arXiv subjects

Shmuel Balberg

Publications and source records attributed to Shmuel Balberg.

27 records · Page 2Linked to original sources

arXiv2001

Neutron Stars with a Stable, Light Supersymmetric Baryon

If a light gluino exists, the lightest gluino-containing baryon, the \OSO, is a possible candidate for self-interacting dark matter. In this scenario, the simplest explanation for the observed ratio $Ω_{dm}/Ω_b \approx 6-10$ is that $m_{S^0} \sim 900$\MeVcs; this is not at present excluded by particle physics. Such an \OSO could be present in neutron stars, with hyperon formation serving as an intermediate stage. We calculate equilibrium compositions and equation of state for high density matter with the \OSO, and find that for a wide range of parameters the properties of neutron stars with the \OSO are consistent with observations. In particular, the maximum mass of a nonrotating star is $1.7-1.8 M_\odot$, and the presence of the \OSO is helpful in reconciling observed cooling rates with hyperon formation.

astro-ph↗

arXiv2000

The Properties of Matter in White Dwarfs and Neutron Stars

White dwarfs and neutron stars are stellar objects with masses comparable to that of our sun. However, as the endpoint stages of stellar evolution, these objects do not sustain any thermonuclear burning and therefore can no longer support the gravitational load of their own mass by generating thermal pressure. Rather, matter in their interiors is compressed to much higher densities than commonly found in normal stars, and pressure is created by degenerate fermion kinetic energy and particle interactions. As a result, white dwarfs and neutron stars offer unique cosmic laboratories for studying matter at very high densities. In this review we discuss the basic properties of condensed matter at extreme densities and summarize the extent to which these properties can be examined by observations of compact objects.

astro-ph↗

arXiv2000

Black Hole Emergence in Supernovae

If a black hole formed in a core-collapse supernova is accreting material from the base of the envelope, the accretion luminosity could be observable in the supernova light curve. Here we continue the study of matter fall back onto a black hole in the wake of a supernova and examine realistic supernovae models which allow for an early emergence of the accretion luminosity. Such cases may provide a direct observational identification of the black hole formed in the aftermath of the explosion. Our approach combines analytic estimates and fully relativistic, radiation-hydrodynamic numerical computations. We employ a numerical hydrodynamical scaling technique to accommodate the diverse range of dynamical time scales in a single simulation. We find that while in typical Type II supernovae heating by radioactive decays dominates the late-time light curve, low-energy explosions of more massive stars should provide an important exception where the accretion luminosity will emerge while it is still relatively large. Our main focus is on the only current candidate for such an observation, the very unusual SN1997D. Due to the low energy of the explosion and the very small ($2\times10^{-3} M_\sun$) inferred mass of Co56 in the ejected envelope, we find that accretion should become the dominant source of its luminosity during the year 2000. The total luminosity at emergence is expected to lie in the range $0.5-3\times10^{36} $ ergs/s, potentially detectable with HST. We also discuss the more favorable case of explosions which eject negligible amounts of radioactive isotopes and find that the black hole is likely to emerge a few tens of days after the explosion, with a luminosity of $\sim 10^{37} $\ergss.

astro-ph↗

arXiv1999

Black Hole Emergence in Supernovae

If a black hole formed in a core-collapse supernova is accreting material from the base of the envelope, the accretion luminosity could be observable in the supernova light curve. We present results of a fully relativistic numerical investigation of the fallback of matter onto a black hole in a supernova and examine conditions which would be favorable for detection of the black hole. In general, heating by radioactive decays is likely to prevent practical detection of the black hole, but we show that low energy explosions of more massive stars may provide an important exception. We emphasize the particular case of SN1997D in NGC1536, for which we predict that the presence of a black hole could be inferred observationally within the next year.

