MG14 - Talk detail

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The evolution of the remnant of the merger of two white dwarfs is still an open problem. In such cases in which the mass of the remnant is larger than the Chandrasekhar limiting mass it is expected that a powerful explosion will ensue, resulting in a Type Ia supernova. In particular, it is expected that for such systems angular momentum loss might bring the remnant to conditions for which a thermonuclear instability develops. Dipole magnetic braking is one of the mechanisms that can drive such loss of angular momentum in magnetized white dwarfs. However, the timescale on which such process occurs is still the matter of an active debate, as it depends on many factors, like the strength of the magnetic field, its angle of inclination with respect to the rotation axis, and the properties of the remnant of the merger. In particular, these include its mass and radius, its angular velocity, and its moment of inertia. Moreover, Super-Chandrasekhar white dwarfs resulting from the coalescence of two otherwise normal stars are surrounded by a Keplerian disk that can be accreted onto the newly formed compact object. Thus, the post-merger evolution of the remnant also depends critically on the accretion rate. Here we compute the post-merger evolution of a Super-Chandraskhar magnetized white dwarf taking into account all the relevant physical processes. These include magnetic torques acting on the star, accretion from the Keplerian disk, and the threading of the magnetic field lines through the disk.

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Abstract

The induced gravitational collapse (IGC) paradigm explains a class of energetic, long-duration gamma-ray bursts associated with Ic supernovae, recently named binary-driven hypernovae. The progenitor is a tight binary system formed of a carbon-oxygen core and a neutron star companion (NS). The supernova ejecta triggers a hypercritical accretion process onto the NS, which reaches in a few seconds the critical mass, and gravitationally collapses to a black hole emitting a GRB. In our previous simulations of this process we adopted a spherically symmetric approximation to compute the features of the hypercritical accretion process. We here present the first estimates of the angular momentum transported by the supernova ejecta, and perform numerical simulations of the angular momentum transfer to the NS during the hyperaccretion process in full general relativity. We show that the NS: i)reaches in a few seconds either mass-shedding limit or the secular axisymmetric instability depending on its initial mass; ii) reaches a maximum dimensionless angular momentum value, [cJ/(GM^2)]_max ≈ 0.7; iii) can support less angular momentum than the one transported by supernova ejecta, hence there is an angular momentum excess which necessarily leads to jetted emission.

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