Moreno Méndez, Enrique 2013

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Title:		An Argument for Weakly Magnetized, Slowly Rotating Progenitors of Long Gamma-Ray Bursts
Authors:		Moreno Méndez, Enrique
Affiliation:		AA(Instituto de Astronomía, Universidad Nacional Autónoma de México, Circuito Exterior, Ciudad Universitaria, Apartado Postal 70-543, 04510, Distrito Federal, Mexico; Argelander-Institut für Astronomie, Bonn University, Auf dem Hügel 71, D-53121 Bonn, Germany)
Publication:		The Astrophysical Journal, Volume 781, Issue 1, article id. 3, 6 pp. (2014). (ApJ Homepage)
Publication Date:		01/2014
Origin:		IOP
Astronomy Keywords:		binaries: close, black hole physics, gamma-ray burst: general, stars: magnetic field, supernovae: general
DOI:		10.1088/0004-637X/781/1/3
Bibliographic Code:		2014ApJ...781....3M

Abstract

Using binary evolution with Case-C mass transfer, the spins of several black holes (BHs) in X-ray binaries (XBs) have been predicted and confirmed (three cases) by observations. The rotational energy of these BHs is sufficient to power up long gamma-ray bursts (GRBs) and hypernovae (HNe) and still leave a Kerr BH behind. However, strong magnetic fields and/or dynamo effects in the interior of such stars deplete their cores from angular momentum preventing the formation of collapsars. Thus, even though binaries can produce Kerr BHs, most of their rotation is acquired from the stellar mantle, with a long delay between BH formation and spin up. Such binaries would not form GRBs. We study whether the conditions required to produce GRBs can be met by the progenitors of such BHs. Tidal-synchronization and Alfvén timescales are compared for magnetic fields of different intensities threading He stars. A search is made for a magnetic field range that allows tidal spin up all the way in to the stellar core but prevents its slow down during differential rotation phases. The energetics for producing a strong magnetic field during core collapse, which may allow for a GRB central engine, are also estimated. An observationally reasonable choice of parameters is found (B <~ 10² G threading a slowly rotating He star) that allows Fe cores to retain substantial angular momentum. Thus, the Case-C mass-transfer binary channel is capable of explaining long GRBs. However, the progenitors must have low initial spin and low internal magnetic field throughout their H-burning and He-burning phases.

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