GRB 060218 and GRBs associated with supernovae lb/c

Abstract

Context. The Swift satellite has given continuous data in the range 0.3-150 keV from 0 s to 106 s for GRB 060218 associated with SN2006aj. This Gamma-Ray Burst (GRB) which has an unusually long duration (T90 ̃ 2100 s) fulfills the Amati relation. These data offer the opportunity to probe theoretical models for GRBs connected with Supernovae (SNe). Aims. We plan to fit the complete y- and X-ray light curves of this long duration GRB, including the prompt emission, in order to clarify the nature of the progenitors and the astrophysical scenario of the class of GRBs associated with SNe Ib/c. Methods. We apply our "fireshell" model based on the formation of a black hole, giving the relevant references. It is characterized by the precise equations of motion and equitemporal surfaces and by the role of thermal emission. Results. The initial total energy of the electron-positron plasma Ee±tot = 2.32 x 1050 erg has a particularly low value, similar to the other GRBs associated with SNe. For the first time, we observe a baryon loading B= 10~2 which coincides with the upper limit for the dynamical stability of the fireshell. The effective CircumBurst Medium (CBM) density shows a radial dependence ∝ r -α with 1.0 ≤ α ≤ 1.7 and monotonically decreases from 1 to 10-6 particles/cm3. This behavior is interpreted as being due to a fragmentation in the fireshell. Analogies with the fragmented density and filling factor characterizing Novae are outlined. The fit presented is particularly significant in view of the complete data set available for GRB 060218 and of the fact that it fulfills the Amati relation. Conclusions. We fit GRB 060218, usually considered as an X-Ray Flash (XRF), as a "canonical GRB" within our theoretical model. The smallest possible black hole, formed by the gravitational collapse of a neutron star in a binary system, is consistent with the especially low energetics of the class of GRBs associated with SNe Ib/c. We provide the first evidence for a fragmentation in the fireshell. This fragmentation is crucial in explaining both the unusually large T90 and the consequently inferred abnormally low value of the CBM effective density. © ESO 2007.