Fermi Large Area Telescope Observations of the Active Galaxy 4C +55.17: Steady, Hard Gamma-Ray Emission and its Implications

TL;DR

This study reports steady, high-energy gamma-ray emission from the active galaxy 4C +55.17, explores its possible nature as a young radio source or blazar, and discusses its potential for very-high-energy observations and EBL constraints.

Contribution

It provides the first detailed gamma-ray analysis of 4C +55.17, examining its emission origin and highlighting its suitability for VHE studies at high redshift.

Findings

Gamma-ray emission extends up to 145 GeV without variability.

The source may be a young radio galaxy or a blazar, based on spectral modeling.

Potential detectability in very-high-energy range by ground-based telescopes.

Abstract

We report Fermi/LAT observations and broad-band spectral modeling of the radio-loud active galaxy 4C +55.17 (z=0.896), formally classified as a flat-spectrum radio quasar. Using 19 months of all-sky survey Fermi/LAT data, we detect a gamma-ray continuum extending up to an observed energy of 145 GeV, and furthermore we find no evidence of gamma-ray variability in the source over its observed history. We illustrate the implications of these results in two different domains. First, we investigate the origin of the steady gamma-ray emission, where we re-examine the common classification of 4C +55.17 as a quasar-hosted blazar and consider instead its possible nature as a young radio source. We analyze and compare constraints on the source physical parameters in both blazar and young radio source scenarios by means of a detailed multiwavelength analysis and theoretical modeling of its broad-band spectrum. Secondly, we show that the gamma-ray spectrum may be formally extrapolated into the very-high energy (VHE; >= 100 GeV) range at a flux level detectable by the current generation of ground-based Cherenkov telescopes. This enables us to place constraints on models of extragalactic background light (EBL) within LAT energies and features the source as a promising candidate for VHE studies of the Universe at an unprecedented redshift of z=0.896.