There is mounting evidence for the binary nature of the progenitors of gamma-ray bursts (GRBs). For a long GRB, the induced gravitational collapse (IGC) paradigm proposes as progenitor, or "in-state", a tight binary system composed of a carbon-oxygen core (CO core ) undergoing a supernova (SN) explosion which triggers hypercritical accretion onto a neutron star (NS) companion. For a short GRB, a NS-NS merger is traditionally adopted as the progenitor. We divide long and short GRBs into two sub-classes, depending on whether or not a black hole (BH) is formed in the merger or in the hypercritical accretion process exceeding the NS critical mass. For long bursts, when no BH is formed we have the sub-class of X-ray flashes (XRFs), with isotropic energy E iso 10 52 erg and rest-frame spectral peak energy E p,i 200 keV. When a BH is formed we have the sub-class of binary-driven hypernovae (BdHNe), with E iso 10 52 erg and E p,i 200 keV. In analogy, short bursts are similarly divided into two sub-classes. When no BH is formed, short gamma-ray flashes (S-GRFs) occur, with E iso 10 52 erg and E p,i 2 MeV. When a BH is formed, the authentic short GRBs (S-GRBs) occur, with E iso 10 52 erg and E p,i 2 MeV. We give examples and observational signatures of these four sub-classes and their rate of occurrence. From their respective rates it is possible that "in-states" of S-GRFs and S-GRBs originate from the "out-states" of XRFs. We indicate two additional progenitor systems: white dwarf-NS and BH-NS. These systems have hybrid features between long and short bursts. In the case of S-GRBs and BdHNe evidence is given of the coincidence of the onset of the high energy GeV emission with the birth of a Kerr BH.

Following the induced gravitational collapse (IGC) paradigm of gamma-ray bursts (GRBs) associated with type Ib/c supernovae, we present numerical simulations of the explosion of a carbon-oxygen (CO) core in a binary system with a neutron-star (NS) companion. The supernova ejecta trigger a hypercritical accretion process onto the NS thanks to a copious neutrino emission and the trapping of photons within the accretion flow. We show that temperatures 1-10 MeV develop near the NS surface, hence electron-positron annihilation into neutrinos becomes the main cooling channel leading to accretion rates 10 −9 -10 −1 M s −1 and neutrino luminosities 10 43 -10 52 erg s −1 (the shorter the orbital period the higher the accretion rate). We estimate the maximum orbital period, P max , as a function of the NS initial mass, up to which the NS companion can reach by hypercritical accretion the critical mass for gravitational collapse leading to black-hole (BH) formation. We then estimate the effects of the accreting and orbiting NS companion onto a novel geometry of the supernova ejecta density profile. We present the results of a 1.4 × 10 7 particle simulation which show that the NS induces accentuated asymmetries in the ejecta density around the orbital plane. We elaborate on the observables associated with the above features of the IGC process. We apply this framework to specific GRBs: we find that X-ray flashes (XRFs) and binary-driven hypernovae (BdHNe) are produced in binaries with P > P max and P < P max , respectively. We analyze in detail the case of XRF 060218.

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We present the first three-dimensional (3D) smoothed-particle-hydrodynamics (SPH) simulations of the induced gravitational collapse (IGC) scenario of long-duration gamma-ray bursts (GRBs) associated with supernovae (SNe). We simulate the SN explosion of a carbon-oxygen core (CO core ) forming a binary system with a neutron star (NS) companion. We follow the evolution of the SN ejecta, including their morphological structure, subjected to the gravitational field of both the new NS (νNS) formed at the center of the SN, and the one of the NS companion. We compute the accretion rate of the SN ejecta onto the NS companion as well as onto the νNS from SN matter fallback. We determine the fate of the binary system for a wide parameter space including different CO core and NS companion masses, orbital periods and SN explosion geometry and energies. We identify, for selected NS nuclear equations-of-state, the binary parameters leading the NS companion, by hypercritical accretion, either to the mass-shedding limit, or to the secular axisymmetric instability for gravitational collapse to a black hole (BH), or to a more massive, fast rotating, stable NS. We also assess whether the binary remains or not gravitationally bound after the SN explosion, hence exploring the space of binary and SN explosion parameters leading to νNS-NS and νNS-BH binaries. The consequences of our results for the modeling of long GRBs, i.e. X-ray flashes (XRFs) and binary-driven hypernovae (BdHNe), are discussed. arXiv:1803.04356v3 [astro-ph.HE]

