Very High Energy Gamma Rays from Ultra Fast Outflows

Abstract

Context. Ultra fast outflows (UFOs) from active galactic nuclei (AGN) are expected to lead to the formation of sub-relativistic strong shocks expanding in a dense circumnuclear medium, and thus have the potential for being efficient particle accelerators, and to be proficient sources of gamma rays and neutrinos. Aims. We investigate the detectability of a sample of nearby identified UFOs in gamma rays and neutrinos with current and next- generation instruments. Methods. We model the acceleration of particles at the strong shocks of UFOs, and estimate the associated gamma-ray and neutrino signal. We adopt our model to investigate the prospects for detection with current and next-generation observatories. Results. We find that several UFOs could be detectable in the very-high-energy (VHE) domain - for example, by the Cherenkov Telescope Array Observatory (CTAO)- even if they remain undetected by Fermi-LAT in the high-energy range. Detectability is favored for hard proton spectra (spectral index α \lessim 3.9), high acceleration efficiencies, and amplified magnetic fields. Our results suggest that next-generation VHE observatories could detect the first gamma-ray signatures of AGN UFOs, providing a new probe of particle acceleration in sub-relativistic shocks

Figures (5)

• Figure 1: Description of the sample of sources, top panel : distributin of the outflow speed, on the middle the distance and on the bottom one the mass-loss rate, in violet stars are indicated the main properties of the detectable UFOs, summarized in Table \ref{['tab:names_fig2']}.
• Figure 2: Schematic representation of the geometry of UFOs and associated shocks.
• Figure 3: Number of UFOs with a gamma-ray signal above the typical sensitivity of the CTAO (50 hours) and below the Fermi-LAT sensitivity. The shock radius is $R_{\rm sh}= 1$ pc. Panels from top to bottom illustrate different assumptions of external density: $n_0 = 10^{2}-10^{3}-10^{4} \mathrm{cm} ^{-3}$. Sensitivities of Fermi-LAT (14 years), LHAASO (5 years) and KM3NeT/ARCA (10 years) are shown.
• Figure 4: Number of UFOs with a gamma-ray signal above the typical sensitivity of the CTAO (50 hours) with dark areas for parameters that lead to a Fermi detection. The target density is $n_0 =10^{2} \mathrm{cm}^{-3}$. Sensitivities of Fermi-LAT (14 years), LHAASO (5 years) and KM3NeT/ARCA (10 years) are shown.
• Figure 5: Ratio of hadronic gamma rays over hadronic + leptonic gamma rays $R_{\rm hadronic}= \frac{F_{\rm had}( > 100 ~\text{GeV})}{F_{\rm had}( > 100 ~\text{GeV})+F_{\rm lep}( > 100 ~\text{GeV})}$ for various values of the shock radius $R_{\rm sh}$ and ambient density $n_0$. In this figure, the bolometric luminosity is fixed at $L_{\rm bol} = 10^{43}~\rm erg/s$, and the electron-to-proton ratio is set to $K_{\rm ep} = 10^{-3}$. The leptonic gamma-ray flux is computed for Inverse Compton with Naima code Kafexhiu_20142010_Aharonian assuming that the electron population follows the same energy distribution as the protons, with normalization $K_{\rm ep}$ at 1 GeV. The AGN photon field considered is described in Sec. \ref{['Section: Model']}, and the maximum energy of electrons is estimated by equating the acceleration timescale to the smaller of the losses timescale and the UFO age.