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

摘要

A scenario for SGRs is introduced in which gravitational radiation reactioneffects drive the dynamics of an ultrashort orbital period X-ray binaryembracing a high-mass donor white dwarf (WD) to a rapidly rotating lowmagnetised massive neutron star (NS) surrounded by a thick, dense and massiveaccretion torus. Driven by GR reaction, sparsely, the binary separationreduces, the WD overflows its Roche lobe and the mass transfer drives unstablethe accretion disk around the NS. As the binary circular orbital period is amultiple integer number ($m$) of the period of the WD fundamental mode (Pons etal. 2002), the WD is since long pulsating at its fundamental mode; and most ofits harmonics, due to the tidal interaction with its NS orbital companion.Hence, when the powerful irradiation glows onto the WD; from the fireballejected as part of the disk matter slumps onto the NS, it is partiallyabsorbed. This huge energy excites other WD radial ($p$-mode) pulsations(Podsiadlowski 1991,1995). After each mass-transfer episode the binaryseparation (and orbital period) is augmented significantly (Deloye & Bildsten2003; Al\'ecyan & Morsink 2004) due to the binary's angular momentumredistribution. Thus a new adiabatic inspiral phase driven by GR reactionstarts which brings the binary close again, and the process repeats. This modelallows to explain most of SGRs observational features: their recurrentactivity, energetics of giant superoutbursts and quiescent stages, andparticularly the intriguing subpulses discovered by BeppoSAX (Feroci et al.1999), which are suggested here to be {\it overtones} of the WD radialfundamental mode (see the accompanying paper: Mosquera Cuesta 2004b).