Letizia Caito

Evidence for a proto-black hole and a double astrophysical component in GRB 101023

Context: It has been recently shown that GRB 090618, observed by AGILE, Coronas Photon, Fermi, Konus, Suzaku and Swift, is composed of two very di erent components: episode 1, lasting 50 s, shows a thermal plus power-law spectrum with a characteristic temperature evolving in time as a power law; episode 2 (the remaining 100 s) is a canonical long GRB. We have associated episode 1 to the progenitor of a collapsing bare core leading to the formation of a black hole: what was defined as a “proto black hole”. Aims: In precise analogy with GRB 090618 we aim to analyze the 89s of the emission of GRB 101023, observed by Fermi, Gemini, Konus and Swift, to see if there are two di erent episodes: the first one presenting a characteristic black-body temperature evolving in time as a broken power law, and the second one consistent with a canonical GRB. Methods: To obtain information on the spectra, we analyzed the data provided by the GBM detector onboard the Fermi satellite, and we used the heasoft package XSPEC and RMFIT to obtain their spectral distribution. We also used the numerical code GRBsim to simulate the emission in the context of the fireshell scenario for episode 2. Results: We confirm that the first episode can be well fit by a black body plus power-law spectral model. The temperature changes with time following a broken power law, and the photon index of the power-law component presents a soft-to-hard evolution. We estimate that the radius of this source increases with time with a velocity of 1:5 10 4 km=s. The second episode appears to be a canonical GRB. By using the Amati and the Atteia relations, we determined the cosmological redshift, z 0:9 0:084(stat:) 0:2(sys:). The results of GRB 090618 are compared and contrasted with the results of GRB 101023. Particularly striking is the scaling law of the soft X-ray component of the afterglow. Conclusions: We identify GRB 090618 and GRB 101023 with a new family of GRBs related to a single core collapse and presenting two astrophysical components: a first one related to the proto-black hole prior to the process of gravitational collapse (episode 1), and a second one, which is the canonical GRB (episode 2) emitted during the formation of the black hole. For the first time we are witnessing the process of a black hole formation from the instants preceding the gravitational collapse up to the GRB emission. This analysis indicates progress towards developing a GRB distance indicator based on understanding the P-GRB and the prompt emission, as well as the soft X-ray behavior of the late afterglow.

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.

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...

A NEW SPECTRAL ENERGY DISTRIBUTION OF PHOTONS IN THE FIRESHELL MODEL OF GRBS

The analysis of various Gamma-Ray Bursts (GRBs) having a low energetics (an isotropic energy Eiso ≲ 1053 ergs) within the fireshell model has shown how the N(E) spectrum of their prompt emission can be reproduced in a satisfactory way by a convolution of thermal spectra. Nevertheless, from the study of very energetic bursts (Eiso ≳ 1054 ergs) such as, for example, GRB 080319B, some discrepancies between the numerical simulations and the observational data have been observed. We investigate a different spectrum of photons in the comoving frame of the fireshell in order to better reproduce the spectral properties of GRB prompt emission within the fireshell model. We introduce a phenomenologically modified thermal spectrum: a thermal spectrum characterized by a different asymptotic power-law index in the low energy region. Such an index depends on a free parameter α, so that the pure thermal spectrum corresponds to the case α = 0. We test this spectrum by comparing the numerical simulations with the observed prompt emission spectra of various GRBs. From this analysis it has emerged that the observational data can be correctly reproduced by assuming a modified thermal spectrum with α = -1.8.

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.