Cas A - A shell-type supernova remnant detected at TeV energies PDF Print
Smoothed sky map of excess counts from the region centered at Cassiopeia A observed with VERITAS for a total of 22 hours in 2007. For more details see Figure 1 below.



Cassiopeia A - A shell-type supernova remnant detected at TeV energies



Reference:  V. A. Acciari et al. (The VERITAS Collaboration), Astrophysical Journal 714: 163-169, 2010

Full text version

ArXiv version: ArXiV:1002.2974

Contact person: Alex Konopelko


Cassiopeia A is the youngest of the historical galactic supernova remnants (SNR). It first appeared in the sky about 1680. Located 3.4 kpc away, the optical shell of 2.5' radius corresponds to a physical size of about 2.5 pc. The synchrotron radiation of Cassiopeia A extends from radio wavelengths through the sub-millimeter and near-infrared all the way to hard X-rays. The morphology of the non-thermal X-ray emission of Cassiopeia A is dominated by faint, well-defined filaments and knots, which are possibly sites of cosmic ray acceleration. These energetic hadronic cosmic rays (CR) can subsequently produce gamma-rays in collisions with ambient gas via π0 decay.

With an exposure of 232 hours, accumulated during the summer months of 1997-1999, HEGRA barely detected TeV gamma-ray emission associated with Cassiopeia A. It was observed with the VERITAS stereoscopic array of four imaging atmospheric Cherenkov telescopes in Arizona for only 22 hours 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 8.3σ. 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 with an index Γ=2.61 ± 0.24 (stat) ± 0.2 (sys). The data are consistent with a point-like source.

Understanding the mechanism of particle acceleration in isolated SNR shocks is of great interest. The question of whether or not the very-high-energy gamma-ray emission of Galactic supernova remnants implies a sufficiently high flux of charged cosmic rays, merging into a steady flux of Galactic cosmic rays, remains one of the most stimulating rationales for ground-based gamma-ray astronomy. The presence of a large flux of high-energy electrons in the reverse-shock region, responsible for the non-thermal radio-to-X-ray emission, will also produce high-energy gamma-ray emission through non-thermal bremsstrahlung and inverse-Compton scattering. Cassiopeia A is one of the best-studied SNR in the Galaxy, and a multitude of observations in different wavebands constrain its physical properties and hence the environment in which particle-acceleration processes operate. The question whether the TeV gamma-rays which we see have a leptonic or hadronic origin motivated recent VERITAS observations of Cassiopeia A. Unfortunately, the limited angular resolution of VERITAS and the current, rather limited, event statistics of gamma rays from Cassiopeia A do not allow us to draw a final conclusion. Further observations of Cassiopeia A with VERITAS along with the Fermi LAT may finally disclose the origin of galactic cosmic rays.


Figures from paper (click to get full size image):




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