Model-independent constraints on superfluidity from the cooling neutron star in Cassiopeia A

Abstract

We present a new model-independent (applicable for a broad range of equations of state) analysis of the neutrino emissivity due to triplet neutron pairing in neutron star cores. We find that the integrated neutrino luminosity of the Cooper Pair Formation (CPF) process can be written as a product of two factors. The first factor depends on the neutron star mass, radius and maximal critical temperature of neutron pairing in the core, TCnmax, but not on the particular superfluidity model; it can be expressed by an analytical formula valid for many nucleon equations of state. The second factor depends on the shape of the critical temperature profile within the star, the ratio of the temperature T to TCnmax, but not on the maximal critical temperature itself. While this second factor depends on the superfluidity model, it obeys several model-independent constraints. This property allows one to analyse the thermal evolution of neutron stars with superfluid cores without relying on a specific model of their interiors. The constructed expressions allow us to perform a self-consistent analysis of spectral data and neutron star cooling theory. We apply these findings to the cooling neutron star in the Cassiopeia A supernova remnant using 14 sets of observations taken over 19 years. We constrain TCnmax to the range of (5 − 10) × 108 K. This value depends weakly on the equation of state and superfluidity model, and will not change much if cooling is slower than the current data suggest. We also constrain the overall efficiency of the CPF neutrino luminosity.

X-ray bounds on cooling, composition, and magnetic field of the Cassiopeia A neutron star and young central compact objects

“…Both fits allow for varying H (but tied across and hence the same value for all observations) and (can be different for each observation). The best-fit converges to = 1.69 Sun , = 13.0 km, and H = 1.67 +0.02 −0.06 × 10 22 cm −2 , which are nearly the same as values found in Wijngaarden et al (2019), and 1 = 0.33 ± 0.03 and 2 = 0.24 ± 0.02, which are consistent with values found in Heinke & Ho (2010); Ho et al (2015); Shternin et al (2021). Figure 5 shows the -confidence contours.…”

“…To account for a correlation between , , and pileup grade migration parameter (see Shternin et al 2021), we iterate initially between two model fits to the Chandra ACIS-S Graded spectra of the CCO in Cassiopeia A. The first fit allows for varying and (with the same values for all 14 observations) and fixed 1 (for observations with frame time of 3.24 s; see Table 2) and 2 (for observations with frame time of 3.04 s).…”

Dark gauge boson production from neutron stars via nucleon-nucleon bremsstrahlung

“…We assume a NS mass to be 1.4 solar mass, which is within the reasonable range for a possible presence of the compact remnant of SN1987A as a neutron star called NS1987A [23]. The other important NS model parameter for NS cooling history is the amount of accreted light elements (such as H or He) in the envelope [23,31,60], which controls the thermal conductivity and hence the relation between the internal temperature and the surface temperature. This amount is quantified by η = g 14 ∆M/M NS [61], where g 14 denotes the surface gravity in units of 10 14 cm s −2 and M NS and ∆M are the NS mass and the total mass of light elements, respectively.…”

Dynamical tides in superfluid neutron stars

“…More exotic phases of matter, like superfluid hyperons and/or colour superconducting quarks may come into play above the nuclear saturation density. In essence, astrophysical observations allow us to explore the hottest superfluids/superconductors in the Universe (Page et al 2011;Shternin et al 2011Shternin et al , 2021.…”