Is the Cassiopeia A neutron star cooling unusually fast?
In plain words
The young neutron star in the Cassiopeia A remnant appeared to cool by a few percent in ten years, much faster than expected. This was explained as neutrons becoming superfluid, but the measurement depends on instrument calibration.
Precise statement
Chandra spectra of the Cas A neutron star (age about $340\ \mathrm{yr}$) have been fit with a surface temperature decline of approximately 1 to 4 percent per decade, the value and significance depending on detector mode, calibration and interstellar absorption modeling. Determine $dT_{s}/dt$ with systematics, and decide whether it requires neutrino emission from Cooper-pair formation at the onset of neutron triplet superfluidity (critical temperature of order $5e8\text{ to }1e9\ \mathrm{K}$) or permits slower standard cooling.
What would settle it
A cross-calibrated multi-instrument temperature history over about 25 yr with the absorption column fit jointly.
Status in the literature
Unverified note
An analysis of all Chandra ACIS-S data found a continued decline constraining superfluid gaps (Shternin et al., MNRAS, 2022), while 2026 papers propose alternative cooling routes such as Urca processes (Potekhin and Yakovlev, JHEAp, 2026).