Roberto Guida

GRB060614: a 'fake' short GRB from a merging binary system

Context. GRB060614 observations by VLT and by Swift have infringed the traditionally accepted gamma-ray burst (GRB) collapsar scenario that purports the origin of all long duration GRBs from supernovae (SN). GRB060614 is the first nearby long durat ion GRB clearly not associated with a bright Ib/c SN. Moreover, its duration (T90 ∼ 100 s) makes it hardly classifiable as a short GRB. It presents strong similarities with GRB970228, the prototype of a new class of “fake” short GRBs that appear to originate from the coalescence of binary neutron stars or white dwarfs spiraled out into the galactic halo. Aims. Within the “canonical” GRB scenario based on the “fireshell” model, we test if GRB060614 can be a “fake” or “disguised” short GRB. We model the traditionally termed “prompt emission” and discriminate the signal originating from the gravitational collapse leading to the GRB from the process occurring in the circumburst medium (CBM). Methods. We fit GRB060614 light curves in Swift’s BAT (15 − 150 keV) and XRT (0.2− 10 keV) energy bands. Within the fireshell model, light curves are formed by two well defined and different components: the proper-GRB (P-GRB), emitted when the fireshell becomes transparent, and the extended afterglow, due to the interaction between the leftover accelerated baryonic and leptonic shell and the CBM. Results. We determine the two free parameters describing the GRB source within the fireshell model: the total e ± plasma energy (E e ± tot = 2.94× 10 51 erg) and baryon loading (B = 2.8× 10 −3 ). A small average CBM density∼ 10 −3 particles/cm 3 is inferred, typical of galactic halos. The first spikelike emission is identified with the P-GRB and the following prolonged emission with the extended afterglow peak. We obtain very good agreement in the BAT (15− 150 keV) energy band, in what is traditionally called “prompt emission”, and in the XRT (0.2− 10 keV) one. Conclusions. The anomalous GRB060614 finds a natural interpretation within our canonic al GRB scenario: it is a “disguised” short GRB. The total time-integrated extended afterglow luminosity is greater than the P-GRB one, but its peak luminosity is smaller since it is deflated by the peculiarly low average CBM density of gal actic halos. This result points to an old binary system, like ly formed by a white dwarf and a neutron star, as the progenitor of GRB060614 and well justifies the absence of an associated SN Ib /c. Particularly important for further studies of the final merging process ar e the temporal structures in the P-GRB down to 0.1 s.

GRB 970228 and a class of GRBs with an initial spikelike emission

Context. The discovery by Swift and HETE-2 of an afterglow emission associated possibly with short GRBs opened the new problematic of their nature and classification. This issue has been further enhanced by the observation of GRB 060614 and by a new analysis of the BATSE catalog which led to the identification of a new class of GRBs with “an occasional softer extended emission lasting tenths of seconds after an initial spikelike emission”. Aims. We plan a twofold task: a) to fit this new class of “hybrid” sources within our “canonical GRB” scenario, where all GRBs are generated by a “common engine” (i.e. the gravitational collapse to a black hole); b) to propose GRB 970228 as the prototype of the above mentioned class, since it shares the same morphology and observational features. Methods. We analyze BeppoSAX data on GRB 970228 within the “fireshell” model and we determine the parameters describing the source and the CircumBurst Medium (CBM) needed to reproduce its light curves in the 40–700 keV and 2–26 keV energy bands. Results. We find that GRB 970228 is a “canonical GRB”, like e.g. GRB 050315, with the main peculiarity of a particularly low

GRB 060218 and GRBs associated with supernovae Ib/c

Context. The Swift satellite has given continuous data in the range 0.3–150 keV from 0 s to 10 6 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 γ- 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 E tot± = 2.32 × 10 50 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 ncbm ∝ r −α with 1.0 < α < 1.7 and monotonically decreases from 1 to 10 −6 particles/cm 3 . 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.

GRB970228 and the class of GRBs with an initial spikelike emission: do they follow the Amati relation?

On the basis of the recent understanding of GRB050315 and GRB060218, we return to GRB970228, the first Gamma‐Ray Burst (GRB) with detected afterglow. We proposed it as the prototype for a new class of GRBs with “an occasional softer extended emission lasting tenths of seconds after an initial spikelike emission”. Detailed theoretical computation of the GRB970228 light curves in selected energy bands for the prompt emission are presented and compared with observational BeppoSAX data. From our analysis we conclude that GRB970228 and likely the ones of the above mentioned new class of GRBs are “canonical GRBs” have only one peculiarity: they exploded in a galactic environment, possibly the halo, with a very low value of CBM density. Here we investigate how GRB970228 unveils another peculiarity of this class of GRBs: they do not fulfill the “Amati relation”. We provide a theoretical explanation within the fireshell model for the apparent absence of such correlation for the GRBs belonging to this new class.