We analyze the early X-ray flares in the GRB "flare-plateau-afterglow" (FPA) phase observed by Swift-XRT. The FPA occurs only in one of the seven GRB subclasses: the binary-driven hypernovae (BdHNe). This subclass consists of long GRBs with a carbon-oxygen core and a neutron star (NS) binary companion as progenitors. The hypercritical accretion of the supernova (SN) ejecta onto the NS can lead to the gravitational collapse of the NS into a black hole. Consequently, one can observe a GRB emission with isotropic energy E iso 10 52 erg, as well as the associated GeV emission and the FPA phase. Previous work had shown that gamma-ray spikes in the prompt emission occur at ∼ 10 15 -10 17 cm with Lorentz gamma factor Γ ∼ 10 2 -10 3 . Using a novel data analysis we show that the time of occurrence, duration, luminosity and total energy of the X-ray flares correlate with E iso . A crucial feature is the observation of thermal emission in the X-ray flares that we show occurs at radii ∼ 10 12 cm with Γ 4. These model independent observations cannot be explained by the "fireball" model, which postulates synchrotron and inverse Compton radiation from a single ultra relativistic jetted emission extending from the prompt to the late afterglow and GeV emission phases. We show that in BdHNe a collision between the GRB and the SN ejecta occurs at 10 10 cm reaching transparency at ∼ 10 12 cm with Γ 4. The agreement between the thermal emission observations and these theoretically derived values validates our model and opens the possibility of testing each BdHN episode with the corresponding Lorentz gamma factor.

We propose that the inner engine of a type I binary-driven hypernova (BdHN) is composed of a Kerr black hole (BH) in a non-stationary state, embedded in a uniform magnetic field B 0 aligned with the BH rotation axis, and surrounded by an ionized plasma of extremely low density of 10 −14 g cm −3 . Using GRB 130427A as a prototype we show that this inner engine acts in a sequence of elementary impulses. Electrons are accelerated to ultra-relativistic energy near the BH horizon and, propagating along the polar axis, θ = 0, they can reach energies of ∼ 10 18 eV, and partially contribute to ultra-high energy cosmic rays (UHECRs). When propagating with θ = 0 through the magnetic field B 0 they give origin by synchrotron emission to GeV and TeV radiation. The mass of BH, M = 2.3M , its spin, α = 0.47, and the value of magnetic field B 0 = 3.48 × 10 10 G, are determined self-consistently in order to fulfill the energetic and the transparency requirement. The repetition time of each elementary impulse of energy E ∼ 10 37 erg, is ∼ 10 −14 s at the beginning of the process, then slowly increasing with time evolution. In principle, this "inner engine" can operate in a GRB for thousands of years. By scaling the BH mass and the magnetic field the same "inner engine" can describe active galactic nuclei (AGN).

On 2018 July 28, GRB 180728A triggered Swift satellites and, soon after the determination of the redshift, we identified this source as a type II binary-driven hypernova (BdHN II) in our model. Consequently, we predicted the appearance time of its associated supernova (SN), which was later confirmed as SN 2018fip. A BdHN II originates in a binary composed of a carbon-oxygen core (CO core ) undergoing SN, and the SN ejecta hypercritically accrete onto a companion neutron star (NS). From the time of the SN shock breakout to the time when the hypercritical accretion starts, we infer the binary separation 3 × 10 10 cm. The accretion explains the prompt emission of isotropic energy 3 × 10 51 erg, lasting ∼ 10 s, and the accompanying observed blackbody emission from a thermal convective instability bubble. The new neutron star (νNS) originating from the SN powers the late afterglow from which a νNS initial spin of 2.5 ms is inferred. We compare GRB 180728A with GRB 130427A, a type I binary-driven hypernova (BdHN I) with isotropic energy > 10 54 erg. For GRB 130427A we have inferred an initially closer binary separation of 10 10 cm, implying a higher accretion rate leading to the collapse of the NS companion with consequent black hole formation, and a faster, 1 ms spinning νNS. In both cases, the optical spectra of the SNe are similar, and not correlated to the energy of the gamma-ray burst. We present three-dimensional smoothed-particle-hydrodynamic simulations and visualisations of the BdHNe I and II.

We recall evidence that long gamma-ray bursts (GRBs) have binary progenitors and give new examples. Binary-driven hypernovae (BdHNe) consist of a carbon-oxygen core (COcore) and a neutron star (NS) companion. For binary periods ∼5 min, the COcore collapse originates the subclass BdHN I characterized by: 1) an energetic supernova (the “SN-rise”); 2) a black hole (BH), born from the NS collapse by SN matter accretion, leading to a GeV emission with luminosity $L_{\rm GeV} = A_{\rm GeV}\, t^{-\alpha _{\rm GeV}}$, observed only in some cases; 3) a new NS (νNS), born from the SN, originating the X-ray afterglow with $L_X = A_{\rm X}\, t^{-\alpha _{\rm X}}$, observed in all BdHN I. We record 378 sources and present for four prototypes GRBs 130427A, 160509A, 180720B and 190114C: 1) spectra, luminosities, SN-rise duration; 2) AX, αX = 1.48 ± 0.32, and 3) the νNS spin time-evolution. We infer a) AGeV, αGeV = 1.19 ± 0.04; b) the BdHN I morphology from time-resolved spectral analysis, three-dimensional simulations, and the GeV emission presence/absence in 54 sources within the Fermi-LAT boresight angle. For 25 sources, we give the integrated and time-varying GeV emission, 29 sources have no GeV emission detected and show X/gamma-ray flares previously inferred as observed along the binary plane. The 25/54 ratio implies the GeV radiation is emitted within a cone of half-opening angle ≈60○ from the normal to the orbital plane. We deduce BH masses 2.3–8.9 M⊙ and spin 0.27–0.87 by explaining the GeV emission from the BH energy extraction, while their time evolution validates the BH mass-energy formula.