Optical versus X–ray afterglows of GRBs: towards understanding the emission processes

Gamma–Ray Bursts (GRBs) are the most distant objects ever detected after the recombination epoch. They consist of a short intense emission episode of gammarays (10 keV–2 MeV) with typical duration between 10−2 and 103 seconds. This is called the “prompt” emission phase. GRBs are classified, according to their observed duration, into short GRBs (lasting less than 2s) and long GRBs (lasting more than 2 s). During the prompt phase GRBs are the brightest objects in the gamma–ray sky. The gamma–ray prompt emission is accompanied by a long lasting emission, called “afterglow”, covering the whole spectral range from the radio to the X– rays. The afterglow emission can be observed up to months after the prompt phase ceased. After the discovery of the GRB afterglow made possible by the Dutch-Italian satellite BeppoSAX, and the confirmation of their cosmological origin, the GRB community reached a general consensus about the nature of these sources which led to the formulation of the so called “standard fireball model”. This model was able, until recently, to account for most of the observational properties of the both the prompt and the afterglow emission. In this scenario, long GRBs are thought to be produced by the core collapse of massive stars. The gamma–ray prompt emission is produced by the “internal shocks” developed by the collisions of different plasma shells ejected by the central engine with different Lorentz factors. The afterglow emission is due to the “external shock” produced by the deceleration of a relativistically expanding fireball by the external medium. The leading radiative mechanism responsible for the prompt and the afterglow emission is synchrotron radiation by electrons accelerated at the internal/external shocks. An important assumption of the standard model is that both the optical and the X–ray afterglows are produced by the same mechanism, taking place in the same region. The launch of the Swift satellite (in November 2004), in synergy with the available network of automatic ground based optical telescopes, signed a remarkable improvement (a kind of “revolution”) of our ”view” of GRB afterglows. Thanks to the fast repointing capabilities of Swift, now X–ray and optical afterglows can be observed starting only few minutes after the prompt GRB emission. Before the launch of Swift, instead, afterglow observations started typically several hours after the burst detection. This new observational window, opened on the early times afterglow emission, unveiled a picture that is much more complex than what had been seen before Swift when the optical and X–ray light curves were usually well described by simple power law decays. The early time light curves observed in the X–rays (and sometimes in the optical), show different phases characterised by different decay indices, chromatic breaks and sudden rebrightenings. Another important finding of Swift is the fact that often the GRB optical light curve does not track the X– ray one. This cannot be explained in the framework of the standard model which assumes that both the X–ray and the optical emission have the same origin and, therefore, should behave similarly. For this reason, in the last few years, several alternative models have been proposed in order to account for the new “afterglow picture” depicted by the Swift observations. Most of these models, however, try to reconcile the observed X–ray and optical light curve complexity through some modifications of the standard afterglow model. Usually, these alternative scenarios assume, as in the standard model, that the optical and X–ray emission are due to the same emission mechanism operating in the same emitting region and therefore suffer of the same main problem of the standard model i.e. they can hardly reproduce the diverse light curves of the optical and X–ray emission of individual GRBs. My thesis is devoted to the study of this issue, i.e. the study of the GRB afterglows to understand the physical mechanisms that produce the observed optical and X–ray emission. The aim of my thesis is to study and to test with the available observations a possible alternative scenario to the standard model that fails to explain the complex behaviour of the X-ray and optical afterglow emission of GRBs. To this aim I studied the intrinsic (i.e. rest frame) afterglow properties simultaneously taking into account the optical and X–ray light curves. This is possible exploiting the rich broad band follow up that is now available for a large number of events. I analysed the optical luminosities of long GRBs finding an unexpected clustering and bimodality of the optical luminosity distributions. I proved that these results are not due to observational selection effects and that the X–ray luminosity are not in agreement with what found in the optical. These results can hardly be explained in the framework of the standard afterglow model. Together with the group I am working with, I analysed the light curve of the optical and X–rays rest frame luminosity of a sample of 33 long GRBs. We modelled the broad band light curve evolution as due to the sum of two separate components, contrary to the usual assumption of a common origin of the optical and X–ray emission. We obtain a good agreement with the observations, accounting for the light curves complexity and diversity. This two component model makes predictions about the broad band spectral energy distribution (SEDs), that I tested analysing the observed SEDs. Through this analysis I confirm that our two component model is consistent with the observed data also form the spectral point of view. This led us to propose a new view of the afterglow emission mechanism following the so called late prompt scenario proposed by Ghisellini et al. 2007. According to our view, the central engine activity lasts for long time (up to months after the trigger) keeping on producing slower shells that are responsible for the emission of optical and X–ray radiation that competes with the standard forward shock emission. This generates the complexity of the observed broad band light curves and explains the diversity between the optical and X–ray temporal evolution. We suggest that the late time activity of the central engine is sustained by the accretion of the material that failed to reach the escape velocity from the exploding progenitor star, and falls back. The presence of this mechanism is strengthened by the similarity between the temporal evolution of the late prompt component, and the expected time profile of the accretion rate of the fall back material.