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Detection Prospects for AGNs with the Cherenkov Telescope Array

Luana Passos-Reis, Elisabete M. de Gouveia Dal Pino, Tarek Hassan, Santiago Pita, Jonathan Biteau, Jean-Philippe Lenain, Atreya Acharyya, Alberto Domínguez, Lucas Gréaux, Luiz Augusto Stuani Pereira, Edivaldo Moura Santos, Paolo Goldoni

TL;DR

The paper develops a variability-informed framework to forecast CTAO detectability of extragalactic AGNs by combining Fermi-LAT spectral extrapolation with a normalized excess variance ($\sigma_{\rm NXS}^{2}$) based on multi-timecale light curves. It processes 4LAC-DR3 sources with light-curve quality control, computes $F_{ m var}$ and $\sigma_{\rm NXS}^{2}$ on 3-, weekly, and 30-day cadences, and uses a 50% completeness threshold, followed by a 3-sigma variability filter to compile a robust sub-sample. An empirical scaling between $\sigma_{\rm NXS,3d}^{2}$ and $\sigma_{\rm NXS,30d}^{2}$ from 87 sources with both measurements enables short-timescale extrapolation to about 320 additional sources, boosting CTAO forecasted detections for 5h and 20h observations. This variability-informed approach enhances the realism of extragalactic VHE sky forecasts and provides insights into the jet physics driving AGN variability across timescales and energy bands.

Abstract

The Cherenkov Telescope Array Observatory (CTAO) will enable detailed studies of Active Galactic Nuclei (AGN) in the very-high-energy (VHE) regime, as the next-generation ground-based gamma-ray observatory, designed to enhance sensitivity and energy coverage (20 GeV -- 300 TeV) over current Imaging Atmospheric Cherenkov Telescopes (IACTs). In the context of the CTAO Science Collaboration, within the AGN Population working group, we developed a variability-based strategy to improve predictions of AGNs detectable by CTAO, using Fermi-LAT data and normalized excess variance (NXS) as a tracer of flux variability. By extrapolating from 30-day to 3-day timescales, we expanded the sample of sources with short-timescale variability estimates from 87 to 407. This approach allows us to identify flaring and distant AGNs that are promising CTAO targets. The results are being used to support the CTAO extragalactic science program and will be included in an upcoming Consortium publication for the AGN Population collaboration.

Detection Prospects for AGNs with the Cherenkov Telescope Array

TL;DR

The paper develops a variability-informed framework to forecast CTAO detectability of extragalactic AGNs by combining Fermi-LAT spectral extrapolation with a normalized excess variance () based on multi-timecale light curves. It processes 4LAC-DR3 sources with light-curve quality control, computes and on 3-, weekly, and 30-day cadences, and uses a 50% completeness threshold, followed by a 3-sigma variability filter to compile a robust sub-sample. An empirical scaling between and from 87 sources with both measurements enables short-timescale extrapolation to about 320 additional sources, boosting CTAO forecasted detections for 5h and 20h observations. This variability-informed approach enhances the realism of extragalactic VHE sky forecasts and provides insights into the jet physics driving AGN variability across timescales and energy bands.

Abstract

The Cherenkov Telescope Array Observatory (CTAO) will enable detailed studies of Active Galactic Nuclei (AGN) in the very-high-energy (VHE) regime, as the next-generation ground-based gamma-ray observatory, designed to enhance sensitivity and energy coverage (20 GeV -- 300 TeV) over current Imaging Atmospheric Cherenkov Telescopes (IACTs). In the context of the CTAO Science Collaboration, within the AGN Population working group, we developed a variability-based strategy to improve predictions of AGNs detectable by CTAO, using Fermi-LAT data and normalized excess variance (NXS) as a tracer of flux variability. By extrapolating from 30-day to 3-day timescales, we expanded the sample of sources with short-timescale variability estimates from 87 to 407. This approach allows us to identify flaring and distant AGNs that are promising CTAO targets. The results are being used to support the CTAO extragalactic science program and will be included in an upcoming Consortium publication for the AGN Population collaboration.

Paper Structure

This paper contains 6 sections, 3 equations, 4 figures.

Table of Contents

  1. Introduction
  2. Extragalactic Source Population Studies
  3. Processing and Selection of Light Curves
  4. Selecting Blazar Targets based on Variability
  5. Determination of the proportionality coefficient between $\sigma_{\rm NXS, 3d}^{2} (\text{NXS 3-days})$ and $\sigma_{\rm NXS, 30d}^{2} (\text{NXS 30 days})$
  6. Discussion and Results

Figures (4)

  • Figure 1: Unfiltered 3-day light curve of Mkn 421. Outliers and unconstrained flux points appear as extreme deviations, including vertical lines that extend beyond the graph boundaries.
  • Figure 2: Filtered 3-day light curve of Mkn 421, after removing outliers and unconstrained points.
  • Figure 3: Normalized excess variance $\sigma_{\rm NXS}^{2}$ (NXS) distributions across different cadences, after applying completeness and variability filters.
  • Figure 4: Correlation between $\sigma_{\rm NXS}^{2}$ on 3-day and 1-month timescales for a sample of 87 AGNs with high-significance variability. The fits enable extrapolation of short-timescale variability from monthly data, providing a practical approach to extend temporal coverage for population studies.