The soft gamma repeaters as very strongly magnetized neutron stars - I. Radiative mechanism for outbursts

Abstract

A radiative model for the soft gamma repeaters and the energetic 1979 March 5 burst is presented. We identify the sources of these bursts with neutron stars the external magnetic fields of which are much stronger than those of ordinary pulsars. Several independent arguments point to a neutron star with Bdipole ~ 5 × 1014 G as the source of the March 5 event. A very strong field can (i) spin down the star to an 8-s period in the ~ 104-yr age of the surrounding supernova remnant N49; (ii) provide enough energy for the March 5 event; (iii) undergo a large-scale interchange instability the growth time of which is comparable to the ~ 0.2-s width of the initial hard transient phase of the March 5 event; (iv) confine the energy that was radiated in the soft tail of that burst; (v) reduce the Compton scattering cross-section sufficiently to generate a radiative flux that is ~ 104 times the (non-magnetic) Eddington flux; (vi) decay significantly in ~ 104–105 yr, as is required to explain the activity of soft gamma repeater sources on this time-scale; and (vii) power the quiescent X-ray emission LX ~ 7 × 1035 erg s–1 observed by Einstein and ROSAT as it diffuses the stellar interior.