# VERITAS discovery of very high energy gamma-ray emission from S3 1227+25 and multiwavelength observations

**Authors:** Atreya Acharyya, Colin Adams, Avery Archer, Priyadarshini Bangale, Wystan Benbow, Aryeh Brill, Jodi Christiansen, Alisha Chromey, Manel Errando, Abe Falcone, Qi Feng, John Finley, Gregory Foote, Lucy Fortson, Amy Furniss, Greg Gallagher, William Hanlon, David Hanna, Olivier Hervet, Claire Hinrichs  
**arXiv:** [2305.02860](https://arxiv.org/abs/2305.02860)  
**Date:** 2023-05-04  
**Summary page:** https://www.summarizepaper.com/en/paper/2305.02860v1/

## Key points

- Very high energy gamma-ray emission detected from blazar S3 1227+25 using VERITAS
- Observations triggered by a hard-spectrum GeV flare detected on May 15, 2015 with Fermi-LAT
- Strong 13$\sigma$ detection with a differential photon spectral index, $\Gamma$ = 3.8 $\pm$ 0.4, and a flux level at 9% of the Crab Nebula above 120 GeV
- Multiwavelength correlations investigated to identify relationships between emission zones and place constraints on dominant mechanisms responsible for blazar emission
- Multiwavelength lightcurves presented over wider time interval from August 4, 2008 until February 1, 2022 including Fermi-LAT data in two distinct energy ranges, radio lightcurves measured with Mets¨ahovi and OVRO, as well as optical lightcurves obtained from ASAS-SN Sky Patrol and ATLAS data
- Evidence of strong correlations between Fermi-LAT lightcurves in different energy regimes indicating both MeV and GeV components of the flare have similar origins
- Evidence of optical and gamma-ray correlations suggesting a single zone model of emission
- Multiwavelength spectral energy distribution well described by a simple one zone leptonic synchrotron self Compton radiation model

## Summary

This study reports the detection of very high energy gamma-ray emission from the blazar S3 1227+25 (VER J1230+253) using the Very Energetic Radiation Imaging Telescope Array System (VERITAS). The observations were triggered by a hard-spectrum GeV flare detected on May 15, 2015 with the Fermi-Large Area Telescope (LAT). A combined five-hour VERITAS exposure on May 16th and May 18th resulted in a strong 13$\sigma$ detection with a differential photon spectral index, $\Gamma$ = 3.8 $\pm$ 0.4, and a flux level at 9% of the Crab Nebula above 120 GeV. This also triggered target of opportunity observations with Swift, optical photometry, polarimetry and radio measurements, in addition to the VERITAS and Fermi-LAT data. The study investigates multiwavelength correlations to identify relationships between emission zones and place constraints on the dominant mechanisms responsible for blazar emission. The authors present multiwavelength lightcurves over a wider time interval from midnight on August 4, 2008 until midnight on February 1, 2022. The lightcurves include Fermi-LAT data in two distinct energy ranges: low energy (0.1–1 GeV) and high energy (1–300 GeV), radio lightcurves measured with Mets¨ahovi and OVRO, as well as optical lightcurves obtained from ASAS-SN Sky Patrol and ATLAS data. Local cross-correlation functions are applied to pairs of two different lightcurves to search for correlations between them. Monte Carlo simulations are used to determine the significance of an obtained LCCF value using artificial light curves that match the probability distribution function (PDF) and power spectral density (PSD) of each observation. The results show evidence of strong correlations between the Fermi-LAT lightcurves in different energy regimes which indicates that both MeV and GeV components of the flare have similar origins. Furthermore, investigation into correlations between multiwavelength observations revealed evidence of optical and gamma-ray correlations suggesting a single zone model of emission. Finally, multiwavelength spectral energy distribution is well described by a simple one zone leptonic synchrotron self Compton radiation model. Overall this study provides valuable insights into blazar emission mechanisms while highlighting importance of investigating multiwavelength correlations to identify relationships between emission zones.