A Decline in the Nonthermal X-ray Emission from Cassiopeia A

TL;DR

The study reports a steady decline and spectral steepening in the nonthermal X-ray emission from Cassiopeia A over a decade, indicating a deceleration of the remnant's forward shock and changes in electron acceleration.

Contribution

This work provides the first detailed measurement of the decline rate and spectral evolution of Cassiopeia A's nonthermal X-ray emission over a decade.

Findings

X-ray flux declined by 17% overall and 21% in the western limb.

Spectral index steepened by about 10%.

Forward shock deceleration estimated at 10-40 km/s/yr.

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 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 forward shock of ~10--40 km/s/yr