A Decline in the Nonthermal X-Ray Emission From Cassiopeia A

Abstract

We present new Chandra ACIS-S3 observations of Cassiopeia A which, when combined with earlier ACIS-S3 observations, show evidence for a steady ∼ 1.5-2% yr −1 decline in the 4.2-6.0 keV X-ray emission between the years 2000 and 2010. The computed flux from exposure corrected images over the entire remnant showed a 17% decline over the entire remnant and a slightly larger (21%) decline from regions along the remnant's western limb. Spectral fits of the 4.2-6.0 keV emission across the entire remnant, forward shock filaments, and interior filaments indicate the remnant's nonthermal spectral powerlaw index has steepened by about 10%, with interior filaments having steeper powerlaw indices. Since TeV electrons, which give rise to the observed X-ray synchrotron emission, are associated with the exponential cutoff portion of the electron distribution function, we have related our results to a change in the cutoff energy and conclude that the observed decline and steepening of the nonthermal X-ray emission is consistent with a deceleration of the remnant's ≃5000 km s −1 forward shock of ≈ 10-40 km s −1 yr −1 .

Multi-year X-Ray Variations of Iron-K and Continuum Emissions in the Young Supernova Remnant Cassiopeia A

“…Flux evolution of the Cassiopeia A was well reproduced by linear decline (Patnaude et al 2011). Therefore, we also fitted time variation of the fluxes with a linear model.…”

“…Helder & Vink (2008) estimated that the fraction of non-thermal component is ∼ 54 % in the 4.2-6 keV band. In the further lower energy band (1-3 keV), no significant X-ray variability in the soft X-ray band (1-3 keV) has been found (e.g., Patnaude et al 2011;Rutherford et al 2013). However, we note the possibility that the 4.2-6 keV and the 1-3 keV band emission may originate from different plasma.…”

“…The spectral and image evolutions are quicker when the age of the remnants are younger since the shock velocity is faster and its braking is larger. Recently, there are several arguments about the X-ray spectral variations from young SNRs (e.g., Uchiyama et al 2007;Uchiyama & Aharonian 2008;Patnaude et al 2011). Mainly, the variable component is considered to be synchrotron X-rays (non-thermal X-rays) caused by high-energy electrons in the amplified magnetic field (∼mG).…”

Observational Signatures of Particle Acceleration in Supernova Remnants

“…28 can be used to couple the flux decline rate with a steepening of the synchrotron spectrum (Patnaude et al, 2011):…”

“…These discrepancies could either point to the invalidity of the simple model for flux decline (see also the discussion in Patnaude et al, 2011), or the invalidity of Eq 27 to describe the overall X-ray synchrotron spectrum of Cas A. For the youngest known supernova remnant G1.9+0.3 the X-ray synchrotron flux is measured to be increasing with a rate of 1.7 ± 1.0 % yr −1 (Carlton et al, 2011).…”