arXiv subjects
Publications and source records attributed to Ignazio Bombaci.
We discuss the role of strangeness on the internal constitution and structural properties of neutron stars. In particular, we report on recent calculations of hyperon star properties derived from microscopic equations of state for hyperonic matter. Next, we discuss the possibility of having a strange quark matter core in a neutron star, or the possible existence of strange quark matter stars, the so-called strange stars.
In a very recent paper Shaposhnikov et al. (ApJ 593, 2003, L35) have extracted very tight constraints on the radius and the mass for the compact star in the low mass X-ray binary 4U~1728-34, from the analysis of a set of Type I X-ray bursts from this source. In the present letter, we perform a systematic comparison between the mass-radius (MR) relation given by Shaposhnikov et al., with the theoretical determination of MR curves for compact stars using some of the most recent and realistic models for the equation of state for stellar dense matter. Our study clearly reveals that the semi-empirical MR relation for the compact star in 4U~1728-34 is not compatible with models of neutrons stars composed of nuclear matter or hyperonic matter, while it is consistent with strange stars or neutron stars with a core of deconfined quark matter (hybrid neutron stars).
We calculate the neutrino mean free path and the Equation of State of pure neutron matter at finite temperature within a selfconsistent scheme based on the Brueckner--Hartree--Fock approximation. We employ the nucleon-nucleon part of the recent realistic baryon-baryon interaction (model NSC97e) constructed by the Nijmegen group. The temperatures considered range from 10 to 80 MeV. We report on the calculation of the mean field, the residual interaction and the neutrino mean free path including short and long range correlations given by the Brueckner--Hartree--Fock plus Random Phase Approximation (BHF+RPA) framework. This is the first fully consistent calculation in hot neutron matter dedicated to neutrino mean free path. We compare systematically our results to those obtain with the D1P Gogny effective interaction, which is independent of the temperature. The main differences between the present calculation and those with nuclear effective interactions come from the RPA corrections to BHF (a factor of about 8) while the temperature lack of consistency accounts for a factor of about 2.
We compute the temperature profiles of accretion discs around rapidly rotating strange stars, using constant gravitational mass equilibrium sequences of these objects, considering the full effect of general relativity. Beyond a certain critical value of stellar angular momentum ($J$), we observe the radius ($r_{\rm orb}$) of the innermost stable circular orbit (ISCO) to increase with J (a property seen neither in rotating black holes nor in rotating neutron stars). The reason for this is traced to the crucial dependence of $dr_{\rm orb}/dJ$ on the rate of change of the radial gradient of the Keplerian angular velocity at $r_{\rm orb}$ with respect to $J$. The structure parameters and temperature profiles obtained are compared with those of neutron stars, as an attempt to provide signatures for distinguishing between the two. We show that when the full gamut of strange star equation of state models, with varying degrees of stiffness are considered, there exists a substantial overlap in properties of both neutron stars and strange stars. However, applying accretion disc model constraints to rule out stiff strange star equation of state models, we notice that neutron stars and strange stars exclusively occupy certain parameter spaces. This result implies the possibility of distinguishing these objects from each other by sensitive observations through future X-ray detectors.
Definitely, an affirmative answer to this question would have implications of fundamental importance for astrophysics (a new class of compact stars), and for the physics of strong interactions (deconfined phase of quark matter, and strange matter hypothesis). In the present work, we use observational data for the newly discovered millisecond X-ray pulsar SAX J1808.4-3658 and for the atoll source 4U 1728-34 to constrain the radius of the underlying compact stars. Comparing the mass-radius relation of these two compact stars with theoretical models for both neutron stars and strange stars, we argue that a strange star model is more consistent with SAX J1808.4-3658 and 4U 1728-34, and suggest that they are likely strange star candidates.
We study the conversion of a neutron star to a strange star as a possible energy source for gamma-ray bursts. We use different recent models for the equation of state of neutron star matter and strange quark matter. We show that the total amount of energy liberated in the conversion is in the range of (1-4) 10^{53} ergs (one order of magnitude larger than previous estimates) and is in agreement with the energy required to power gamma-ray burst sources at cosmological distances.
