Impact of axions on the Cassiopea A neutron star cooling

The observed anomalous steady decrease in surface temperature of the supernova remnant Cassiopeia A (Cas A), which was reported about ten years ago, has generated much debate. Several exotic cooling scenarios have been proposed using non-standard assumptions about the physics and evolution of this neutron star (NS). At present, significant corrections have been made to the observational data, which make it possible to numerically simulate the Cas A NS cooling process in the framework of the scenario of minimal neutrino cooling. If there is an additional source of cooling, such as axion emission, the steepness of the Cas A NS surface temperature drop will increase with the growth of the axion-nucleon interaction strength. This makes it possible to limit the minimum value of the axion decay constant $f_a$ using the condition that the NS surface temperature should be within the 99% confidence interval obtained from the observational data. Two types of axion models are considered: the Kim-Shifman-Weinstein-Zakharov -- KSVZ model and the Dean-Fischler-Srednitsky-Zhitnitsky --DFSZ model. The above criterion gives a lower limit on the axion decay constant, $f_a>3\times 10^7$ GeV and $f_a>4.5\times 10^8$ GeV for KSVZ and DFSZ axions, respectively.