The cooling of the Central Compact Object in Cas A from 2006 to 2020

TL;DR

This study analyzes 14 years of Chandra data to measure the cooling rate of the Cas A CCO, finding a significant temperature decrease that may be affected by calibration uncertainties.

Contribution

First measurement of the Cas A CCO's cooling rate over 14 years using multiple observations and accounting for instrumental effects.

Findings

Temperature decreases by approximately 2900-4500 K per year.

Cooling rate of about 1.5-2.3% per decade.

Possible systematic effects influencing the observed cooling trend.

Abstract

We report on the study of six Chandra observations (four epochs) of the Central Compact Object (CCO) in the Cassiopeia A supernova remnant with the ACIS instrument in the subarray mode. This mode minimizes spectrum-distorting instrumental effects such as pileup. The data were taken over a time span ~ 14 years. If a non-magnetic carbon atmosphere is assumed for this youngest known CCO, then the temperature change is constrained to be $\dot{T}=-2900\pm 600$ K yr$^{-1}$ or $\dot{T}=-4500\pm 800 $ K yr$^{-1}$ ($1\sigma$ uncertainties) for constant or varying absorbing hydrogen column density. These values correspond to cooling rates of $-1.5 \pm 0.3$% per 10yr and $-2.3 \pm 0.4$% per 10yr, respectively. We discuss an apparent increase in the cooling rate in the last five years and the variations of the inferred absorbing hydrogen column densities between epochs. Considered together, these changes could indicate systematic effects such as caused by, e.g., an imperfect calibration of the increasing contamination of the ACIS filter.