On the Magnetic Fields and Particle Acceleration in Cassiopeia A

Abstract

We investigate the non-thermal X-ray emission from Cas A, using BeppoSAX, CGRO-OSSE, and Chandra data. For the hard X-ray continuum we test the model proposed by Laming, which invokes nonthermal 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. For such high magnetic fields the synchrotron losses are severe enough that most of the electron populations responsible for the radio emission have maximum electron energies well below the limit for which X-ray synchrotron emission is important. However, we do suggest that the rim surrounding Cas A, seen in Chandra continuum images, is X-ray synchrotron emission. 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 ∼ 57 − 40 TeV. This magnetic field strength is lower than the average magnetic field, but higher than what may be expected from shocked interstellar medium, suggesting either a high magnetic field in the wind of the progenitor, or rapid, post shock, field amplification by non-linear growth of plasma waves. 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. In contrast, the inverse Compton emission is dominated by emission 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 ∼ 70 cm −3 . Pion decay is therefore likely to be the dominant emission from Cas A at TeV energies.

Prospects for Cherenkov Telescope Array Observations of the Young Supernova Remnant RX J1713.7−3946

“…This can lead to strong modifications of the shock structure and a nonlinear coupling between the shock flow and CR acceleration. Some observational results are consistent with the predictions of such nonlinear DSA (NLDSA) models (Bamba et al 2003(Bamba et al , 2005a2005b, Vink & Laming 2003Warren et al 2005;Uchiyama et al 2007;Helder et al 2009). However, whether these models can fully explain all aspects revealed by the accumulating multiwavelength observations still remains in doubt.…”

A Review of X-ray Observations of Supernova Remnants

“…The widths can be converted to a combination of magnetic field (B) and electron energy (E), assuming that the width is determined by synchrotron losses (curved lines, right). Together with the observed photon energy (straight lines) this gives approximate E and B estimates (after [32]). are primarily nuclear cosmic rays, unfortunately observations of SNRs lead primarily to the detection of electron cosmic rays due to synchrotron or inverse Compton radiation.…”

“…Finally, however, a 500 ks BeppoSAX observation of Cas A resulted in a better than 3σ detection of those lines (Fig. 3) [19,32] with a flux of (3.3 ± 0.3) × 10 −5 ph cm −2 s −1 , with some uncertainty due to the uncertain shape of the hard Xray continuum. Assuming an age for Cas A of 320 yr [38].…”

“…The recent discovery of X-ray synchrotron emission from a number of SNRs, SN 1006 4 [1,53,43], Cas A [32], RCW 86 [54], RXJ 1713.7-3946 [55,56], and G266.2-1.2 ("Vela jr") [57], have extended the observed electron energies from the GeV range (from radio observations) to 10-100 TeV energies. Although this is additional proof that electrons are accelerated to high energies, the observed energy range falls short of the "knee" energy [4].…”

“…However, the spatial resolution of Chandra has lead to resolving the narrow X-ray synchrotron structures at the shock fronts of Cas A and SN 1006 [32,59], from which one can infer the interior magnetic fields close to the shock front, if one has an estimate of the shock velocity [60,61], as illustrated in Fig. 6.…”