III. THE GLOBAL PICTURE - GENERALLY ACCEPTED INGREDIENTS

There are several generally accepted ingredients in practically all current GRB models.

Relativistic Motion: Practically all current GRB models involve a relativistic motion with a Lorentz factor, > 100. This is essential to overcome the compactness problem (see Section IVA below). At first this understanding was based only on theoretical arguments. However, now there are direct observational proofs of this concept: It is now generally accepted that both the radio scintillation [131] and the lower frequency self-absorption [187] provide independent estimates of the size of the afterglow, ~ 10¹⁷ cm, two weeks after the burst. These observations imply that the afterglow has indeed expanded relativistically. Sari and Piran [372] suggested that the optical flash accompanying GRB 990123 provided a direct evidence for ultra-relativistic motion with ~ 100. Soderberg and Ramirez-Ruiz [390] find a higher value: 1000 ± 100. However, these interpretations are model dependent.

The relativistic motion implies that we observe blue shifted photons which are significantly softer in the moving rest frame. It also implies that when the objects have a size R the observed emission arrives on a typical time scale of R / c ² (see Section IVB). Relativistic beaming also implies that we observe only a small fraction (1 / ) of the source. As I discussed earlier (see Section IID and also IVC) this has important implications on our ability to estimate the total energy of GRBs.

While all models are based on ultra-relativistic motion, none explains convincingly (this is clearly a subjective statement) how this relativistic motion is attained. There is no agreement even on the nature of the relativistic flow. While in some models the energy is carried out in the form of kinetic energy of baryonic outflow in others it is a Poynting dominated flow or both.

Dissipation In most models the energy of the relativistic flow is dissipated and this provides the energy needed for the GRB and the subsequent afterglow. The dissipation is in the form of (collisionless) shocks, possibly via plasma instability. There is a general agreement that the afterglow is produced via external shocks with the circumburst matter (see VII). There is convincing evidence (see e.g. Fenimore et al. [94], Piran and Nakar [311], Ramirez-Ruiz and Fenimore [328], Sari and Piran [370] and Section VIA below) that in most bursts the dissipation during the GRB phase takes place via internal shocks, namely shocks within the relativistic flow itself. Some (see e.g. Dar [70], Dermer and Mitman [78], Heinz and Begelman [162], Ruffini et al. [358]) disagree with this statement.

Synchrotron Radiation: Most models (both of the GRB and the afterglow) are based on Synchrotron emission from relativistic electrons accelerated within the shocks. There is a reasonable agreement between the predictions of the synchrotron model and afterglow observations [140, 291, 439]. These are also supported by measurements of linear polarization in several optical afterglows (see Section IIB2). As for the GRB itself there are various worries about the validity of this model. In particular there are some inconsistencies between the observed special slopes and those predicted by the synchrotron model (see [321] and Section IIA1). The main alternative to Synchrotron emission is synchrotron-self Compton [124, 431] or inverse Compton of external light [42, 211, 383, 384]. The last model requires, of course a reasonable source of external light.

Jets and Collimation: Monochromatic breaks appear in many afterglow light curves. These breaks are interpreted as "jet breaks" due to the sideways beaming of the relativistic emission [292, 344, 374] (when the Lorentz factor drops below 1 / ₀ the radiation is beamed outside of the original jet reducing the observed flux) and due to the sideways spreading of a beamed flow [344, 374]. An alternative interpretation is of a viewing angles of a "universal structured jet" [219, 347, 446] whose energy varies with the angle. Both interpretations suggest that GRBs are beamed. However, they give different estimates of the overall rate and the energies of GRBs (see Section VIII below). In either case the energy involved with GRBs is smaller than the naively interpreted isotropic energy and the rate is higher than the observed rate.

A (Newborn) Compact Object If one accepts the beaming interpretation of the breaks in the optical light curve the total energy release in GRBs is ~ 10⁵¹ ergs [105, 291]. It is higher if, as some models suggest, the beaming interpretation is wrong or if a significant amount of additional energy (which does not contribute to the GRB or to the afterglow) is emitted from the source. This energy, ~ 10⁵¹ ergs, is comparable to the energy released in a supernovae. It indicates that the process must involve a compact object. No other known source can release so much energy within such a short time scale. The process requires a dissipation of ~ 0.1 m within the central engine over a period of a few seconds. The sudden appearance of so much matter in the vicinity of the compact object suggest a violent process, one that most likely involves the birth of the compact object itself.

Association with Star Formation and SNe: Afterglow observations, which exist for a subset of relatively bright long bursts, show that GRBs arise within galaxies with a high star formation rate (see [83] and Section IIC1). Within the galaxies the bursts distribution follows the light distribution [36]. This has lead to the understanding that (long) GRB arise from the collapse of massive stars (see Section IXD). This understanding has been confirmed by the appearance of SN bumps in the afterglow light curve (see Section IIC4 earlier) and in particular by the associations of SN 1999bw with GRB 980425 and of SN 2003dh with GRB 030329.

Summary: Based on these generally accepted ideas one can sketch the following generic GRB model: GRBs are a rare phenomenon observed within star forming regions and associated with the death of massive stars and the birth of compact objects. The -rays emission arises from internal dissipation within a relativistic flow. This takes place at a distances of ~ 10¹³ - 10¹⁵ cm from the central source that produces the relativistic outflow. Subsequent dissipation of the remaining energy due to interaction with the surrounding circumburst matter produces the afterglow. The nature of the "inner engine" is not resolved yet, however, a the association with SN (like 1998bw and 2003dh) shows that long GRBs involve a a collapsing star. Much less is known on the origin of short GRBs.