Fermi-LAT observation of nonblazar AGNs

Abstract

Context. Fermi-LAT has recently detected γ-ray emission from active galactic nuclei which do not show clear evidence for optical blazar characteristics or have jets pointing away from the observer (non-blazar AGNs). These are interesting γ-ray emitters providing an alternative approach to studying high energy emission processes. Aims. Investigation of the spectral and temporal properties of γ-ray emission from non-blazar AGNs using the recent Fermi-LAT observational data. Methods. The data collected by Fermi-LAT during 2008-2015, from the observations of 26 non-blazar AGNs, including eleven FRI and ten FRII radio galaxies/SSRQs and five NLSY1s are analyzed using the new PASS 8 event selection and instrument response function. Possible spectral changes above GeV energies are investigated with a detailed spectral analysis. Light curves generated with normal and adaptive time bins are used to study the γ-ray flux variability. Results. Non-blazar AGNs have a γ-ray photon index in the range of 1.84 − 2.86 and a flux varying from a few times 10 −9 photon cm −2 s −1 to 10 −7 photon cm −2 s −1 . Over long time periods, power-law provides an adequate description of the γ-ray spectra of almost all sources. Significant curvature is observed in the γ-ray spectra of NGC 1275, NGC 6251, SBS 0846+513 and PMN J0948+0022 and their spectra are better described by log-parabola or power-law with exponential cut-off models. The γ-ray spectra of PKS 0625-25 and 3C 380 show a possible deviation from a simple power-law shape, indicating a spectral cutoff around the observed photon energy of E cut = 131.2 ± 88.04 GeV and E cut = 55.57 ± 50.74 GeV, respectively. Our analysis confirms the previous finding of an unusual spectral turnover in the γ-ray spectrum of Cen A: the photon index changes from Γ = 2.75± 0.02 to 2.31± 0.1 at 2.35± 0.08 GeV. In the Γ − L γ plane, the luminosity of non-blazar AGNs is spread in the range of (10 41 − 10 47 ) erg s −1 , where the lowest luminosity have FRI radio galaxies (but typically appear with a harder photon index) and the highest-SSRQs/NLSY1s (with softer photon indexes). We confirm the previously reported short-timescale flux variability of NGC 1275 and 3C 120. The γ-ray emission from NLSY1s, 1H 0323+342, SBS 0846+513 and PMN J0948+0022, is variable, showing flares in short scales sometimes accompanied by a moderate hardening of their spectra (e.g., on MJD 56146.8 the γ-ray photon index of SBS 0846+513 was Γ = 1.73 ± 0.14). 3C 111, Cen A core, 3C 207, 3C 275.1, 3C 380, 4C+39.23B, PKS 1502+036 and PKS 2004-447 show a long-timescale flux variability in the γ-ray band.

Morphology of TeV Gamma-Ray Emission from the Kiloparsec-scale Jets in Radio Galaxies

“…The spectrum is well described by a simple power law with a spectral index 2.52 ± 0.13 stat ± 0.20 sys in the energy range 0.25−6 TeV (Abdalla et al 2018). This spectrum smoothly connects to the hard GeV γray component observed by the Fermi satellite (Abdo et al 2010, Sahakyan et al 2013, Brown et al 2017, Sahakyan et al 2018. At least a part of the TeV γ-ray emission has been localized to be produced within the kpc scale jet in Cen A (Sanchez et al 2018), confirming the essential role of relativistic particles also in the kpc scale jets of radio galaxies.…”

Faint γ-ray sources at low redshift: the radio galaxy IC 1531

“…IC 1531 shares the properties of the LAT detected FR Is (see Torresi et al 2018, and references therein). These sources are continuously detected by LAT and show low-level or no variability in γ-rays (Fermi- LAT Collaboration et al 2015;Sahakyan et al 2018). The notable exception is NGC 1275 which couples a long-term continuous brightening with short-term (down to few days) flaring activity (Tanada et al 2018).…”

Radio Galaxies at VHE Energies

“…In the HE range Fermi-LAT has detected about 20 RGs [e.g., 129]. Six of them are also known as VHE emitters (see Fig.…”