A Spatial and Spectral Study of Nonthermal Filaments in Historical Supernova Remnants: Observational Results withChandra

Abstract

The outer shells of young supernova remnants (SNRs) are the most plausible acceleration sites of high-energy electrons with the diffusive shock acceleration (DSA) mechanism. We studied spatial and spectral properties close to the shock fronts in four historical SNRs (Cas A, Kepler's remnant, Tycho's remnant, and RCW 86) with excellent spatial resolution of Chandra. In all of the SNRs, hard X-ray emissions were found on the rims of the SNRs, which concentrate in very narrow regions (so-called "filaments"); apparent scale widths on the upstream side are below or in the order of the point spread function of Chandra, while 0.5-40 arcsec (0.01-0.4 pc) on the downstream side with most reliable distances. The spectra of these filaments can be fitted with both thermal and nonthermal (power-law and SRCUT) models. The former requires unrealistic high temperature ( 2 keV) and low abundances ( 1 solar) for emission from young SNRs and may be thus unlikely. The latter reproduces the spectra with best-fit photon indices of 2.1-3.8, or roll-off frequencies of (0.1-28)×10 17 Hz, which reminds us of the synchrotron emission from electrons accelerated via DSA. We consider various physical parameters as functions of the SNR age, including the previous results

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.…”

“…In this case, the flux ratio between synchrotron X-rays and inverse Compton gamma rays suggests a magnetic field strength of B∼10-20μG. At first sight, this kind of value is inconsistent with the simple interpretation for the observed time variability (Uchiyama et al 2007) and thin filamentary structures (Bamba et al 2005b;Parizot et al 2006) of synchrotron X-rays through a fast energy loss of high-energy electrons, which typically requires B100 μG. Recent X-ray observations of the SNR RXJ0852.0−4622, which show very similar features to RXJ1713.7−3946, have however recovered a shallow steepening of the synchrotron index behind the shock as a function of radius, suggesting an average magnetic field near the shock of only a fewμG (Kishishita et al 2013).…”

Nonthermal particles and photons in starburst regions and superbubbles

“…A strong non-relativistic shock may transfer up to 10% of the ram pressure into magnetic fields as it is illustrated in the left panel of Figure 7. There are convincing observational evidences for strong non-adiabatic amplification of magnetic fields in the vicinities of forward shocks of a number of young SNRs including Cas A, Tycho, SN 1006, RXJ 1713.7-3946 and others obtained via arcsecond angular resolution imaging of synchrotron structures in these SNRs with Chandra (see, e.g., Vink 2012, Bamba et al 2005, Uchiyama et al 2007, Reynolds et al 2012, Helder et al 2012, Ressler et al 2014, Tananbaum et al 2014. The mechanisms of magnetic field amplification which are associated with CR-driven instabilities were thoroughly discussed and reviwed in Blandford and Eichler (1987), Bell and Lucek (2001), Malkov and Drury (2001), Zweibel (2003), Bell (2004), Lemoine and Pelletier (2010), Schure et al (2012), Blasi (2013), Bykov et al (2013a).…”

Observational Signatures of Particle Acceleration in Supernova Remnants

“…As Table 2 shows, all supernova remnants seem to have magnetic fields higher than this; Cas A and Tycho have even considerably higher magnetic fields. This has been taken as evidence that some form of magnetic field amplification is operating in the vicinity of shocks of young supernova remnants (Vink and Laming, 2003;Berezhko et al, 2003b;Bamba et al, 2005;Ballet, 2006). Their are several means by which this magnetic field amplification occurs (see elsewhere in this volume), but a mechanism that received a lot of attention is the so-called Bell's instability (Bell, 2004).…”