Binary-driven hypernova model gains observational support
The change of paradigm in gamma-ray burst (GRBs) physics and astrophysics introduced by the binary driven hypernova (BdHN) model, proposed and applied by the ICRA-ICRANet-INAF members in collaboration with the University of Ferrara and the University of Côte d'Azur, has gained further observational support from the X-ray emission in long GRBs. These novel results are presented in the new article, published on April 20, 2020, in the Astrophysical Journal, co-authored by J. A. Rueda, Remo Ruffini, Mile Karlica, Rahim Moradi, and Yu Wang.
The GRB emission is composed by episodes: from the hard X-ray trigger and the gamma-ray prompt emission, to the high-energy emission in GeV, recently observed also in TeV energies in GRB 190114C, to the X-ray afterglow. The traditional model of GRBs attempts to explain the entire GRB emissions from a single-component progenitor, i.e., from the emission of a relativistic jet originating from a rotating black hole (BH). Differently, the BdHN scenario proposes GRBs originate from a cataclysmic event in the last evolutionary stage of a binary system composed of a carbon-oxygen (CO) star and a neutron star (NS) companion in close orbit. The gravitational collapse of the iron core of the CO star produces a supernova (SN) explosion ejecting the outermost layers of the star, and at the same time, a newborn NS (vNS) at its center. The SN ejecta trigger a hypercritical accretion process onto the NS companion and onto the vNS. Depending on the size of the orbit, the NS may reach, in the case of short orbital periods of the order of minutes, the critical mass for gravitational collapse, hence forming a newborn BH. These systems where a BH is formed are called BdHN of type I. For longer periods, the NS gets more massive but it does not form a BH. These systems are BdHNe II. Three-dimensional simulations of all this process showing the feasibility of its occurrence, from the SN explosion to the formation of the BH, has been recently made possible by the collaboration between ICRANet and the group of Los Alamos National Laboratory (LANL) guided by Prof. C. L. Fryer (see Figure 1).
The role of the BH for the formation of the high-energy GeV emission has been recently presented in the Astrophysical Journal. There, the "inner engine" composed of a Kerr BH, with a magnetic field aligned with the BH rotation axis immersed in a low-density ionized plasma, gives origin, by synchrotron radiation, to the beamed emission in the MeV, GeV, and TeV, currently observed only in some BdHN I, by the Fermi-LAT and MAGIC instruments. In the new publication, the ICRA-ICRANet team addresses the interaction of the vNS with the SN due to hypercritical accretion and pulsar-like emission. They show that the fingerprint of the vNS appears in the X-ray afterglow of long GRBs observed by the XRT detector on board the Niels Gehrels Swift observatory. Therefore, the vNS and the BH have well distinct and different roles in the long GRB observed emission.
The emission from the magnetized vNS and the hypercritical accretion of the SN ejecta into it, gives origin to the afterglow observed in all BdHN I and II subclasses. The early (~few hours) X-ray emission during the afterglow phase is explained by the injection of ultra-relativistic electrons from the vNS into the expanding ejecta, producing synchrotron radiation (see Figure 2). The magnetic field inferred from the synchrotron analysis agrees with the expected toroidal/longitudinal magnetic field component of the vNS. Furthermore, from the analysis of the XRT data of these GRBs at times t>10^4 s, it has been shown that the power-law decaying luminosity is powered by the vNS rotational energy loss by the torque acted upon it by its dipole+quadrupole magnetic. From this, it has been inferred that the vNS possesses a magnetic field of strength ~ 10^12 to 10^13 G, and a rotation period of the order of a millisecond (see Figure 3). It is shown that the inferred millisecond rotation period of the vNS agrees with the conservation of angular momentum in the gravitational collapse of the iron core of the CO star which the vNS came from.
The inferred structure of the magnetic field of the "inner engine" agrees with a scenario in which, along the rotational axis of the BH, it is rooted in the magnetosphere left by the NS that collapsed into a BH.
On the equatorial plane, the field is magnified by magnetic flux conservation.