Gapless Neutron Superfluidity Can Explain the Late Time Cooling of Transiently Accreting Neutron Stars

The current interpretation of the observed late time cooling of transiently accreting neutron stars in low-mass x-ray binaries during quiescence requires the suppression of neutron superfluidity in their crust at variance with recent ab initio many-body calculations of dense matter. Focusing on the two emblematic sources KS 1731-260 and MXB 1659-29, we show that their thermal evolution can be naturally explained by considering the existence of a neutron superflow driven by the pinning of quantized vortices. Under such circumstances, we find that the neutron superfluid can be in a gapless state in which the specific heat is dramatically increased compared to that in the classical BCS state assumed so far, thus delaying the thermal relaxation of the crust. We perform neutron-star cooling simulations taking into account gapless superfluidity, and we obtain excellent fits to the data, thus reconciling astrophysical observations with microscopic theories. The imprint of gapless superfluidity on other observable phenomena is briefly discussed.

Figure 1

Evolution of the effective surface temperature in electronvolts of KS 1731-260 (as seen by an observer at infinity) as a function of the time in days after a 12.5 year outburst. Symbols represent observational data with error bars. The dotted and solid lines are models considering BCS and gapless superfluidity, respectively, using the realistic pairing calculations of Ref. [39]. The dashed line was obtained assuming BCS superfluidity with the fine-tuned “Deep” gap of Ref. [29].

Figure 2

Same as Fig. 1 for MXB 1659-29 after the first outburst. The shaded area corresponds to the second accretion phase (which occurred in 2015) and its subsequent cooling phase: The cooling curves within this region depict the expected behavior had this outburst not occurred.