Jorge A. Rueda

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.

Relativistic Thomas-Fermi treatment of compressed atoms and compressed nuclear matter cores of stellar dimensions

The Feynman-Metropolis-Teller treatment of compressed atoms is extended to the relativistic regimes. Each atomic configuration is confined by a Wigner-Seitz cell and is characterized by a positive electron Fermi energy. The nonrelativistic treatment assumes a pointlike nucleus and infinite values of the electron Fermi energy can be attained. In the relativistic treatment there exists a limiting configuration, reached when the Wigner-Seitz cell radius equals the radius of the nucleus, with a maximum value of the electron Fermi energy (E{sub e}{sup F}){sub max}, here expressed analytically in the ultrarelativistic approximation. The corrections given by the relativistic Thomas-Fermi-Dirac exchange term are also evaluated and shown to be generally small and negligible in the relativistic high-density regime. The dependence of the relativistic electron Fermi energies by compression for selected nuclei are compared and contrasted to the nonrelativistic ones and to the ones obtained in the uniform approximation. The relativistic Feynman-Metropolis-Teller approach here presented overcomes some difficulties in the Salpeter approximation generally adopted for compressed matter in physics and astrophysics. The treatment is then extrapolated to compressed nuclear matter cores of stellar dimensions with A{approx_equal}(m{sub Planck}/m{sub n}){sup 3}{approx}10{sup 57} or M{sub core}{approx}M{sub {circle_dot}}. A new family of equilibrium configurations exists for selected values ofmorexa0» the electron Fermi energy varying in the range 0<E{sub e}{sup F}{<=}(E{sub e}{sup F}){sub max}. Such configurations fulfill global but not local charge neutrality. They have electric fields on the core surface, increasing for decreasing values of the electron Fermi energy reaching values much larger than the critical value E{sub c}=m{sub e}{sup 2}c{sup 3}/(e{Dirac_h}) for E{sub e}{sup F}=0. We compare and contrast our results with the ones of Thomas-Fermi model in strange stars.«xa0less

On binary-driven hypernovae and their nested late X-ray emission

Context: The induced gravitational collapse (IGC) paradigm addresses the very energetic (10^{52}-10^{54}erg) long gamma-ray bursts (GRBs) associated to supernovae (SNe). Unlike the traditional collapsar model, an evolved FeCO core with a companion neutron star (NS) in a tight binary system is considered as the progenitor. This special class of sources, here named binary driven hypernovae (BdHNe), presents a composite sequence composed of four different episodes [...]. Aims: We first compare and contrast the steep decay, the plateau and the power-law decay of the X-ray luminosities of three selected BdHNe [...]. Second, to explain the different sizes and Lorentz factors of the emitting regions of the four Episodes, [...]. Finally, we show the possible role of r-process, which originates in the binary system of the progenitor.. Methods: We compare and contrast the late X-ray luminosity of the above three BdHNe. We examine correlations between the time at the starting point of the constant late power-law decay, t^*_a, the average prompt luminosity, , and the luminosity at the end of the plateau, L_a. We analyze a thermal emission (~0.97-0.29 keV), observed during the X-ray steep decay phase of GRB 090618. Results: The late X-ray luminosities of the three BdHNe [...] show a precisely constrained nested structure [...]. Conclusions: We confirm a constant slope power-law behavior for the late X-ray luminosity in the source rest-frame, which may lead to a new distance indicator for BdHNe. These results, as well as the emitter size and Lorentz factor, appear to be inconsistent with the traditional afterglow model based on synchrotron emission from an ultra-relativistic [...] collimated jet outflow. We argue, instead, for the possible role of r-process, originating in the binary system, to power the mildly relativistic X-ray source.The induced gravitational collapse (IGC) paradigm addresses energetic (1052-1054 erg), long gamma-ray bursts (GRBs) associated to supernovae (SNe) and proposes as their progenitors tight binary systems composed of an evolved FeCO core and a companion neutron star (NS). Their emission is characterized by four specific episodes: Episode 1, corresponding to the on-set of the FeCO SN explosion and the accretion of the ejecta onto the companion NS; Episode 2, related the collapse of the companionNS to a black hole (BH) and to the emission of a long GRB; Episode 3, observed in X-rays and characterized by a steep decay, a plateau phase and a late power-law decay; Episode 4, corresponding to the optical SN emission due to the 56Ni decay. We focus on Episode 3 and we show that, from the thermal component observed during the steep decay of the prototype GRB 090618, the emission region has a typical dimension of ~1013 cm, which is inconsistent with the typical size of the emitting region of GRBs, e.g., ~1016 cm. We propose, therefore, that the X-ray afterglow emission originates from a spherically symmetric SN ejecta expanding at G ˜ 2 or, possibly, from the accretion onto the newly formed black hole, and we name these systems “binary driven hypernovae” (BdHNe). This interpretation is alternative to the traditional afterglow model based on the GRB synchrotron emission from a collimated jet outflow, expanding at ultra-relativistic Lorentz factor of G ~ 102-103 and originating from the collapse of a single object. We show then that the rest-frame energy band 0.3-10 keV X-ray luminosities of three selected BdHNe, GRB 060729, GRB 061121, and GRB 130427A, evidence a precisely constrained “nested” structure and satisfy precise scaling laws between the average prompt luminosity, 〈Liso〉, and the luminosity at the end of the plateau, La, as functions of the time at the end of the plateau. All these features extend the applicability of the “cosmic candle” nature of Episode 3. The relevance of r-process in fulfilling the demanding scaling laws and the nested structure are indicated.

