Verification of Cas A Neutron Star Cooling Rate Using Chandra HRC-S Observations

Abstract The young neutron star (NS) in the Cassiopeia A (Cas A) supernova remnant is a fascinating test for theories of NS cooling. Chandra observations have indicated that its surface temperature is declining rapidly, by about 2% per decade, using 20 yr of data, if a uniform carbon atmosphere is assumed for the NS. This rapid decline may be caused by the neutrons in the NS core transitioning from a normal to a superfluid state. However, most of the Cas A NS observations were performed by the Chandra ACIS detectors, which suffer complicated systematic effects. Here, we test the cooling of the Cas A NS with Chandra High Resolution Camera (HRC) data over 25 yr. The Chandra HRC detector has independent systematics, serving as a cross-check. Assuming a fixed hydrogen column density ( N H ), we infer the cooling rate of the Cas A NS to be 0.57 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.27</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.26</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> % per decade. Allowing the N H to vary with time (as estimated using ACIS data), the cooling rate is 1.11 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.28</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.25</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> % per decade. These cooling rates are smaller than measured using ACIS data, implying systematic uncertainties have not been eradicated from either or both data sets. However, we have verified the decline in the absorbed flux from the Cas A NS using an independent instrument, at &gt;3 σ level (4.7% ± 1.5% over 10 yr). Additionally, the weaker cooling rate of Cas A NS inferred from HRC data sets eliminates the tension with the theoretically predicted cooling, and can be explained by the reduced efficiency of the neutrino emission accompanying the Cooper pair breaking and formation process in neutron triplet-state superfluid.