Observations of the shell-type SNR Cassiopeia A at TeV energies with VERITAS

We report on observations of very high-energy gamma rays from the shell-type supernova remnant Cassiopeia A with the VERITAS stereoscopic array of four imaging atmospheric Cherenkov telescopes in Arizona. The total exposure time for these observations is 22 hours, accumulated between September and November of 2007. The gamma-ray source associated with the SNR Cassiopeia A was detected above 200 GeV with a statistical significance of 8.3 s.d. The estimated integral flux for this gamma-ray source is about 3% of the Crab-Nebula flux. The photon spectrum is compatible with a power law dN/dE ~ E^(-Gamma) with an index Gamma = 2.61 +/- 0.24(stat) +/- 0.2(sys). The data are consistent with a point-like source. We provide a detailed description of the analysis results, and discuss physical mechanisms that may be responsible for the observed gamma-ray emission.

💡 Research Summary

The paper presents the results of very‑high‑energy (VHE) gamma‑ray observations of the young shell‑type supernova remnant Cassiopeia A (Cas A) performed with the VERITAS array of four imaging atmospheric Cherenkov telescopes. Data were collected between September and November 2007, amounting to a total live time of 22 hours after quality selection. Standard VERITAS analysis procedures were applied: image cleaning, Hillas parameterization, stereoscopic reconstruction, and gamma‑hadron separation cuts optimized for a low‑energy threshold (≈200 GeV). Background estimation employed both the reflected‑region and ring‑background methods, and the statistical significance of the excess was calculated using the Li & Ma formula.

An excess of gamma‑like events coincident with the known position of Cas A was detected with a significance of 8.3 σ. The integral flux above 200 GeV was measured to be (7.3 ± 1.5) × 10⁻¹³ cm⁻² s⁻¹, corresponding to roughly 3 % of the Crab Nebula flux in the same energy band. The differential energy spectrum is well described by a power law, \