Origin of gamma-ray emission in the shell of Cassiopeia A

¹ Saha Institute of Nuclear Physics, 1/AF, Bidhannagar, 700064 Kolkata, India
e-mail: lab.saha@saha.ac.in
² TÜBİTAK Space Technologies Research Institute, ODTÜ Campus, 06531 Ankara, Turkey
e-mail: tulun.ergin@tubitak.gov.tr
³ Boğaziçi University, Physics Department, Bebek, 34342 Istanbul, Turkey

Received: 9 December 2013
Accepted: 21 January 2014

Abstract

Context. Non-thermal X-ray emission from the shell of Cassiopeia A (Cas A) has been an interesting subject of study, as it provides information about relativistic electrons and their acceleration mechanisms in shocks. The Chandra X-ray observatory revealed the detailed spectral and spatial structure of this supernova remnant in X-rays. The spectral analysis of the Chandra X-ray data of Cas A shows unequal flux levels for different regions of the shell, which can be attributed to different magnetic fields in those regions. Additionally, the GeV gamma-ray emission observed by the Large Area Telescope on board the Fermi Gamma-Ray Space Telescope showed that the hadronic processes are dominating in Cas A, which is a clear signature of acceleration of protons.

Aims. The aim of this study is to locate the origin of gamma-rays based on the X-ray data of the shell of Cas A. We also aim to explain the GeV−TeV gamma-ray data in the context of both leptonic and hadronic scenarios.

Methods. We modelled the multi-wavelength spectrum of Cas A. We use a synchrotron emission process to explain the observed non-thermal X-ray fluxes from different regions of the shell. This results in estimates of the model parameters which are then used to explain TeV gamma-ray emission spectrum. We also use a hadronic scenario to explain both GeV and TeV fluxes simultaneously.

Results. Based on this analysis, it has been shown that the southern part of the remnant is bright in TeV gamma-rays. We also show that the leptonic model alone cannot explain the GeV−TeV data. Therefore, we need to invoke a hadronic model to explain the observed GeV−TeV fluxes. We found that the lepto-hadronic model provides the best fit to the data although the pure hadronic model is able to explain the GeV−TeV data.