astro-ph↗

arXiv1998

Roles of Hyperons in Neutron Stars

We examine the roles the presence of hyperons in the cores of neutron stars may play in determining global properties of these stars. The study is based on estimates that hyperons appear in neutron star matter at about twice the nuclear saturation density, and emphasis is placed on effects that can be attributed to the general multi-species composition of the matter, hence being only weakly dependent on the specific modeling of strong interactions. Our analysis indicates that hyperon formation not only softens the equation of state but also severely constrains its values at high densities. Correspondingly, the valid range for the maximum neutron star mass is limited to about 1.5-1.8 $M_\odot$, which is a much narrower range than available when hyperon formation is ignored. Effects concerning neutron star radii and rotational evolution are suggested, and we demonstrate that the effect of hyperons on the equation of state allows a reconciliation of observed pulsar glitches with a low neutron star maximum mass. We discuss the effects hyperons may have on neutron star cooling rates, including recent results which indicate that hyperons may also couple to a superfluid state in high density matter. We compare nuclear matter to matter with hyperons and show that once hyperons accumulate in neutron star matter they reduce the likelihood of a meson condensate, but increase the susceptibility to baryon deconfinement, which could result in a mixed baryon-quark matter phase.

astro-ph↗

arXiv1998

Implications of Hyperon Pairing for Cooling of Neutron Stars

The implications of hyperon pairing for the thermal evolution of neutron stars containing hyperons are investigated. The outcome of cooling simulations are compared for neutron star models composed only of nucleons and leptons, models including hyperons, and models including pairing of hyperons. We show that lambda and neutron pairing suppresses all possible fast neutrino emission processes in not too massive neutron stars. The inclusion of lambda pairing yields better agreement with X-ray observations of pulsars. Particularly, the surface temperatures deduced from X-ray observations within the hydrogen atmosphere model are more consistent with the thermal history of neutron stars containing hyperons, if the critical temperature for the onset of lambda and nucleon pairing is not too small.

astro-ph↗

arXiv1997

S-wave Pairing of $Λ$ Hyperons in Dense Matter

In this work we calculate the $^1S_0$ gap energies of $Λ$ hyperons in neutron star matter. The calculation is based on a solution of the BCS gap equation for an effective G-matrix parameterization of the $Λ-Λ$ interaction with a nuclear matter background, presented recently by Lanskoy and Yamamoto. We find that a gap energy of a few tenths of MeV is expected for $Λ$ Fermi momenta up to about 1.3 fm$^{-1}$. Implications for neutron star matter are examined, and suggest the existence of a $Λ$ $^1S_0$ superfluid between the threshold baryon density for $Λ$ formation and the baryon density where the $Λ$ fraction reaches $15-20%$.

nucl-th↗

arXiv1997

Constraints on the Existence of Strange Quark Stars

Creation of strange quark stars through strong interaction deconfinement is studied based on modern estimates of hyperon formation in neutron stars. The hyperon abundance is shown to be large enough so that if strange quark matter (SQM) is the true ground state of matter, the deconfinement density should be at most 2.5-3 times the nuclear saturation density. If so, deconfinement occurs in neutron stars at birth, and all neutron stars must be strange quark stars. Alternatively, sould observation indicate that some neutron stars have a baryonic interior, SQM is unlikely to be absolutely stable.

astro-ph↗

arXiv1997

An Effective Equation of State for Dense Matter with Strangeness

An effective equation of state (EoS) which generalizes the Lattimer-Swesty equation for nuclear matter is presented for matter at supernuclear densities including strange baryons. It contains an adjustable baryon potential energy density, based on models of local potentials for the baryon-baryon interactions. The features of the equation rely on the properties of nuclei for the nucleon-nucleon interactions, and mainly on experimental data from hypernuclei for the hyperon-nucleon and hyperon-hyperon interactions. The EoS is used to calculate equilibrium compositions and thermodynamic properties of high density matter with strangeness in two astrophysical contexts: neutron star matter (transparent to neutrinos) and proto-neutron star matter (opaque to neutrinos). The effective EoS reproduces typical properties of high density matter found in theoretical microscopic models. Of these, the main result is that hyperons appear in both types of matter at about twice the nuclear saturation density, and that their appearance significantly softens the EoS. The range of maximal masses of neutron stars found in a comprehensive parameter survey is 1.4-1.7 $M_\odot$. Another typical result is that the maximal mass of a proto-neutron star with strange baryons is higher than that of an evolved neutron star (opposite to the case of nuclear matter), setting the stage for a "delayed collapse" scenario.

nucl-th↗