arXiv subjects

G. E. Brown

Publications and source records attributed to G. E. Brown.

At least 73 records · Page 4Linked to original sources

arXiv1999

High Mass Black Holes in Soft X-Ray Transients: Gap in Black Hole Masses ?

We suggest that high-mass black holes; i.e., black holes of several solar masses, can be formed in binaries with low-mass main-sequence companions, provided that the hydrogen envelope of the massive star is removed in common envelope evolution which begins only after the massive star has finished He core burning. Our evolution scenario naturally explains the gap (low probability region) in the observed black hole masses.

astro-ph↗

arXiv1999

The Blandford-Znajek Process as a Centeral Engine for a Gamma Ray Burst

We investigate the possibility that gamma-ray bursts are powered by a central engine consisting of a black hole with an external magnetic field anchored in a surrounding disk or torus. The energy source is then the rotation of the black hole, and it is extracted electromagnetically via a Poynting flux, a mechanism first proposed by Blandford and Znajek (1997) for AGN. Accounting both for the maximum rotation energy of the hole and for the efficiency of electromagnetic extraction, we find that a maximum of 9% of the rest mass of the hole can be converted to a Poynting flow, i.e. the energy available to produce a gamma-ray burst is 1.6 X 10^{53}M/M_{sun}erg for a black hole of mass M. We show that the black holes formed in a variety of gamma-rayburst scenarios probably contain the required high angular momentum. To extract the energy from a black hole in the required time \lsim1000s a field of 10^{15}G near the black hole is needed. We give an example of a disk-plus-field structure that both delivers the required field and makes the Poynting flux from the hole dominate that ofthe disk. Thereby we demonstrate that the Poynting energy extracted need not be dominated by the disk, nor is limited to the binding energy of the disk. This means that the Blandford-Znajek mechanism remains a very good candidate for powering gamma-ray bursts.

astro-ph↗

arXiv1999

Blandford-Znajek process as a gamma ray burst central engine

We investigate the possibility that gamma-ray bursts are powered by a central engine consisting of a black hole with an external magnetic field supported by a surrounding disk or torus. The rotational energy of the black hole can be extracted electromagnetically as a Poynting flux, a mechanism proposed by Blandford and Znajek(1977). Recently observed magnetars indicate that some compact objects have very high magnetic fields, up to $10^{15}$ G, which is required to extract the energy within the duration of a GRB, i.e., in 1000 s or less. We demonstrate also that the Poynting flux need not be substantially dominated by the disk.

astro-ph↗

arXiv1999

Brightness from the Blackest Night: Bursts of Gamma Rays and Gravity Waves from Black Hole Binaries

We use recent results in binary stellar evolution to argue that binaries with at least one black hole dominate the rate of compact-object mergers. Two phenomena generally attributable to such mergers, gamma-ray bursts and gravity-wave bursts, are therefore likely to originate from near the event horizon of a black hole. In addition to sheer numbers, black holes have an added advantage over neutron stars in both phenomena. For gamma-ray bursts, the presence of an event horizon eases the baryon pollution problem, because energy can be stored into rotation until most baryons have been swallowed, and then released into a cleaner environment via the Blandford-Znajek process. For gravity-wave bursts, black holes offer higher luminosities due to their higher masses, thus enabling detection out to larger distances, which leads to a 30-fold increase in the predicted LIGO event rate.

astro-ph↗

arXiv1999

Two-Pion Exchange Interaction Between Constituent Quarks

The two-pion exchange interaction between constituent quarks is shown to enhance the effect of the the isospin dependent spin-spin component of the one-pion exchange interaction, and to cancel out its tensor component. It therefore provides a partial explanation for the phenomenological observation that the hyperfine interaction between constituent quarks is well described by a flavor dependent spin-spin interaction, which is attractive at short and repulsive at long range. The spin-orbit component of the two-pion exchange interaction is stronger than and has the opposite sign from that associated with the linear confining interaction in the $P-$shell multiplets.

