Fermi-LAT observation of nonblazar AGNs

¹ ICRANet, Piazza della Repubblica 10, 65122 Pescara, Italy
e-mail: narek@icra.it
² ICRANet Armenia, Marshall Baghramian Avenue 24a, Yerevan 0019, Republic of Armenia
³ Yerevan State University, Alek Manukyan Str. 1, Yerevan 0025, Republic of Armenia

Received: 16 November 2017
Accepted: 27 January 2018

Abstract

Context. Fermi Large Area Telescope (Fermi-LAT) has recently detected γ-ray emission from active galactic nuclei (AGN) that do not show clear evidence for optical blazar characteristics or have jets pointing away from the observer (nonblazar AGNs). These are interesting γ-ray emitters providing an alternative approach to studying high energy emission processes.

Aims. This paper investigates the spectral and temporal properties of γ-ray emission from nonblazar AGNs using the recent Fermi-LAT observational data.

Methods. The data collected by Fermi-LAT during 2008–2015, from the observations of 26 nonblazar AGNs, including 11 Fanaroff–Riley Type I (FRI) and ten FRII radio galaxies and steep spectrum radio quasars (SSRQs) and five narrow line seyfert 1s (NLSy1s) are analysed 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, the 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 by the 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 cut-off 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 nonblazar AGNs is spread in the range of (10⁴¹–10⁴⁷) erg s⁻¹, where those with the lowest luminosity are FRI radio galaxies (but typically appear with a harder photon index) and those with the highest luminosity have are 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., for MJD 56146.8 the γ-ray photon index of SBS 0846 + 513 was Γ = 1.73 ± 0.14). Non-blazar AGNs 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.