PRL 106, 081101 (2011)
Selected for a Viewpoint in Physics PHYSICAL REVIEW LETTERS
week ending 25 FEBRUARY 2011
Rapid Cooling of the Neutron Star in Cassiopeia A Triggered by Neutron Superﬂuidity in Dense Matter Dany Page,1 Madappa Prakash,2 James M. Lattimer,3 and Andrew W. Steiner4 ´ ´ ´ Instituto de Astronomıa, Universidad Nacional Autonoma de Mexico, Mexico D.F. 04510, Mexico 2 Department of Physics and Astronomy, Ohio University, Athens, Ohio 45701-2979, USA 3 Department of Physics and Astronomy, State University of New York at Stony Brook, Stony Brook, New York 11794-3800, USA 4 Joint Institute for Nuclear Astrophysics, National Superconducting Cyclotron Laboratory and, Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824, USA (Received 29 November 2010; published 22 February 2011) We propose that the observed cooling of the neutron star in Cassiopeia A is due to enhanced neutrino emission from the recent onset of the breaking and formation of neutron Cooper pairs in the 3 P2 channel. We ﬁnd that the critical temperature for this superﬂuid transition is ’ 0:5 Â 109 K. The observed rapidity of the cooling implies that protons were already in a superconducting state with a larger critical temperature. This is the ﬁrst direct evidence that superﬂuidity and superconductivity occur at supranuclear densities within neutron stars. Our prediction that this cooling will continue for several decades at the present rate can be tested by continuous monitoring of this neutron star. DOI: 10.1103/PhysRevLett.106.081101 PACS numbers: 97.60.Jd, 95.30.Cq, 26.60.Àc 1
The neutron star in Cassiopeia A (Cas A), discovered in 1999 in the Chandra ﬁrst light observation  targeting the supernova remnant, is the youngest known in the Milky Way. An association with the historical supernova SN 1680  gives Cas A an age of 330 yr, in agreement with its kinematic age . The distance to the remnant is estimated to be 3:4þ0:3 kpc . The thermal soft x-ray spectrum of À0:1 Cas A is well ﬁt by a nonmagnetized carbon atmosphere model, with a surface temperature of 2 Â 106 K and an emitting radius of 8–17 km . These results raise Cas A to the rank of the very few isolated neutron stars with a well determined age and a reliable surface temperature, thus allowing for detailed modeling of its thermal evolution and the determination of its interior properties . Analyzing archival data from 2000–2009, Heinke and Ho  recently reported that Cas A’s surface temperature has rapidly decreased from 2:12 Â 106 to 2:04 Â 106 K [8,9]. This rate of cooling is signiﬁcantly larger than expected from the modiﬁed Urca (‘‘MU’’) process [10,11] or a medium modiﬁed Urca . It is also unlikely to be due to any of the fast neutrino () emission processes (such as direct Urca processes from nucleons or hyperons, or emission from Bose condensates or gapless quark matter) since the visible effects of those become apparent over the thermal relaxation time scale of the crust ; i.e., 30–100 yr, much earlier than the age of Cas A, and exhibit a slow evolution at later times. We interpret Cas A’s cooling within the ‘‘minimal cooling’’ paradigm  and suggest it is due to the recent triggering of enhanced neutrino emission resulting from the neutron 3 P2 pairing in the star’s core. Our numerical calculations and analytical analysis imply a critical temperature TC ’ 0:5 Â 109 K for the triplet neutron superﬂuidity. 0031-9007=11=106(8)=081101(4)
The essence of the minimal cooling paradigm is the a priori exclusion of all fast -emission mechanisms, thus restricting emission to the ‘‘standard’’ MU process and nucleon bremsstrahlung processes . However, effects of pairing, i.e., neutron superﬂuidity and/or proton superconductivity, are included. At temperatures just below the critical temperature Tc of a pairing phase transition, the continuous breaking and formation of Cooper pairs , referred to...
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