6 Burst models

$\rm NS-NS$ or $\rm NS-BH$ binary mergers and "collapsar'' or "hypernova'' events continue to be the leading models for the energy source of GRBs. Rees (1999) described what he termed the "best buy'' model, which involves a $\rm NS-BH$ binary merger and a magnetically powered jet. Woosley (1999) reported a series of calculations and hydrodynamic simulations that explore various stages of the collapsar scenario, including the production of a hydrodynamic jet (although the jet might also be magnetically powered in this scenario, if magnetic fields were included).

The increasingly strong evidence that the bursts detected by BeppoSAX originate in galaxies undergoing star formation, and may occur near or in the star-forming regions themselves, favors the collapsar model and disfavors the binary merger model as the explanation for long, softer, smoother bursts. Simulations of the kicks given to $\rm NS-NS$ and $\rm NS-BH$binaries by the SNe that form them shows that most binary mergers are expected to occur well outside any galaxy (Bulik & Belczynski 1999). This is particularly the case, given that the GRB host galaxies identified so far have small masses, as discussed earlier, and therefore low escape velocities. The fact that all of the optical afterglows of the BeppoSAX bursts are coincident with the disk of the host galaxy therefore also disfavors the binary merger model as the explanation for the long, softer, smoother bursts.

Current models of the bursts themselves fall into three general categories: Those that invoke a central engine, those that invoke internal shock waves in a relativistically expanding wind, and those that invoke a relativistic external shock wave. Dermer (1999) argued that the external shock wave model explains many of the observed properties of the bursts. By contrast, Fenimore (1999; see also Fenimore et al. 1999) argued that several features of GRBs, such as the large gaps seen in burst time histories, cannot be explained by the external shock wave model, and that the bursts must therefore be due either to a central engine or to internal shocks in a relativistically expanding wind. Either way, the intensity and spectral variations seen during the burst must originate at a central engine. This implies that the lifetime of the central engine must in many cases be  s, which poses a severe difficulty for $\rm NS-NS$ or $\rm NS-BH$ binary merger models, if such models are invoked to explain the long, softer, smoother bursts, and may pose a problem for the collapsar model. Fenimore (1999) reported at this meeting that he finds no evidence of relativistic expansion in the time histories and spectra of the GRBs themselves, presenting a possible difficulty for the internal shock wave model.

One puzzle about the bursts themselves is: Why are GRB spectra so smooth? The shock synchrotron model agrees well with observed burst spectra. But this agreement is surprising, since strong deviations from the simplest spectral shape are expected due to inverse Compton scattering, and if forward and reverse shock contributions to the prompt gamma-ray emission occur simultaneously or at different times (Tavani 1999).

Another puzzle is: Why is the spread in the peak energy $E_{\rm peak}$of the burst spectra so narrow? In the external shock model, this requires that all GRBs have nearly the same ultra-relativistic value of $\Gamma$. The narrow range in $E_{\rm peak}$ is, if anything, more difficult to understand in the internal shock model. If $\Delta \Gamma/\Gamma \ll 1$ in the relativistic outflow, the range in $E_{\rm peak}$ will be narrow, but then it is hard to understand why most of the energy of the relativistic outflow is dissipated during the burst rather than in the afterglow. Conversely, if $\Delta \Gamma/\Gamma \gg 1$ in the relativistic outflow, most of the energy of the relativistic outflow is dissipated during the burst rather than in the afterglow, but then one expects a wide range of $E_{\rm peak}$'s. This is a hint - like the problem discussed earlier that one would expect strong beaming to produce a large special relativistic Doppler redshift, yet this is not seen in burst spectra - that there may be something missing in our picture of the dissipation and radiation mechanisms in GRBs.

  

Figure: The radio through X-ray spectrum of the afterglow of GRB 980329. All measurements have been scaled to a common time, approximately three days after the GRB. The solid curve is the best fit spectrum for an isotropic fireball that expands into a homogeneous external medium, extincted by dust at a redshift of z = 3.5. The dotted curve is the un-extincted spectrum. From Lamb et al. (1999)