Unveiling the origin of the gamma-ray emission in NGC 1068 with the Cherenkov Telescope Array

Abstract

Several observations are revealing the widespread occurrence of mildly relativistic wide-angle AGN winds strongly interacting with the gas of their host galaxy. Such winds are potential cosmic-ray accelerators, as supported by gamma-ray observations of the nearby Seyfert galaxy NGC 1068 with the Fermi gamma-ray space telescope. The non-thermal emission produced by relativistic particles accelerated by the AGN-driven wind observed in the circum-nuclear molecular disk of such galaxy is invoked to produce the gamma-ray spectrum. The AGN wind model predicts a hard spectrum that extend in the very high energy band which differs significantly from those corresponding to other models discussed in the literature, like starburst or AGN jet. We present dedicated simulations of observations through the Cherenkov Telescope Array (CTA), the next-generation ground based gamma-ray observatory, of the very high energy spectrum of the Seyfert galaxy NGC 1068 assuming the AGN wind and the AGN jet models. We demonstrate that, considering 50 hours of observations, CTA can be effectively used to constrain the two different emission models, providing important insight into the physics governing the acceleration of particles in non-relativistic AGNdriven outflows. This analysis strongly motivates observations of Seyfert and starburst galaxies with CTA in order to test source population models of the extragalactic gamma-ray and neutrino backgrounds.

Constraints on Gamma-Ray and Neutrino Emission from NGC 1068 with the MAGIC Telescopes

“…The shaded gray band indicates the upper (p=2, E cut =6×10 6 GeV, and ξ=0.25) and lower (p=2, E cut =3×10 5 GeV, and ξ=0.2) bounds of the gamma-ray emission predicted by the AGN wind model as proposed by Lamastra et al (2016). For the sake of clarity, the predictions of the revised AGN wind model (Lamastra et al 2019) are not shown, as they do not differ from that by Lamastra et al (2016) at energies smaller than 10 TeV. For comparison, the spectrum predicted by the AGN wind model that is obtained by assuming one of the combinations of CR proton spectral parameters compatible with the MAGIC upper limits (p=2, E cut =8×10 3 GeV, and ξ=0.2, see Figure 3), is shown with the dark gray line.…”

“…Both processes contribute in turn to the uncertainty in the computation of p and E cut . As discussed in Lamastra et al (2019), absorption due to the EBL and to the infrared emission from the starburst ring surrounding the acceleration regions affects the gamma-ray spectrum only above E10 TeV, thus the constraints shown in Figure 3 can be considered robust.…”

“…At the same time, the improved sensitivity of the next generation of ground-based gamma-ray observatories, like the Cherenkov Telescope Array, will allow us to disentangle the different emission mechanisms. In particular, simulations of 50 hr of observations of NGC 1068 with CTA have shown that leptonic and hadronic models could be distinguished (Lamastra et al 2019).…”

“…Studies on the star-forming properties of AGN host galaxies indicate that the level of star formation in AGN hosts can be either elevated, as in starbursts, or normal, as in quiescently star-forming galaxies, or suppressed, as in passive spheroids (see Lamastra et al 2013b;Gatti et al 2015;Rodighiero et al 2015 and references therein). In active galaxies, non-thermal radiation in the gamma-ray band may also be produced by the interaction of particles (protons and electrons) accelerated in AGN-driven outflows (wind and jet) with the ISM and interstellar radiation fields (e.g., Lenain et al 2010;Tamborra et al 2014;Lamastra et al 2016Lamastra et al , 2019. Indeed, weak misaligned radio jets, and wide-angle AGN-driven outflows have been observed in star-forming AGNs detected in the GeV band by Fermi-LAT (Gallimore et al 1996;Elmouttie et al 1998;García-Burillo et al 2014;Zschaechner et al 2016).…”

A Cloud-based Architecture for the Cherenkov Telescope Array Observation Simulations: Optimization, Design, and Results

“…The application of the energy dispersion matrix for sources requiring a detailed spectral treatment in the softer energies (e.g. E 500 GeV) and will be shown (Romano et al 2018;Lamastra et al 2019) to increase the run time by a factor of 5-10.…”