A white dwarf-neutron star relativistic binary model for soft gamma-ray repeaters

摘要

A scenario for soft gamma-ray repeaters (SGRs) is introduced in which gravitational radiation reaction (GRR) effects drive the dynamics of an ultrashort orbital period X-ray binary embracing a high-mass,donor white dwarf (WD) to a rapidly rotating low magnetized massive neutron star (NS) surrounded by a thick, dense and massive accretion torus. Driven by GRR, over timescales of Delta T-rep similar to 10 years, the binary separation reduces, the WD overflows its Roche lobe and the mass transfer drives unstable the accretion disk around the NS. As the binary circular orbital period is a multiple integer number (m) of the,period of the WD fundamental mode,37 the WD is since long pulsating at its fundamental mode; and most of its harmonies, due to the tidal interaction with its NS orbital companion. Hence, when the powerful irradia- tion glows onto the WD; from the fireball ejected as part of the disk matter slumps onto the NS, it is partially absorbed. This huge energy excites other WD radial (p-mode) pulsations. 34,35 After each mass-transfer episode the binary separation (and orbital period) is augmented significantly(1,5) due to the binary's angular momentum redistribution. Thus a new adiabatic inspiral phase driven by GRR reaction starts which brings the binary close again, and the process repeats after a time span Delta T-rep. This model allows to explain most of SGRs observational features: their recurrent activity, energetics of giant superoutbursts and quiescent stages, and particularly the intriguing subpulses discovered by BeppoSAX,(10) which are suggested here to be overtones of the WD radial fundamental mode (see the accompanying paper).