The Blackholic energy and the canonical Gamma‐Ray Burst IV: the “long,” “genuine short” and “fake—disguised short” GRBs

We report some recent developments in the understanding of GRBs based on the theoretical framework of the “fireshell” model, already presented in the last three editions of the “Brazilian School of Cosmology and Gravitation.” After recalling the basic features of the “fireshell model,” we emphasize the following novel results: 1) the interpretation of the X‐ray flares in GRB afterglows as due to the interaction of the optically thin fireshell with isolated clouds in the CircumBurst Medium (CBM); 2) an interpretation as “fake—disguised” short GRBs of the GRBs belonging to the class identified by Norris & Bonnell; we present two prototypes, GRB 970228 and GRB 060614; both these cases are consistent with an origin from the final coalescence of a binary system in the halo of their host galaxies with particularly low CBM density ncbm∼10−3 particles/cm3; 3) the first attempt to study a genuine short GRB with the analysis of GRB 050509B, that reveals indeed still an open question; 4) the interpretation of the GR...

The canonical Gamma‐Ray Bursts and their “precursors”

The fireshell model for Gamma‐Ray Bursts (GRBs) naturally leads to a canonical GRB composed of a proper‐GRB (P‐GRB) and an afterglow. P‐GRBs, introduced by us in 2001, are sometimes considered “precursors” of the main GRB event in the current literature. We show in this paper how the fireshell model leads to the understanding of the structure of GRBs, with precise estimates of the time sequence and intensities of the P‐GRB and the of the afterglow. It leads as well to a natural classification of the canonical GRBs which overcomes the traditional one in short and long GRBs.

Short and canonical GRBs

Within the “fireshell” model for the Gamma‐Ray Bursts (GRBs) we define a “canonical GRB” light curve with two sharply different components: the Proper‐GRB (P‐GRB), emitted when the optically thick fireshell of electron‐positron plasma originating the phenomenon reaches transparency, and the afterglow, emitted due to the collision between the remaining optically thin fireshell and the CircumBurst Medium (CBM). We outline our “canonical GRB” scenario, with a special emphasis on the discrimination between “genuine” and “fake” short GRBs.

GRB 050315: A step in the proof of the uniqueness of the overall GRB structure

Using the Swift data of GRB 050315, we progress in proving the uniqueness of our theoretically predicted Gamma‐Ray Burst (GRB) structure as composed by a proper‐GRB, emitted at the transparency of an electron‐positron plasma with suitable baryon loading, and an afterglow comprising the “prompt radiation” as due to external shocks. Detailed light curves for selected energy bands are theoretically fitted in the entire temporal region of the Swift observations ranging over 106 seconds.

The canonical Gamma‐Ray Bursts: long, “fake”‐“disguised” and “genuine” short bursts

The Gamma‐Ray Bursts (GRBs) offer the unprecedented opportunity to observe for the first time the blackholic energy extracted by the vacuum polarization during the process of gravitational collapse to a black hole leading to the formation of an electron‐positron plasma. The uniqueness of the Kerr‐Newman black hole implies that very different processes originating from the gravitational collapse a) of a single star in a binary system induced by the companion, or b) of two neutron stars, or c) of a neutron star and a white dwarf, do lead to the same structure for the observed GRB. The recent progress of the numerical integration of the relativistic Boltzmann equations with collision integrals including 2‐body and 3‐body interactions between the particles offer a powerful conceptual tool in order to differentiate the traditional “fireball” picture, an expanding hot cavity considered by Cavallo and Rees, as opposed to the “fireshell” model, composed of an internally cold shell of relativistically expanding el...

Prompt emission and X‐ray flares: the case of GRB 060607 A

GRB 060607 A is a very distant and energetic event. Its main peculiarity is that the peak of the near‐infrared (NIR) afterglow has been observed with the REM robotic telescope, allowing to estimate the initial Lorentz gamma factor within the fireball forward shock model. We analyze GRB 060607 A within the fireshell model. The initial Lorentz gamma factor of the fireshell can be obtained adopting the exact solutions of its equations of motion, dealing only with the BAT and XRT observations, that are the basic contribution to the afterglow emission, up to a distance from the progenitor r∼1018 cm. According to the “canonical GRB” scenario we interpret the whole prompt emission as the peak of the afterglow emission, and we show that the observed temporal variability of the prompt emission can be produced by the interaction of the fireshell with overdense CircumBurst Medium (CBM) clumps. This is indeed the case also of the X‐ray flares which are present in the early phases of the afterglow light curve.