Limit on the Axion Decay Constant from the Cooling Neutron Star in Cassiopeia A

Understand

The observed rapid cooling of the neutron star (NS) located at the center of the supernova remnant Cassiopeia A (Cas A) can be explained in the minimal NS cooling scenario.

• This consequence may be changed if there exists an extra cooling source, such as axion emission.
• In this work, we study the Cas A NS cooling in the presence of axion emission, taking account of the temperature evolution in the whole life of the Cas A NS.
• We obtain a lower limit on the axion decay constant, $f_a\gtrsim (5-7)\times 10^8$ GeV, if the star has an envelope with a thin carbon layer.