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

Federico García, Deborah N. Aguilera, and Gustavo E. Romero.

Serie: Trabajos publicados del IAR ; no. 1262

Resumen: 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 [aprox.] 10⁸ G). However, the most common manifestation of neutron stars are pulsars, whose magnetic field distribution peaks at B [aprox.] 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 M ≳ 10^-10 M_⊙ yr^-1, surface magnetic fields can decay up to four orders of magnitude in [aprox.]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