What is the earliest evidence for the existence of superfluidity in neutron stars?
I'm about to present the subject in a seminar and I'm under the impression that at the moment there is no conclusive evidence the superfluidity actually exists in neutron stars (although it is widely believed to be so?).
I've found this review by Chamel (2017), but it doesn't seem to be very conclusive.
Apart from the fast cooling of Cas A, that is still not a conclusive evidence, the most convincing evidence (but also the earliest!) comes from observations of pulsar glitches. In particular, the observed quasi-exponential post-glitch relaxation indicates the coupling with an internal superfluid, the large glitch amplitudes and large glitch activity of some pulsars indicate the presence of a large angular momentum reservoir in the form of internal superfluid currents. All this is discussed in the review of Antonelli et al. Insights into the physics of neutron star interiors from pulsar glitches, where you can easily find the original works of the late 60s, but also the recent interpretation of the pulsar glitch phenomenon.
Link to the review: https://arxiv.org/abs/2301.12769v3
