computed the observed energy and spatial distribution of X-ray photons from the pair halos and check the feasibility of pair halo detection by current and future X-ray observatories. The results show that current X-ray missions could register a positive signal for the pair halos created under some conditions. For a magnetic field (B) of ∼1 μG, the X-ray could be observable for a seed gamma-ray energy of ${E}_{{\gamma }_{0}}\gtrsim 50$ TeV. If ${E}_{{\gamma }_{0}}\sim 100$ TeV, the observable range of B would be B ≳ 300 nG.

Recent radio surveys have discovered a large number of low-luminosity core-dominated radio galaxies that are much more abundant than those at higher luminosities. These objects will be too faint in γ-rays to be detected individually by Fermi. Nevertheless, they may contribute significantly to the unresolved extragalactic γ-ray background. We consider here the possible contribution of these core-dominated radio galaxies to the diffuse extragalactic γ-ray background. Using published data available for all 45 of the radio galaxies listed as detected counterparts in the Fermi FL8Y source list update to the 3FGL catalog, we have searched for radio maps that can resolve the core flux from the total source flux. Using high-resolution radio maps we were able to obtain core fluxes for virtually every source. We then derived a relation between core radio flux and γ-ray flux that we extrapolated to sources with low radio luminosities that are known to be highly core-dominated. We then employed a very recent determination of the luminosity function for core-dominated radio galaxies in order to obtain the contribution of all possible γ-ray-emitting radio galaxies to the unresolved extragalactic γ-ray background. We find this contribution to be possibly non-negligible, 4%–18% of the unresolved γ-ray background observed using the Fermi-LAT telescope.

We report on Owens Valley Radio Observatory (OVRO), WISE, Swift and Fermi-LAT observations of the high redshift blazar CGRaBS J0733+0456, from which significant flux variations in radio and infrared (IR), as well as γ-ray domains, are detected. Particularly, the amplitude of long-term IR variation is over one order of magnitude, and the IR variability timescale can be constrained as short as a few hours in the source frame. The IR and γ-ray light curves are found to be rather similar, and the strong quasi-simultaneous IR and γ-ray flares are proven to be unique among the nearby γ-ray sources. This is the first time that a γ-ray blazar at redshift z ≥ 3 with multi-wavelength flux variations (flares) is identified. Broadband spectral energy distributions in different flux states are constructed and theoretically described. The γ-ray flares from some blazars as distant as redshift ∼5 are expected to be detectable for Fermi-LAT.

Centaurus A (Cen A) is the nearest active radio galaxy, which has kiloparsec-scale jets and giant lobes detected by various instruments in radio and X-ray frequency ranges. The Fermi-Large Area Telescope and High Energy Stereoscopic System (HESS) confirmed that Cen A is a very high-energy (VHE; >0.1 TeV) γ-ray emitter with a known spectral softening in the energy range from a few GeV to TeV. In this work, we consider a synchrotron self-Compton model in the nucleus for the broadband spectrum below the break energy and an external Compton model in kiloparsec-scale jets for the γ-ray excess. Our results show that the observed γ-ray excess can be suitably described by the inverse Compton scattering of the starlight photons in the kiloparsec-scale jets, which is consistent with the recent tentative report by HESS on the spatial extension of the TeV emission along the jets. Considering the spectral fitting results, the excess can only be seen in Cen A, which is probably due to two factors: (1) the host galaxy is approximately 50 times more luminous than other typical radio galaxies and (2) the core γ-ray spectrum quickly decays above a few MeV due to the low maximum electron Lorentz factor of γ_c = 2.8 × 10³ resulting from the large magnetic field of 3.8 G in the core. By the comparison with other γ-ray detected radio galaxies, we found that the magnetic field strength of relativistic jets scales with the distance from the central black holes d with B(d) ∝ d^{−0.88 ± 0.14}.

PKS 1510–089 is a bright and active γ-ray source that showed strong and complex γ-ray flares in mid-2015 during which the Major Atmospheric Gamma Imaging Cerenkov telescopes detected variable very high energy (photon energies >100 GeV) emission. We present long-term multifrequency radio, optical, and γ-ray light curves of PKS 1510–089 from 2013 to 2018, and results of an analysis of the jet kinematics and linear polarization using 43 GHz Very Long Baseline Array data observed between late 2015 and mid-2017. We find that a strong radio flare trails the γ-ray flares in 2015, showing an optically thick spectrum at the beginning and becoming optically thin over time. Two laterally separated knots of emission are observed to emerge from the radio core nearly simultaneously during the γ-ray flares. We detect an edge-brightened linear polarization near the core in the active jet state in 2016, similar to the quiescent jet state in 2008–2013. These observations indicate that the γ-ray flares may originate from compression of the knots by a standing shock in the core and the jet might consist of multiple complex layers showing time-dependent behavior, rather than of a simple structure of a fast jet spine and a slow jet sheath.