arXiv subjects

I. Bombaci

Publications and source records attributed to I. Bombaci.

28 records · Page 2Linked to original sources

arXiv2003

Gamma Ray Bursts from delayed collapse of neutron stars to quark matter stars

We propose a model to explain how a Gamma Rays Burst can take place days or years after a supernova explosion. Our model is based on the conversion of a pure hadronic star (neutron star) into a star made at least in part of deconfined quark matter. The conversion process can be delayed if the surface tension at the interface between hadronic and deconfined-quark-matter phases is taken into account. The nucleation time (i.e. the time to form a critical-size drop of quark matter) can be extremely long if the mass of the star is small. Via mass accretion the nucleation time can be dramaticaly reduced and the star is finally converted into the stable configuration. A huge amount of energy, of the order of 10$^{52}$--10$^{53}$ erg, is released during the conversion process and can produce a powerful Gamma Ray Burst. The delay between the supernova explosion generating the metastable neutron star and the new collapse can explain the delay proposed in GRB990705 and in GRB011211.

astro-ph↗

arXiv2002

Equation of state and magnetic susceptibility of spin polarized isospin asymmetric nuclear matter

Properties of spin polarized isospin asymmetric nuclear matter are studied within the framework of the Brueckner--Hartree--Fock formalism. The single-particle potentials of neutrons and protons with spin up and down are determined for several values of the neutron and proton spin polarizations and the asymmetry parameter. It is found an almost linear and symmetric variation of the single-particle potentials as increasing these parameters. An analytic parametrization of the total energy per particle as a function of the asymmetry and spin polarizations is constructed. This parametrization is employed to compute the magnetic susceptibility of nuclear matter for several values of the asymmetry from neutron to symmetric matter. The results show no indication of a ferromagnetic transition at any density for any asymmetry of nuclear matter.

nucl-th↗

arXiv2002

Strange star candidates

In this talk we discuss possible signatures for the existence of strange quark matter stars (strange stars).

astro-ph↗

arXiv2002

Timing evolution of accreting strange stars

It has been suggested that the QPO phenomenon in LMXB's could be explained when the central compact object is a strange star. In this work we investigate within a standard model for disk accretion whether the observed clustering of spin frequencies in a narrow band is in accordance with this hypothesis. We show that frequency clustering occurs for accreting strange stars when typical values of the parameters of magnetic field initial strength and decay time, accretion rate are chosen. In contrast to hybrid star accretion no mass clustering effect is found.

astro-ph↗

arXiv2001

Asymmetric Nuclear Matter from Extended Brueckner-Hartree-Fock Approach

The properties of isospin-asymmetric nuclear matter have been investigated in the framework of the extended Brueckner-Hartree-Fock approximation at zero temperature. Self-consistent calculations using the Argonne $V_{14}$ interaction are reported for several asymmetry parameters $β= \frac{N - Z}{A}$ ranging from symmetric nuclear matter to pure neutron matter. The binding energy per nucleon fulfills the $β^2$ law in the whole asymmetry range. The symmetry energy is calculated for different densities and discussed in comparison with other predictions. At the saturation point it is in fairly good agreement with the empirical value. The present approximation, based on the Landau definition of quasiparticle energy, is investigated in terms of the Hugenholtz-Van Hove theorem, which is proved to be fulfilled with a good accuracy at various asymmetries. The isospin dependence of the single-particle properties is discussed, including mean field, effective mass, and mean free path of neutrons and protons. The isospin effects in nuclear physics and nuclear astrophysics are briefly discussed.

nucl-th↗

arXiv2000

Rapidly rotating strange stars for a new equation of state of strange quark matter

For a new equation of state of strange quark matter, we construct equilibrium sequences of rapidly rotating strange stars in general relativity. The sequences are the normal and supramassive evolutionary sequences of constant rest mass. We also calculate equilibrium sequences for a constant value of $Ω$ corresponding to the most rapidly rotating pulsar PSR 1937 + 21. In addition to this, we calculate the radius of the marginally stable orbit and its dependence on $Ω$, relevant for modeling of kilo-Hertz quasi-periodic oscillations in X-ray binaries.

astro-ph↗

arXiv1999

Is SAX J1808.4-3658 a Strange Star ?

One of the most important questions in the study of compact objects is the nature of pulsars, including whether they are composed of $β$-stable nuclear matter or strange quark matter. Observations of the newly discovered millisecond X-ray pulsar \sax with the Rossi X-Ray Timing Explorer place firm constraint on the radius of the compact star. Comparing the mass - radius relation of \sax with the theoretical mass - radius relation for neutron stars and for strange stars, we find that a strange star model is more consistent with SAX J1808.4-3658, and suggest that it is a likely strange star candidate.

hep-ph↗

arXiv1998

Equilibrium Sequences of Rotating Neutron Stars for New Microscopic Equations of State

For four newly suggested microscopic equations of state of neutron star matter, we construct equilibrium sequences of rapidly rotating neutron stars in general relativity. The sequences are the normal and supramassive evolutionary sequences of constant rest mass. We find that for these equations of state the maximum (gravitational) mass rotating models occur (in central density and rotation rate $Ω$) before the maximum--$Ω$ models. We calculate equilibrium sequences for a constant value of $Ω$ corresponding to the most rapidly rotating pulsar PSR 1937+21. Also calculated is the radius of the marginally stable orbit and its dependence on $Ω$.

astro-ph↗

arXiv1997

Microscopic nuclear equation of state with three-body forces and neutron star structure

We calculate static properties of non-rotating neutron stars (NS's) using a microscopic equation of state (EOS) for asymmetric nuclear matter, derived from the Brueckner-Bethe-Goldstone many-body theory with explicit three-body forces. We use the Argonne AV14 and the Paris two-body nuclear force, implemented by the Urbana model for the three-body force. We obtain a maximum mass configuration with $ M_{max} = 1.8 M_{\sun}$ ($M_{max} = 1.94 M_{\sun}$) when the AV14 (Paris) interaction is used. They are both consistent with the observed range of NS masses. The onset of direct Urca processes occurs at densities $n \geq 0.65~fm^{-3}$ for the AV14 potential and $n \geq 0.54~fm^{-3}$ for the Paris potential. Therefore, NS's with masses above $M^{Urca} = 1.4 M_{\sun}$ for the AV14 and $M^{Urca} = 1.24 M_{\sun}$ for the Paris potential can undergo very rapid cooling, depending on the strength of superfluidity in the interior of the NS. The comparison with other microscopic models for the EOS shows noticeable differences.

astro-ph↗

arXiv1996

Microscopic Nuclear Equation of State with Three-Body Forces and Neutron Star Structure

We calculate static properties of non-rotating neutron stars (NS's) using a microscopic equation of state (EOS) for asymmetric nuclear matter. The EOS is computed in the framework of the Brueckner--Bethe--Goldstone many--body theory. We introduce three-body forces in order to reproduce the correct saturation point of nuclear matter. A microscopic well behaved EOS is derived. We obtain a maximum mass configuration with $M_{max} = 1.8 M_\odot$, a radius $R = 9.7$ km and a central density $n_c = 1.34~fm^{-3}$. We find the proton fraction exceeds the critical value $x^{Urca}$, for the onset of direct Urca processes, at densities $n \geq 0.45~fm^{-3}$. Therefore, in our model, NS's with masses above $M^{Urca} = 0.96 M_\odot$ can undergo very rapid cooling depending on whether or not nucleon superfluidity in the interior of the NS takes place. A comparison with other microscopic models for the EOS is done, and neutron star structure is calculated for these models too.

nucl-th↗