title: Strong neutrino cooling by cycles of electron capture and β- decay in neutron star crusts.
abstract: The temperature in the crust of an accreting neutron star, which comprises its outermost kilometre, is set by heating from nuclear reactions at large densities, neutrino cooling and heat transport from the interior. The heated crust has been thought to affect observable phenomena at shallower depths, such as thermonuclear bursts in the accreted envelope. Here we report that cycles of electron capture and its inverse, β(-) decay, involving neutron-rich nuclei at a typical depth of about 150 metres, cool the outer neutron star crust by emitting neutrinos while also thermally decoupling the surface layers from the deeper crust. This 'Urca' mechanism has been studied in the context of white dwarfs and type Ia supernovae, but hitherto was not considered in neutron stars, because previous models computed the crust reactions using a zero-temperature approximation and assumed that only a single nuclear species was present at any given depth. The thermal decoupling means that X-ray bursts and other surface phenomena are largely independent of the strength of deep crustal heating. The unexpectedly short recurrence times, of the order of years, observed for very energetic thermonuclear superbursts are therefore not an indicator of a hot crust, but may point instead to an unknown local heating mechanism near the neutron star surface.
authors.affiliation: 1] National Superconducting Cyclotron Laboratory, Michigan State University, 640 South Shaw Lane, East Lansing, Michigan 48824, USA [2] Joint Institute for Nuclear Astrophysics, 225 Nieuwland Science Hall, University of Notre Dame, Notre Dame, Indiana 46556, USA [3] Department of Physics and Astronomy, Michigan State University, 567 Wilson Road, East Lansing, Michigan 48824, USA.
authors.forename: H
authors.initials: H
authors.lastname: Schatz
authors.affiliation: Indian Institute of Technology Ropar, Nangal Road, Rupnagar (Ropar), Punjab 140 001, India.
authors.forename: S
authors.initials: S
authors.lastname: Gupta
authors.affiliation: 1] Joint Institute for Nuclear Astrophysics, 225 Nieuwland Science Hall, University of Notre Dame, Notre Dame, Indiana 46556, USA [2] Theoretical Division, MS B214, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA.
authors.forename: P
authors.initials: P
authors.lastname: Möller
authors.affiliation: 1] Joint Institute for Nuclear Astrophysics, 225 Nieuwland Science Hall, University of Notre Dame, Notre Dame, Indiana 46556, USA [2] Department of Physics, 225 Nieuwland Science Hall, University of Notre Dame, Notre Dame, Indiana 46556, USA.
authors.forename: M
authors.initials: M
authors.lastname: Beard
authors.affiliation: 1] National Superconducting Cyclotron Laboratory, Michigan State University, 640 South Shaw Lane, East Lansing, Michigan 48824, USA [2] Joint Institute for Nuclear Astrophysics, 225 Nieuwland Science Hall, University of Notre Dame, Notre Dame, Indiana 46556, USA [3] Department of Physics and Astronomy, Michigan State University, 567 Wilson Road, East Lansing, Michigan 48824, USA.
authors.forename: E F
authors.initials: EF
authors.lastname: Brown
authors.affiliation: 1] Joint Institute for Nuclear Astrophysics, 225 Nieuwland Science Hall, University of Notre Dame, Notre Dame, Indiana 46556, USA [2] Department of Physics and Astronomy, Michigan State University, 567 Wilson Road, East Lansing, Michigan 48824, USA.
authors.forename: A T
authors.initials: AT
authors.lastname: Deibel
authors.affiliation: Departamento de Física Nuclear, Instituto de Física da Universidade de São Paulo, Caixa Postal 66318, 05315-970 São Paulo, Brazil.
authors.forename: L R
authors.initials: LR
authors.lastname: Gasques
authors.affiliation: 1] Physics Division, Oak Ridge National Laboratory, PO Box 2008, Oak Ridge, Tennessee 37831-6354, USA [2] Department of Physics and Astronomy, University of Tennessee, 401 Nielsen Physics Building, 1408 Circle Drive, Knoxville, Tennessee 37996-1200, USA.
authors.forename: W R
authors.initials: WR
authors.lastname: Hix
authors.affiliation: 1] National Superconducting Cyclotron Laboratory, Michigan State University, 640 South Shaw Lane, East Lansing, Michigan 48824, USA [2] Joint Institute for Nuclear Astrophysics, 225 Nieuwland Science Hall, University of Notre Dame, Notre Dame, Indiana 46556, USA [3] Department of Physics and Astronomy, Michigan State University, 567 Wilson Road, East Lansing, Michigan 48824, USA.
authors.forename: L
authors.initials: L
authors.lastname: Keek
authors.affiliation: 1] National Superconducting Cyclotron Laboratory, Michigan State University, 640 South Shaw Lane, East Lansing, Michigan 48824, USA [2] Joint Institute for Nuclear Astrophysics, 225 Nieuwland Science Hall, University of Notre Dame, Notre Dame, Indiana 46556, USA [3] Department of Physics and Astronomy, Michigan State University, 567 Wilson Road, East Lansing, Michigan 48824, USA.
authors.forename: R
authors.initials: R
authors.lastname: Lau
authors.affiliation: 1] Joint Institute for Nuclear Astrophysics, 225 Nieuwland Science Hall, University of Notre Dame, Notre Dame, Indiana 46556, USA [2] Institute for Nuclear Theory, University of Washington, Physics and Astronomy Building, Box 351550, Seattle, Washington 98195-1550, USA.
authors.forename: A W
authors.initials: AW
authors.lastname: Steiner
authors.affiliation: 1] Joint Institute for Nuclear Astrophysics, 225 Nieuwland Science Hall, University of Notre Dame, Notre Dame, Indiana 46556, USA [2] Department of Physics, 225 Nieuwland Science Hall, University of Notre Dame, Notre Dame, Indiana 46556, USA.
authors.forename: M
authors.initials: M
authors.lastname: Wiescher