A search for Centaurus A-like features in the spectra of Fermi-LAT detected radio galaxies

Abstract

Motivated by the detection of a hardening in the γ-ray spectrum of the radio galaxy Centaurus A, we have analysed ∼10 years of Fermi-LAT observations of 26 radio galaxies to search for similar spectral features. We find that the majority of the radio galaxies' γ-ray spectral energy distributions are best fitted with a simple power-law model, and no spectral hardening similar to that found in Centaurus A was detected. We show that, had there been any such spectral features present in our sample of radio galaxies, they would have been seen, but note that 7 of the radio galaxies (3C 111, 3C 120, 3C 264, IC 4516, NGC 1218, NGC 2892 and PKS 0625-35) show evidence for flux variability on 6-month timescales, which makes the detection of any steady spectral features difficult. We find a strong positive correlation (r = 0.9) between the core radio power at 5 GHz and the γ-ray luminosity and, using a simple extrapolation to TeV energies, we expect around half of the radio galaxies studied will be detectable with the forthcoming Cherenkov Telescope Array.

A search for γ-ray emission from a sample of local Universe low-frequency selected radio galaxies

“…Fig. B4 displays γ -ray flux plotted against core dominance; this is consistent with Angioni et al (2019) and Rulten et al (2020), who find no correlation between γ -ray luminosity and core dominance in their analysis of the known 4FGL radio galaxies. Although these authors use a different method to calculate core dominance to Liuzzo et al (2009), which we use, all use radio measurements at similar frequencies (5 and 8.4 GHz, respectively).…”

“…For the STRGs where we have values for core dominance, these are generally smaller than those of the 4FGL galaxies, with none of them above 1, although the sample size is small. Both 4C 29.41 and 3C 272.1 have core dominance values below 1, indicating that any emission from these radio galaxies may not originate from processes in the kpc-scale jet, similar to the emission causing the spectral component seen in the core of Cen A, as discussed in Brown et al (2017), and searched for in Rulten et al (2020) (for example, emission from a parsec-scale inner jet, or an additional population of cosmic rays). In the case of 3C 293, a 95 per cent confidence limit is in place of a measurement with a maximum value of 1.42.…”

“…Launched in 2008, the Fermi Large Area Telescope (Fermi-LAT) has been observing the whole sky over an effective γ -ray energy range of 100 MeV to approximately 300 GeV for over a decade. Varying levels of γ -ray emission have been detected in both FR I and FR II radio galaxies (Abdo et al 2009;Rulten, Brown & Chadwick 2020). There is evidence for γ -ray production in both the radio core of the galaxy, from the parsec-scale jet (Angioni et al 2019), and the kpc-scale jets for both galaxy types (Abdo et al 2010;Grandi & Torresi 2012).…”

“…This result is consistent with the results of Angioni et al (2019), where they find that pc-scale jets drive the γ -ray emission in galaxies that are not dominated by Doppler-boosted emission. Rulten et al (2020) examined 26 Fermi-LAT-detected radio galaxies for Cen A-like spectral features, but found no evidence of spectral hardening in the 10 yr average spectra among the radio galaxies that showed no evidence for variability. In this work, we search for γ -ray emission from a sample of close, lowfrequency radio galaxies that are unlikely to be dominated by Doppler-boosted γ -ray emission (blazar-like emission).…”

High energy emission component, population, and contribution to the extragalactic gamma-ray background of gamma-ray emitting radio galaxies

“…In fact, TeV gamma-ray emission from the Cen A kpc-scale lobe was detected by HESS (H. E. S. S. Collaboration et al 2020), suggesting particle acceleration in the kpc-scale diffuse or knot regions. Rulten et al (2020) surveyed the Fermi/LAT spectra of radio galaxies, but only Cen A shows an upturned spectrum. Considering that the peak frequency of the high energy component of Cen A is the lowest among radio galaxies and thus it is the faintest in gamma-rays, future surveys may find other gamma-ray faint radio galaxies with upturned spectra.…”