GRB 090618: The First Example of a Neutron Star Gravitational Collapse to a Black Hole Induced by a Type Ib/c Supernova

A novel concept has recently been proposed for explaining the temporal coincidence of some gamma ray bursts (GRBs) with an associated supernova (SN) in terms of the gravitational collapse of a neutron star (NS) onto a black hole (BH), induced by a type Ib/c SN explosion. We applied these considerations to the exceptional case of GRB 090618, for which there is evidence of an SN ∼ 10 days after the GRB occurrence. We calculated the accretion rate and total accreted mass onto an NS from an SN Ib/c that originated from a companion evolved star. These calculations show that the NS reaches the critical mass in a few seconds and undergoes gravitational collapse onto a BH, leading to the emission of a GRB. We find for the mass of the NS companion, MNS, and for the SN core progenitor, Mcore, the following mass ranges: 1.8 MNS /M� 2. 1a nd 3≤ Mcore/M� ≤ 8. Finally, we discuss the complementarity of these considerations to alternative processes explaining long and short GRBs.

SGR 0418+5729, Swift J1822.3-1606, and 1E 2259+586 as massive, fast-rotating, highly magnetized white dwarfs

Following Malheiro et al. (2012) we describe the so-called low magnetic field magnetars, SGR 0418+5729, Swift J1822.3‐1606, as well as the AXP prototype 1E 2259+586 as massive fast rotating highly magnetized white dwarfs. We give bounds for the mass, radius, moment of inertia, and magnetic field for these sourc es by requesting the stability of realistic general relativ istic uniformly rotating configurations. Based on these parameters, we impr ove the theoretical prediction of the lower limit of the spin down rate of SGR 0418+5729; for a white dwarf close to its maximum stable we obtain the very stringent interval for the spindown rate of 4.1× 10 −16 < ˙ P < 6× 10 −15 , where the upper value is the known observational limit. A lower limit has been also set for Swift J1822.3-1606 for which a fully observationally accepted spin-down rate is still lacking. The white dwarf model provides for this source ˙ P≥ 2.13× 10 −15 , if the star is close to its maximum stable mass. We also present the theoretical expectation of the infrared, optical and ultraviolet emission of these objects and show their consistency with the current available observational data. We give in addition the frequencies at which absorption features could be present in the spectrum of these sources as the result of t he scattering of photons with the quantized electrons by the surface magnetic field.

Induced gravitational collapse at extreme cosmological distances: the case of GRB 090423

Context. The induced gravitational collapse (IGC) scenario has been introduced in order to explain the most energetic gamma ray bursts (GRBs), Eiso = 10 52 10 54 erg, associated with type Ib/c supernovae (SNe). It has led to the concept of binary-driven hypernovae (BdHNe) originating in a tight binary system composed by a FeCO core on the verge of a SN explosion and a companion neutron star (NS). Their evolution is characterized by a rapid sequence of events: 1) the SN explodes, giving birth to a new NS ( NS). The accretion of SN ejecta onto the companion NS increases its mass up to the critical value; 2) the consequent gravitational collapse is triggered, leading to the formation of a black hole (BH) with GRB emission; 3) a novel feature responsible for the emission in the GeV, X-ray, and optical energy range occurs and is characterized by specific power-law behavior in their luminosity evolution and total spectrum; 4) the optical observations of the SN then occurs. Aims. We investigate whether GRB 090423, one of the farthest observed GRB at z = 8:2, is a member of the BdHN family. Methods. We compare and contrast the spectra, the luminosity evolution, and the detectability in the observations by Swift of GRB 090423 with the corresponding ones of the best known BdHN case, GRB 090618. Results. Identification of constant slope power-law behavior in the late X-ray emission of GRB 090423 and its overlapping with the corresponding one in GRB 090618, measured in a common rest frame, represents the main result of this article. This result represents a very significant step on the way to using the scaling law properties, proven in Episode 3 of this BdHN family, as a cosmological standard candle. Conclusions. Having identified GRB 090423 as a member of the BdHN family, we can conclude that SN events, leading to NS formation, can already occur, namely at 650 Myr after the Big Bang. It is then possible that these BdHNe stem from 40 60 M binaries. They are probing the Population II stars after the completion and possible disappearance of Population III stars.

