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Molecular absorption of Cherenkov light at CTAO

G. Voutsinas, M. Dalchenko, M. Gaug, O. Gueta, T. Montaruli, R. Zanin

Abstract

The Cherenkov Telescope Array Observatory (CTAO) is the next-generation observatory for high energy γ-ray astronomy with unprecedented sensitivity and accuracy. Accurate estimation and mitigation of systematic uncertainties are crucial for its scientific performance. Atmospheric properties significantly influence both the generation and extinction of Cherenkov light generated by gamma and cosmic rays interacting in the atmosphere. This study provides a detailed analysis of molecular extinction processes, including Rayleigh scattering and molecular absorption, and their impact on the transmission of Cherenkov light. We examine typical summer and winter behaviour of Rayleigh scattering and seasonal and event-driven variations of the main absorbing molecules, such as ozone and nitrogen oxides, at the two CTAO array sites. Using simulations, we assess the effects of these variations on image intensity and trigger effective area, particularly during dynamic atmospheric events like stratosphere-to-troposphere transport. Based on our findings, we propose an atmospheric monitoring and calibration strategy to ensure that the CTAO meets its systematic uncertainty requirements, particularly for low-energy gamma-ray observations.

Molecular absorption of Cherenkov light at CTAO

Abstract

The Cherenkov Telescope Array Observatory (CTAO) is the next-generation observatory for high energy γ-ray astronomy with unprecedented sensitivity and accuracy. Accurate estimation and mitigation of systematic uncertainties are crucial for its scientific performance. Atmospheric properties significantly influence both the generation and extinction of Cherenkov light generated by gamma and cosmic rays interacting in the atmosphere. This study provides a detailed analysis of molecular extinction processes, including Rayleigh scattering and molecular absorption, and their impact on the transmission of Cherenkov light. We examine typical summer and winter behaviour of Rayleigh scattering and seasonal and event-driven variations of the main absorbing molecules, such as ozone and nitrogen oxides, at the two CTAO array sites. Using simulations, we assess the effects of these variations on image intensity and trigger effective area, particularly during dynamic atmospheric events like stratosphere-to-troposphere transport. Based on our findings, we propose an atmospheric monitoring and calibration strategy to ensure that the CTAO meets its systematic uncertainty requirements, particularly for low-energy gamma-ray observations.
Paper Structure (14 sections, 5 equations, 16 figures, 2 tables)

This paper contains 14 sections, 5 equations, 16 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Rayleigh scattering
  3. Molecular absorption
  4. Data sources
  5. Mixing ratios variations
  6. Ozone
  7. Nitrogen oxides
  8. Production of molecular absorption profiles
  9. Optical depths for different profiles
  10. Impact on CTAO performance
  11. Full simulations
  12. Systematic uncertainties affecting the study
  13. Discussion on calibration strategy
  14. Conclusions

Figures (16)

  • Figure 1: Comparison of the Rayleigh scattering integrated optical depth at 400 nm as a function of altitude between the two preliminary summer and winter reference profiles of the CTAO-North array site. The red line corresponds to the array site altitude, i.e. 2158 m. The middle panel shows the optical depth ratio between summer and winter. The bottom panel shows the number density ratio.
  • Figure 2: Comparison of the Rayleigh scattering integrated optical depth at 400 nm as a function of altitude between the two indicative summer and winter reference profiles for CTAO-S. The red line corresponds to the ground level (alt. 2147 m). The middle panel shows the optical depth ratio between summer and winter. The bottom panel shows the number density ratio.
  • Figure 3: Nighttime ozone mass mixing ratio over the Southern and Northern CTAO sites, in the time period 2020-2024, at a pressure level of 700 hPa.
  • Figure 4: Time - altitude evolution of a STT ozone mass transport event that took place at CTAO-South array site in June 2020.
  • Figure 5: Ozone mass mixing ratio over the CTAO-North site during the years 2021-2024.
  • ...and 11 more figures