On the Magnetic Fields and Particle Acceleration in Cassiopeia A

Abstract

We investigate the non-thermal X-ray emission from Cas A. For the hard X-ray continuum we test the model proposed by Laming, which invokes bremsstrahlung from electrons accelerated by lower hybrid plasma waves. The justification for this model comes from our determination of a lower limit to the average magnetic field of B > 0.5 mG, which suggest that most of the electron populations are too much affected by synchrotron losses to produce X-ray synchrotron emission. An exception is the X-ray emission from the rim surrounding Cas A, seen in Chandra continuum images. The width of this rim of 1.5" to 4", can be used to infer the magnetic field near the shock front, for which we estimate B = 0.08 - 0.16 mG, and electron energies of about 57 - 40 TeV. This is lower than the average magnetic field, but higher than what may be expected from shocked interstellar medium, suggesting a high pre-shock magnetic field, or rapid, post shock, field amplification. Combining the two magnetic field measurements we have constructed a simple two zone model. Most of the radio emission comes from inside Cas A, where the magnetic field is strong, whereas the inverse Compton emission comes from near the shock front. Only for our lower limit on the magnetic field strength near the shock front is it possible to explain the recent detection of TeV emission by HEGRA with inverse Compton, for which, in addition, we have to assume a rather high far infrared photon density that should be twice as high as our best estimate of roughly 70 cm^{-3}. Pion decay is therefore likely to be the dominant emission from Cas A at TeV energies.