Cooling of young neutron stars in GRB associated to supernovae

Context. The traditional study of neutron star cooling has been generally applied to quite old objects such as the Crab Pulsar (957 years) or the central compact object in Cassiopeia A (330 years) with an observed surface temperature ∼10 6 K. However, recent observations of the late (t = 10 8 –10 9 s) emission of the supernovae (SNe) associated to GRBs (GRB-SN) show a distinctive emission in the X-ray regime consistent with temperatures ∼10 7 –10 8 K. Similar features have been also observed in two Type Ic SNe SN 2002ap and SN 1994I that are not associated to GRBs. Aims. We advance the possibility that the late X-ray emission observed in GRB-SN and in isolated SN is associated to a hot neutron star just formed in the SN event, here defined as a neo-neutron star. Methods. We discuss the thermal evolution of neo-neutron stars in the age regime that spans from ∼1 min (just after the proto-neutron star phase) all the way up to ages <10–100 yr. We examine critically the key factor governing the neo-neutron star cooling with special emphasis on the neutrino emission. We introduce a phenomenological heating source, as well as new boundary conditions, in order to mimic the high temperature of the atmosphere for young neutron stars. In this way we match the neo-neutron star luminosity to the observed late X-ray emission of the GRB-SN events: URCA-1 in GRB980425-SN1998bw, URCA-2 in GRB030329-SN2003dh, and URCA-3 in GRB031203-SN2003lw. Results. We identify the major role played by the neutrino emissivity in the thermal evolution of neo-neutron stars. By calibrating our additional heating source at early times to ∼10 12 –10 15 erg/g/s, we find a striking agreement of the luminosity obtained from the cooling of a neo-neutron stars with the prolonged (t = 10 8 –10 9 s) X-ray emission observed in GRB associated with SN. It is therefore appropriate a revision of the boundary conditions usually used in the thermal cooling theory of neutron stars, to match the proper conditions of the atmosphere at young ages. The traditional thermal processes taking place in the crust might be enhanced by the extreme high-temperature conditions of a neo-neutron star. Additional heating processes that are still not studied within this context, such as e + e − pair creation by overcritical fields, nuclear fusion, and fission energy release, might also take place under such conditions and deserve further analysis. Conclusions. Observation of GRB-SN has shown the possibility of witnessing the thermal evolution of neo-neutron stars. A new campaign of dedicated observations is recommended both of GRB-SN and of isolated Type Ic SN.

General relativistic white dwarfs and their astrophysical implications

We consider applications of general relativistic uniformly-rotating white dwarfs to several astrophysical phenomena related to the spin-up and the spin-down epochs and to delayed type Ia supernova explosions of super-Chandrasekhar white dwarfs, where we estimate the “spinning down” lifetime due to magnetic-dipole braking. In addition, we describe the physical properties of Soft Gamma Repeaters and Anomalous X-Ray Pulsars as massive rapidly-rotating highly-magnetized white dwarfs. Particularly we consider one of the so-called low-magnetic-field magnetars SGR 0418+5729 as a massive rapidly-rotating highly-magnetized white dwarf and give bounds for the mass, radius, moment of inertia, and magnetic field by requiring the general relativistic uniformly-rotating configurations to be stable.

On the Maximum Mass of General Relativistic Uniformly Rotating White Dwarfs

The properties of uniformly rotating white dwarfs are analyzed within the framework of general relativity. Hartles formalism is applied to construct self-consistently the internal and external solutions to the Einstein equations. The mass, the radius, the moment of inertia and quadrupole moment of rotating white dwarfs have been calculated as a function of both the central density and rotation period of the star. The maximum mass of rotating white dwarfs for stable configurations has been obtained.

Relativistic Feynman-Metropolis-Teller treatment at finite temperatures

The Feynman-Metropolis-Teller treatment of compressed atoms has been recently generalized to relativistic regimes and applied to the description of static and rotating white dwarfs in general relativity. We present here the extension of this treatment to the case of finite temperatures and construct the corresponding equation of state (EOS) of the system; applicable in a wide regime of densities that includes both white dwarfs and neutron star outer crusts. We construct the mass-radius relation of white dwarfs at finite temperatures obeying this new EOS and apply it to the analysis of ultra low-mass white dwarfs with