Exploring jet-launching conditions for supergiant fast X-ray transients

¹ Instituto Argentino de Radioastronomía (CCT La Plata, CONICET), C.C.5, (1894) Villa Elisa, Buenos Aires, Argentina
e-mail: fgarcia@iar-conicet.gov.ar
² Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque, B1900 FWA La Plata, Argentina
³ Laboratorio Tandar, CNEA, Av. Gral. Paz 1499, 1430 San Martín, Buenos Aires, Argentina & CONICET, Argentina
⁴ German Aerospace Center, Institute for Space Systems, Robert Hooke Str. 7, 28359 Bremen, Germany

Received: 29 November 2013
Accepted: 13 April 2014

Abstract

Context. In the magneto-centrifugal mechanism for jet formation, accreting neutron stars are assumed to produce relativistic jets only if their surface magnetic field is weak enough (B ~ 10⁸ G). However, the most common manifestation of neutron stars are pulsars, whose magnetic field distribution peaks at B ~ 10¹² G. If the neutron star magnetic field has at least this strength at birth, it must decay considerably before jets can be launched in binary systems.

Aims. We study the magnetic field evolution of a neutron star that accretes matter from the wind of a high-mass stellar companion so that we can constrain the accretion rate and the impurities in the crust, which are necessary conditions for jet formation.

Methods. We solved the induction equation for the diffusion and convection of the neutron star magnetic field confined to the crust, assuming spherical accretion in a simpliflied one-dimensional treatment. We incorporated state-of-the-art microphysics, including consistent thermal evolution profiles, and assumed two different neutron star cooling scenarios based on the superfluidity conditions at the core.

Results. We find that in this scenario, magnetic field decay at long timescales is governed mainly by the accretion rate, while the impurity content and thermal evolution of the neutron star play a secondary role. For accretion rates Ṁ ≳ 10^-10 M_⊙ yr^-1, surface magnetic fields can decay up to four orders of magnitude in ~10⁷ yr, which is the timescale imposed by the evolution of the high-mass stellar companion in these systems. Based on these results, we discuss the possibility of transient jet-launching in strong wind-accreting high-mass binary systems like supergiant fast X-ray transients.