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E. The Supranova Model

Vietri and Stella [426] suggested that GRBs take place when a "supermassive" (or supramassive as Vietri and Stella [426] call it) neutron star (namely a neutron star that is above the maximal cold nonrotating neutron star mass) collapses to a black hole. The collapse can take place because the neutron star losses angular momentum via a pulsar wind and it looses the extra support of the centrifugal force. Alternatively the supramassive neutron star can simply cool and become unstable if rotation alone is not enough to support it. The neutron star could also become over massive and collapse if it accretes slowly matter from a surrounding accretion disk [427]. In this latter case the time delay from the SN could be very large and the SNR will not play any role in the GRB or its afterglow.

The Supranova model is a two step event. First, there is a supernova, which may be more energetic than an average one, in which the supermassive neutron star forms. Then a few weeks or months later this neutron star collapses producing the GRB. While both the Supranova and the Collapsar (or hypernova) events are associated with Supernovae or Supernovae like event the details of the model are very different. First, while in the Collapsar model one expect a supernova bump on the afterglow light curve, such a bump is not expected in the Supranova model unless the time delay is a few days. On the other hand while it is not clear in the Collapsar model how does the Fe needed for the Fe X-ray lines reach the implied large distances form the center, it is obvious in this model, as the supernova shell was ejected to space several month before the GRB. As mentioned earlier (see Section IIC4) the association of GRB 030329 with SN 2003dh [167, 395] is incompatible with the Supranova model. Proponents of this model, argue however, that there might be a distribution of delay times between the first and second collapses.

The models are also very different in their physical content. First in the Supranova model the GRB jet does not have to punch a whole through the stellar envelope. Instead the ejecta propagates in almost free space polluted possibly by a pulsar wind [149, 183]. In both models, like in many other models, the GRB is powered by accretion of a massive accretion disk surrounding the newborn black hole. This accretion disk forms, from the debris of the collapsing neutron star at the same time that the black hole is formed. Again, the time scale of the burst is determined by the accretion time of this disk. Narayan et al. [277] (see also Section IXA) point however that long lived (50 sec) accretion disks must be large and hence extremely inefficient. This may pose a problem for this model.

Königl and Granot [183], Guetta and Granot [149] and Inoue et al. [178] considered the effects a strong pulsar wind (that may exist after the SN and before the second collapse) on this scenario. The pulsar wind can have several effects. First it would produce a denser highly magnetized medium into which the GRB jet propagates. The strong magnetic field will be amplified by the afterglow shock. This resolves the problem of the source of the strong magnetic field needed for the synchrotron afterglow model. This can also explain the high energy emission detected by EGRET in GRB 940217 (Hurley [172] and Section IIA1) by Inverse Compton scattering on the pulsar wind bubble photons. On the other hand the density of this wind matter (~ 103 cm-3) might be too high for the spherical model. Note however, that this depends on the time delay as t-3. However, the pulsar wind won't be spherical and one would expect that it will form an elongated supernova shell cavity within which the pulsar wind is bounded. If, as expected, the pulsar jet coincides with the GRB jet then the relativistic ejecta will move along the elongated direction of this shell.

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