Very high-energy gamma-ray signature of ultrahigh-energy cosmic ray acceleration in Centaurus A

Abstract

The association of at least a dozen ultrahigh-energy cosmic ray (UHECR) events with energy ≳ 55 EeV detected by the Pierre Auger Observatory from the direction of Centaurus-A, the nearest radio galaxy, supports the scenario of UHECR acceleration in the jets of radio galaxies. In this work, we model radio to very high energy (VHE,≳ 100 GeV) γ-ray emission from Cen A, including GeV hardness detected by Fermi–LAT and TeV emission detected by the High Energy Stereoscopic System (HESS). We consider two scenarios: (i) two-zone synchrotron self-Compton (SSC) and external-Compton (EC) models, (ii) two-zone SSC, EC, and photohadronic emission from cosmic ray interactions. The GeV hardness observed by Fermi–LAT can be explained using these two scenarios, where zone 2 EC emission is very important. Hadronic emission in scenario (ii) can explain VHE data with the same spectral slope as obtained through fitting UHECRs from Cen A. The peak luminosity in cosmic ray proton at 1 TeV, to explain the VHE γ-ray data is ≈2.5 × 1046 erg s−1. The bolometric luminosity in cosmic ray protons is consistent with the luminosity required to explain the origin of 13 UHECR signal events that are correlated with Cen A.

Fornax A, Centaurus A, and other radio galaxies as sources of ultrahigh energy cosmic rays

“…The correlation with AGN in A18 would also be improved by including the contribution from the lobes of Centaurus A, which are estimated to be at least as bright as the core in γ-rays (Abdo et al 2010b). Furthermore, although there may be a direct relation between the observed γ-rays and UHECRs (Sahu et al 2012;Yang et al 2012;Joshi et al 2018), γ-ray luminosity may not be the best proxy for UHECR luminosity.…”

“…Mathews & Brighenti 2008). Based on the UHECRs arriving at Earth with energies above 55 EeV and directions clustered around Centaurus A, Joshi et al (2018) estimate an UHECR luminosity of ∼ 10 39 erg s −1 . The jet powers in Centaurus A, Fornax A and M87 exceed this value by orders of magnitude.…”

“…GZK and hadronic γ-ray losses might still matter for γ-ray emission. In fact, UHECRs in the Centaurus A lobes are thought to produce some of the observed γ-ray flux (Sahu et al 2012;Joshi et al 2018).…”

Where are the Ultra High Energy Cosmic Ray (UHECR) originated?

“…The presence in the shadow area of well active galactic sources as Vela (the brightest galactic gamma source in Fermi GeV sky, see on the top Egret gamma map), LMC, SMC, Crab and NGC 253, Fornax D, might be the origin of this very local anti-galactic center emission. or radio band, contrary to the alternative proposal of much distant AGN [22], [23] sources, whose charged nuclei random walk have a time of flight (from a hundred Mpc distances) that should suffer from a huge (million years) delay respect to the direct gamma photons; therefore, most far UHECR are very possibly uncorrelated [9] with their present gamma observations [24]. The main message in conclusion is that the lightest nuclei as Helium might be the first and the main responsible courier of UHECR, source of the hot spot clustering observed by TA and Auger, in the last decade.…”

Sub-parsec and parsec VHE emission from the core of LLAGNs

“…The spectrum of the core of Cen A hardens at GeV energies, suggestive of a new gamma-ray emitting component [2] different from the one producing the radio to GeV emission typically modelled with synchrotron self-Compton (SSC) emission in the subparsec jet. The VHE component of Cen A has been investigated with jet models considering leptonic emission produced in the kpc knots of the outflows [3], as well as with jet hadronic and two-zone lepto-hadronic scenarios ([4], [5]). In this talk, we focus on the potential contribution to the VHE spectrum of Cen A, of radiation produced black hole (BH) accretion flow region [6,7] and the circumnuclear disc (CND) of Cen A [8].…”