Barbara Patricelli

A double component in GRB 090618: a proto-black hole and a genuinely long GRB

Context. The joint X-ray and gamma-ray observations of GRB 090618 by very many satellites offer an unprecedented possibility of testing crucial aspects of theoretical models. In particular, they allow us to test (a) in the process of gravitational collapse, the formation of an optically thick e + e − -baryon plasma self-accelerating to Lorentz factors in the range 200 < Γ < 3000; (b) its transparency condition with the emission of a component of 10 53−54 baryons in the TeV region and (c) the collision of these baryons with the circumburst medium (CBM) clouds, characterized by dimensions of 10 15−16 cm. In addition, these observations offer the possibility of testing a new understanding of the thermal and power-law components in the early phase of this GRB. Aims. We test the fireshell model of GRBs in one of the closest (z = 0.54) and most energetic (Eiso = 2.90 × 10 53 erg) GRBs, namely GRB 090618. It was observed at ideal conditions by several satellites, namely Fermi, Swift, Konus-WIND, AGILE, RT-2, and Suzaku, as well as from on-ground optical observatories. Methods. We analyzed the emission from GRB 090618 using several spectral models, with special attention to the thermal and powerlaw components. We determined the fundamental parameters of a canonical GRB within the context of the fireshell model, including the identification of the total energy of the e + e − plasma, E e + e− tot , the proper GRB (P-GRB), the baryon load, the density and structure of the CBM. Results. We find evidence of the existence of two different episodes in GRB 090618. The first episode lasts 50 s and is characterized by a spectrum consisting of a thermal component, which evolves between kT = 54 keV and kT = 12 keV, and a power law with an average index γ = 1.75 ± 0.04. The second episode, which lasts for ∼100 s, behaves as a canonical long GRB with a Lorentz gamma factor at transparency of Γ= 495, a temperature at transparency of 29.22 keV and with a characteristic size of the surrounding clouds of Rcl ∼ 10 15−16 cm and masses of ∼10 22−24 g. Conclusions. We support the recently proposed two-component nature of GRB 090618, namely, episode 1 and episode 2, with a specific theoretical analysis. We furthermore illustrate that episode 1 cannot be considered to be either a GRB or a part of a GRB event, but it appears to be related to the progenitor of the collapsing bare core, leading to the formation of the black hole, which we call a “proto-black hole”. Thus, for the first time, we are witnessing the process of formation of a black hole from the phases just preceding the gravitational collapse all the way up to the GRB emission.

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.

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

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.

Searching for Gamma-Ray counterparts to Gravitational Waves from merging binary neutron stars with the Cherenkov Telescope Array

The merger of binary neutron star (BNS) systems are predicted to be progenitors of short gamma-ray bursts (GRBs); the definitive probe of this association came with the recent detection of gravitational waves (GWs) from a BNS merger by Advanced LIGO and Advanced Virgo (GW170817), in coincidence with the short GRB 170817A observed by Fermi-GBM and INTEGRAL. Short GRBs are also expected to emit very-high energy (VHE, > 100 GeV) photons and VHE electromagnetic (EM) upper limits have been set with observations performed by ground-based gamma-ray detectors and during the intense EM follow-up campaign associated with GW170817/GRB 170817A. In the next years, the searches for VHE EM counterparts will become more effective thanks to the Cherenkov Telescope Array (CTA): this instrument will be fundamental for the EM follow-up of transient GW events at VHE, owing to its unprecedented sensitivity, rapid response (few tens of seconds) and capability to monitor large sky areas via survey-mode operation. We present a comprehensive study on the prospects for joint GW and VHE EM observations of merging BNSs with Advanced LIGO, Advanced Virgo and CTA, based on detailed simulations of the multi-messenger emission and detection. We propose a new observational strategy optimized on the prior assumptions about the EM emission. The method can be further generalized to include other electromagnetic emission models. The proposed strategy will allow CTA to cover most of the region of the GW skymap for the intermediate and most energetic on-axis GRBs associated to the GW event. We estimate the expected joint GW and VHE EM detection rates and we found this rate goes from 0.08 up to 0.5 events per year for the most energetic EM sources. We also found that the with proposed method we can improve the GW sky coverage and the probability of detecting the EM counterparts with respect to the commonly used strategies.

GRB 080916C AND THE HIGH-ENERGY EMISSION IN THE FIRESHELL SCENARIO

In this paper we discuss a possible explanation for the high energy emission (up to ~ GeV) seen in GRB 080916C. We propose that the GeV emission is originated by the collision between relativistic baryons in the fireshell after the transparency and the nucleons located in molecular clouds near the burst site. This collision should give rise pion production, whose immediate decay provides high energy photons, neutrinos and leptons. Using a public code (SYBILL) we simulate these relativistic collisions in their simple form, so that we can draw our preliminar results in this paper. We will present moreover our hypothesis that the delayed onset of this emission identifies in a complete way the P-GRB emission.

Black Holes in Gamma Ray Bursts

Within the fireshell model, Gamma Ray Bursts (GRBs) originate from an optically thick e± plasma created by vacuum polarization process during the formation of a Black Hole (BH). Here we briefly recall the basic features of this model, then we show how it is possible to interpret GRB observational properties within it. In particular we present, as a specific example, the analysis of GRB 050904 observations of the prompt emission light curve and spectrum in the Swift BAT energy band (15–150 keV).

The Electrodynamics of the Core and the Crust components in Neutron Stars

We study the possibility of having a strong electric field (E) in Neutron Stars. We consider a system composed by a core of degenerate relativistic electrons, protons and neutrons, surrounded by an oppositely charged leptonic component and show that at the core surface it is possible to have values of E of the order of the critical value for electron‐positron pair creation, depending on the mass density of the system. We also describe Neutron Stars in general relativity, considering a system composed by the core and an additional component: a crust of white dwarf—like material. We study the characteristics of the crust, in particular we calculate its mass Mcrust. We propose that, when the mass density of the star increases, the core undergoes the process of gravitational collapse to a black hole, leaving the crust as a remnant; we compare Mcrust with the mass of the baryonic remnant considered in the fireshell model of GRBs and find that their values are compatible.

Prospects for joint GW and high-energy EM observations of BNS mergers

— With the recent detection of two transient gravitational wave (GW) signals by the Advanced LIGO interferometers the era of GW astronomy has begun. The two events, labeled GW150914 and GW151226, are both consistent with the inspiral and the merger of a binary system of black holes (BBH). Besides the merger of BBH systems, one of the most promising candidates for the direct GW detection is the coalescence of binary neutron stars (BNS) and black holes (NSBH). These mergers are thought to be connected with short Gamma Ray Bursts (GRBs), but a definitive probe of this association is still missing. Combined observations of gravitational and electromagnetic (EM) signals from these events will provide a unique opportunity to unveil the progenitors of short GRBs and study the physics of compact objects. In particular, large field-of-view instruments such as Fermi will be crucial to observe the high-energy electromagnetic counterparts of transient gravitational wave signals and provide a robust identification based on a precise sky localization. We present the prospects for joint GW and high-energy EM observations of merging BNS systems with Advanced LIGO and Virgo and with Fermi.