There are about 2000 gamma ray burst (GRB) events known to us with data pouring in at the rate of one per day. While the afterglows of GRBs in radio, optical and X-ray bands are successfully explained by the fireball model, a significant difficulty with the proposed mechanisms for GRBs is that a small amount ($\le 10^{-6} M_{\odot}$) of baryons in the ejecta can be involved. There are very few models that fulfill this criteria together with other observational features, among which are the differentially rotating collapsed object model and the "supernova" model. These models generally invoke rapidly rotating neutron stars, and may be subject to uncertainties in the formation mechanisms and the equations of state of neutron stars. According to Spruit, the problem of making a GRB from an X-ray binary is reduced to finding a plausible way to make the star rotate differentially. We suggest that a model of strange star (SS) can naturally explain many of these bursts with not only their low baryon content, but the differential rotation which leads to an enhanced magnetic field that surfaces up and is responsible for GRBs.
Kilohertz quasi-periodic brightness oscillations (kHz QPOs) observed in certain X-ray burst sources may represent Keplerian frequencies in the inner regions of the accretion disk in such systems. If this assumption is strictly adhered to, we show here that if the central accretor in stellar X-ray burst sources is a strange star (made up of u, d and s quarks in beta equilibrium, referred to as strange matter) then the calculated QPO frequencies are reconcilable with the observed QPO frequencies (corresponding to the highest frequency of 1.22 kHz, observed so far from the source 4U 1636-53) only for particular values of the QCD-related parameters which describe the equation of state of strange matter. We demonstrate that QPO frequencies in the very high range (1.9-3.1) kHz can be understood in terms of a (non- magnetized) strange star X-ray binary (SSXB) rather than a neutron star X-ray binary (NSXB). Future discovery of such high frequency QPOs from X-ray burst sources will constitute a new astrophysical di- agnostic for identifying solar mass range stable strange stars in our galaxy.
The discovery of kilohertz quasi-periodic oscillations (kHz QPOs) in low-mass X-ray binaries (LMXBs) with the Rossi X-ray Timing Explorer has stimulated extensive studies of these sources. Recently, Osherovich & Titarchuk suggest a new model for kHz QPOs and the related correlations between kHz QPOs and low frequency features in LMXBs. Here we use their results to study the mass - radius relation for the atoll source 4U 1728-34. We find that, if this model is correct, 4U 1728-34 is possibly a strange star rather a neutron star.
We derive an equation of state (EOS) for strange matter, starting from an interquark potential which (i) has asymptotic freedom built into it, (ii) shows confinement at zero baryon density and deconfinement at high density and (iii) gives a stable configuration for chargeless, beta--stable quark matter. This EOS is then used to calculate the structure of Strange Stars, and in particular their mass-radius relation. Our present results confirm and reinforce the recent claim by Li et al (Astron. and Astrophys. 303 (1995) L1) and Bombaci (Phys.Rev. C55 (1997) 1587) that the compact objects associated with the x-ray pulsar Her X-1, and with the x-ray burster 4U 1820-30 are strange stars.
We investigate the structure of neutron stars shortly after they are born, when the entropy per baryon is of order 1 or 2 and neutrinos are trapped on dynamical timescales. In all cases, the thermal effects for an entropy per baryon of order 2 or less are small when considering the maximum neutron star mass. Neutrino trapping, however, significantly changes the maximum mass due to the abundance of electrons. When matter is allowed to contain only nucleons and leptons, trapping decreases the maximum mass by an amount comparable to, but somewhat larger than, the increase due to finite entropy. When matter is allowed to contain strongly interacting negatively charged particles, in the form of strange baryons, a kaon condensate, or quarks, trapping instead results in an increase in the maximum mass of order $0.2M_\odot$, which adds to the effects of finite entropy. The presence of negatively-charged particles has two major implications. First, the value of the maximum mass will decrease during the early evolution of a neutron star as it loses trapped neutrinos, so that if a black hole forms, it either does so immediately after the bounce or it is delayed for a neutrino diffusion timescale of $\sim 10$ s. The latter case is most likely if the maximum mass of the hot star with trapped neutrinos is near $1.5M_\odot$. In the absence of negatively-charged hadrons, black hole formation would be due to accretion and therefore is likely to occur only immediately after bounce. Second, the appearance of hadronic negative charges results in a general softening of the equation of state that may be observable in the neutrino luminosities and average energies. Further, these additional negative charges decrease the electron fraction and may be observed in the relative excess of electron neutrinos compared to other neutrinos.