Revisiting the TeV detection prospects for radio galaxies

TL;DR

This paper assesses the future capabilities of the Cherenkov Telescope Array (CTA) to detect radio galaxies at TeV energies, predicting a significant increase in known sources and potential for new discoveries in high-energy astrophysics.

Contribution

It expands previous work by using the larger 4FGL catalog to simulate CTA observations, predicting detection of 11 new TeV radio galaxies and the first detection of FR II types at these energies.

Findings

CTA could detect 11 new TeV radio galaxies with 50-hour observations.

Current TeV observatories may already detect 2 Fermi-LAT radio galaxies.

Detection of extended VHE emission from Fornax A is feasible with >50 hours exposure.

Abstract

Radio galaxies host relativistic jets oriented away from our line of sight, making them challenging targets for Very High Energy (VHE, E$>$100 GeV) $\gamma$-ray detectors. Indeed, out of $\sim100$ extragalactic sources detected at $E>100$ GeV, only six are radio galaxies. The new Cherenkov Telescope Array (CTA) will provide order-of-magnitude improvements in sensitivity with respect to the present generation of ground-based $\gamma$-ray observatories, opening new frontiers for high-energy studies of radio galaxies. In a previous paper, we studied the CTA detection prospects of misaligned jets from the Third Fermi-LAT catalog (3FGL) sample. In this work, we take advantage of the expanded sample from the Fourth Fermi-LAT catalog (4FGL), including roughly double the number of sources. We simulate CTA observations of 41 $\gamma$-ray radio galaxies, extrapolating their GeV spectrum into the TeV range assuming different spectral shapes. We predict that the CTA will detect eleven new TeV radio galaxies with an observational campaign of 50 hours per source, under the realistic assumption of a spectral cutoff at 0.5 TeV. This would increase the sample of VHE radio galaxies by a factor of three, and result in the first detection of FR II radio galaxies at these energies. By simulating CTA observations with 5 hours exposure, we predict that two Fermi-LAT radio galaxies should already be within reach of current TeV observatories. Finally we have investigated the prospects for a CTA detection of extended VHE emission from the lobe-dominated FR I Fornax A, and predict that this will be possible for integration times $\gtrsim\!50$ hours. We conclude that the CTA will significantly impact our understanding of misaligned jets at TeV energies, allowing us to perform population studies and compare the two main radio galaxy subclasses for the first time in this energy band.