Radio Spectral Index Variations and Physical Conditions in Kepler’s Supernova Remnant

Abstract

A new epoch of VLA measurements of Kepler's supernova remnant was obtained to make accurate measurements of the radio spectral index variations and polarization. We have compared these new radio images with Hα, infrared, and X-ray data to better understand the three dimensional structure and dynamics of Kepler, and to better understand the physical relationships between the various nonthermal and thermal plasmas. Spatial variations in the radio spectral index from −0.85 to −0.6 are observed between 6 cm and 20 cm. The mean spectral index is −0.71. The mean percent polarization is 3.5% at 20 cm and 6% at 6 cm. There is a strong correspondence between the radial and azimuthal profiles of the radio, X-ray, Hα, and infrared emission in different locations around the remnant although there is no single, global pattern. Spectral tomography shows that the flat-and steep-spectrum radio emissions have distinct structures. The flat-spectrum radio emission is found either at a larger radius than or coincident with the steep-spectrum emission. We interpret these spectral components as tracing forward-and reverse-shocked material, respectively. The flat-spectrum radio emission can alternatively be interpreted as the bow-shocked material (reshocked by the forward shock) from the progenitor's motion through the interstellar medium. The Hα and IR images are very similar. Their leading edges are coincident and are either in front of or coincident with the leading edges of the X-ray and radio emission. The X-ray emission matches the Hα and IR emission in places, and in other places traces the steep-spectrum radio emission. In the north there is also an anticorrelation in the azimuthal profiles around the -2remnant of the flat-spectrum radio emission and the thermal X-ray, Hα, and IR emissions. We suggest that this could be due to a relative weakening of the particle acceleration at the forward shock due to Alfvén wave damping in regions of high density.

Electron injection by whistler waves in non-relativistic shocks

“…Also, the degree of linear polarization should be smaller in SNR rims with strong synchrotron emission, as we expect the field direction near the shock to be randomized by the amplified turbulence (Stroman & Pohl 2009). On larger scales, the radial magnetic fields inferred from radio polarization measurements of synchrotron-emitting regions of SNR shocks are also consistent with our model (Dickel et al 1991, DeLaney et al 2002.…”

A Multifrequency Radio Study of Supernova Remnant G292.0+1.8 and Its Pulsar Wind Nebula

“…The calculation of spectral indices was carried out through spatial tomography (for discussion and implementation of this approach, see KatzStone & Rudnick 1997;Crawford et al 2001;DeLaney et al 2002). Spatial tomography between two images, I 1 and I 2 , involves scaling I 2 by a trial spectral index t (where I / ), and then subtracting this scaled image from I 1 .…”

Acceleration of cosmic rays by young core-collapse supernova remnants

“…The largest and strongest secondary shock in the system is the reverse shock (RS) that propagates through the ejecta. High-resolution radio and X-ray observations support the notion that particle acceleration can also occur at RS (Gotthelf et al 2001;Rho et al 2002;DeLaney et al 2002;Sasaki et al 2006;Helder & Vink 2008), but only recently has this possibility been considered in theoretical calculations (Zirakashvili & Aharonian 2010;Zirakashvili & Ptuskin 2012;Telezhinsky et al 2012a,b), which relax the restricting assumptions about the magnetic field (MF) in the RS region (e.g. Ellison et al 2005).…”