Quiescent thermal emission from neutron stars in low-mass X-ray binaries

¹ Laboratorio TandarCNEA-CONICET, Av. Gral Paz 1499, 1430 San Martín, Buenos Aires, Argentina
² Deutsches Zentrum für Luft-und Raumfahrt, DLR-RY, Robert-Hooke Str. 7, 28359 Bremen, Germany
e-mail: deborah.aguilera@dlr.de
³ Departament de Física Aplicada, Universitat d’Alacant, Ap. Correus 99, 03080 Alacant, Spain

Received: 16 September 2013
Accepted: 20 December 2014

Abstract

Context. We monitored the quiescent thermal emission from neutron stars in low-mass X-ray binaries after active periods of intense activity in X-rays (outbursts).

Aims. The theoretical modeling of the thermal relaxation of the neutron star crust may be used to establish constraints on the crust composition and transport properties, depending on the astrophysical scenarios assumed.

Methods. We numerically simulated the thermal evolution of the neutron star crust and compared them with inferred surface temperatures for five sources: MXB 1659−29, KS 1731−260, XTE J1701−462, EXO 0748−676  and IGR J17480−2446.

Results. We find that the evolution of MXB 1659−29, KS 1731−260 and EXO 0748−676 can be well described within a deep crustal cooling scenario. Conversely, we find that the other two sources can only be explained with models beyond crustal cooling. For the peculiar emission of XTE J1701−462 we propose alternative scenarios such as residual accretion during quiescence, additional heat sources in the outer crust, and/or thermal isolation of the inner crust due to a buried magnetic field. We also explain the very recent reported temperature of IGR J17480−2446 with an additional heat deposition in the outer crust from shallow sources.