Among the exotic objects that populate our Universe, neutron stars stand out for their extreme conditions of density, temperature, magnetic field and matter composition: indeed, these compact objects (whose density greatly surpass that of atomic nuclei) provide a unique ground to test our understanding of the densest phases of hadronic matter. The astrophysical manifestations of neutron stars provide us with a plethora of fascinating phenomena that can be observed over the entire electromagnetic spectrum as well as via the emission of gravitational waves and neutrinos.
A pulsar, a magnetized neutron star in fast rotation, emits a beam of focused radiation that is detected as a regular pulse. Although the regularity of the pulsed signal surpasses that of atomic clocks, in many cases sporadic increases of the rotational frequency are observed, known as glitches. The largest ones can be explained by the presence of a neutron superfluid in the interior of the star: this superfluid component can flow independently from the crust and can temporarily store angular momentum, which is then abruptly released to the observable crust, thus spinning it up and causing a glitch. Therefore, pulsar glitches are direct observational evidence for the presence of bulk nuclear superfluidity in ultra-dense matter.
In an article published in Nature Astronomy, a group of researchers lead by prof. Pierre Pizzochero (of the Theory group at the Physics Department of the University of Milan and associated to Group IV of INFN) and including the PhD student Marco Antonelli have presented the first consistent model for the reservoir of angular momentum. The model is based on the interaction of quantized vortex lines (naturally present in the bulk of a rotating superfluid) with the nuclear lattice in the crust and it shows a robust inverse relation between the pulsar mass and the largest allowed glitch. When coupled to observations of the maximum recorded glitch, it allows to constrain the mass of the glitching neutron star.
The relevance of the study is twofold: while it enables to give an estimate of the mass of isolated neutron stars, the model also unifies the description of ‘giant’ glitches correlating the different observed phenomenology to the stellar mass. Future observations of glitches in binary systems will allow to falsify or calibrate the model, which in turn will constrain various microscopic parameters of nuclear superfluidity, whose theoretical treatment is still an open issue.
P. M. Pizzochero, M. Antonelli, B. Haskell and S. Seveso
Constraints on pulsar masses from the maximum observed glitch
Nature Astronomy 2017 (DOI: 10.1038/s41550-017-0
