Maria Grazia Bernardini

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 071227: an additional case of a disguised short burst

Context. Observations of gamma-ray bursts (GRBs) have shown an hybridization between the two classes of long and short bursts. In the context of the fireshell model, the GRB light curves are formed by two different components: the proper GRB (P-GRB) and the extended afterglow. Their relative intensity is linked to the fireshell baryon loading B. The GRBs with P-GRB predominance are the short ones, the remainders are long. A new family of disguised short bursts has been identified: long bursts with a protracted low instantaneous luminosity due to a low density CircumBurst Medium (CBM). In the 15–150 keV energy band GRB 071227 exhibits a short duration (about 1.8 s) spike-like emission followed by a very soft extended tail up to one hundred seconds after the trigger. It is a faint (Eiso = 5.8 × 10 50 ) nearby GRB (z = 0.383) that does not have an associated type Ib/c bright supernova (SN). For these reasons, GRB 071227 has been classified as a short burst not fulfilling the Amati relation holding for long burst. Aims. We check the classification of GRB 071227 provided by the fireshell model. In particular, we test whether this burst is another example of a disguised short burst, after GRB 970228 and GRB 060614, and, for this reason, whether it fulfills the Amati relation. Methods. We simulate GRB 071227 light curves in the Swift BAT 15–50 keV bandpass and in the XRT (0.3–10 keV) energy band within the fireshell model. Results. We perform simulations of the tail in the 15–50 keV bandpass, as well as of the first part of the X-ray afterglow. This infers that: E e

Analysis of GRB 080319B and GRB 050904 within the Fireshell Model: Evidence for a Broader Spectral Energy Distribution

The observation of GRB?080319B, with an isotropic energy E iso = 1.32 ? 1054 erg, and GRB?050904, with E iso = 1.04 ? 1054 erg, offers the possibility of studying the spectral properties of the prompt radiation of two of the most energetic gamma-ray bursts (GRBs). This allows us to probe the validity of the fireshell model for GRBs beyond 1054 erg, well outside the energy range where it has been successfully tested up to now (1049-1053 erg). We find that in the low-energy region, the prompt emission spectra observed by Swift Burst Alert Telescope (BAT) reveals more power than theoretically predicted. The opportunities offered by these observations to improve the fireshell model are outlined in this paper. One of the distinguishing features of the fireshell model is that it relates the observed GRB spectra to the spectrum in the comoving frame of the fireshell. Originally, a fully radiative condition and a comoving thermal spectrum were adopted. An additional power law in the comoving thermal spectrum is required due to the discrepancy of the theoretical and observed light curves and spectra in the fireshell model for GRBs 080319B and 050904. A new phenomenological parameter ? is correspondingly introduced in the model. We perform numerical simulations of the prompt emission in the Swift BAT bandpass by assuming different values of ? within the fireshell model. We compare them with the GRB?080319B and GRB?050904 observed time-resolved spectra, as well as with their time-integrated spectra and light curves. Although GRB?080319B and GRB?050904 are at very different redshifts (z = 0.937 and z = 6.29, respectively), a value of ? = ?1.8 for both of them leads to a good agreement between the numerical simulations and the observed BAT light curves, time-resolved and time-integrated spectra. Such a modified spectrum is also consistent with the observations of previously analyzed less energetic GRBs and reasons for this additional agreement are given. Perspectives for future low-energy missions are outlined.

The Blackholic energy: long and short Gamma‐Ray Bursts (New perspectives in physics and astrophysics from the theoretical understanding of Gamma‐Ray Bursts, II)

We outline the confluence of three novel theoretical fields in our modeling of Gamma‐Ray Bursts (GRBs): 1) the ultrarelativistic regime of a shock front expanding with a Lorentz gamma factor ∼ 300; 2) the quantum vacuum polarization process leading to an electron‐positron plasma originating the shock front; and 3) the general relativistic process of energy extraction from a black hole originating the vacuum polarization process. There are two different classes of GRBs: the long GRBs and the short GRBs. We here address the issue of the long GRBs. The theoretical understanding of the long GRBs has led to the detailed description of their luminosities in fixed energy bands, of their spectral features and made also possible to probe the astrophysical scenario in which they originate. We are specially interested, in this report, to a subclass of long GRBs which appear to be accompanied by a supernova explosion. We are considering two specific examples: GRB980425/SN1998bw and GRB030329/SN2003dh. While these sup...

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.

Black Hole Physics and Astrophysics: The GRB-Supernova Connection and URCA-1 - URCA-2

We outline the confluence of three novel theoretical fields in our modeling of Gamma-Ray Bursts (GRBs): 1) the ultrarelativistic regime of a shock front expanding with a Lorentz gamma factor

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

\sim 300

GRB 980425, SN1998BW and the EMBH model

; 2) the quantum vacuum polarization process leading to an electron-positron plasma originating the shock front; and 3) the general relativistic process of energy extraction from a black hole originating the vacuum polarization process. There are two different classes of GRBs: the long GRBs and the short GRBs. We here address the issue of the long GRBs. [...] We are specially interested, in this report, to a subclass of long GRBs which appear to be accompanied by a supernova explosion. We are considering two specific examples: GRB980425/SN1998bw and GRB030329/SN2003dh [...].

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

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.