hep-ph↗

arXiv1999

The Formation of High-Mass Black Holes in Low Mass X-ray Binaries

In this note we suggest that high-mass black holes; i.e., black holes of several solar masses, can be formed in binaries with low-mass main-sequence companions, provided that the hydrogen envelope of the massive star is removed in common envelope evolution which begins only after the massive star has finished He core burning. That is, the massive star is in the supergiant stage, which lasts only $\sim 10^4$ years, so effects of mass loss by He winds are small. Since the removal of the hydrogen envelope of the massive star occurs so late, it evolves essentially as a single star, rather than one in a binary. Thus, we can use evolutionary calculations of Woosley & Weaver (1995) of single stars. We find that the black holes in transient sources can be formed from stars with ZAMS masses in the interval $20-35\msun$. The black hole mass is only slightly smaller than the He core mass, typically $\sim 7\msun$.

astro-ph↗

arXiv1999

Contribution of High-Mass Black Holes to Mergers of Compact Binaries

We consider the merging of high-mass black-hole binaries of the Cyg X-1 type, in which the O-star companion becomes a neutron star following a supernova explosion. Our estimated rate of mergers, $\sim 2\times 10^{-5} yr^{-1}$ for the Galaxy, is relatively low because of the paucity of high-mass black holes. None the less, because of their high masses, these black-hole, neutron-star binaries could contribute importantly to the merging sought by LIGO. From stellar evolutionary calculations including mass loss, we estimate that a ZAMS mass of $\sim 80 \msun$ is necessary before a high-mass black hole can result in a massive binary. We estimate the detection rate for LIGO of high-mass black hole neutron-star mergers to be a factor $\sim 40$ greater than that for binary neutron stars. We suggest how our high cut-off mass can be reconciled with the requirements of nucleosynthesis, and show that a bimodal distribution with masses of black holes can account, at least qualitatively, for the many transient sources which contain high-mass black holes.

astro-ph↗

arXiv1999

A Schematic Model For Density-Dependent Vector Meson Masses

A schematic two-level model consisting of a "collective" bosonic state and an "elementary" meson is constructed that provides interpolation from a hadronic description (a la Rapp/Wambach) to B/R scaling for the description of properties of vector mesons in dense medium. The development is based on a close analogy to the degenerate schematic model of Brown for giant resonances in nuclei.

nucl-th↗

arXiv1998

Medium Dependence of the Vector-Meson Mass: Dynamical and/or Brown-Rho Scaling?

We discuss the similarities and differences for the theories of Rapp, Wambach and collaborators (called R/W in short) and those based on Brown-Rho scaling (called B/R), as applied to reproduce the dileptons measured by the CERES collaboration in the CERN experiments. In both theories the large number of dileptons at invariant masses $\sim$~$m_ρ/2$ are shown to be chiefly produced by a density-dependent $ρ$-meson mass. In R/W the medium dependence is dynamically calculated using hadronic variables defined in the matter-free vacuum. In B/R scaling it follows from movement towards chiral symmetry restoration due to medium-induced vacuum change, and is described in terms of constituent (or quasiparticle) quarks. We argue that the R/W description should be reliable up to densities somewhat beyond nuclear density, where hadrons are the effective variables. At higher density there should be a crossover to constituent quarks as effective variables scaling according to B/R. In the crossover region, the two descriptions must be ``dual''.

nucl-th↗

arXiv1998

K^+ versus Λflow in relativistic heavy-ion collisions

We study $K^+$ and $Λ$ flow in heavy-ion collisions at beam energies of about 2A GeV. We present our results in both the `traditional' (i.e., in terms of the average transverse momentum in the reaction plane) as well as `modern' (i.e., in terms of coefficients of the Fourier analysis of azimuthal distributions) methods of flow analysis. We find significant differences between the $K^+$ and the $Λ$ flow: while the $Λ$ flow is basically similar to that of nucleons, the $K^+$ flow almost disappears. This difference is attributed chiefly to their different mean field potentials in dense matter. The comparisons with the experimental data, as well as theoretical results from independent calculations, indicate clearly the pivotal roles of both $K^+$ and $Λ$ medium effects. We emphasize that similar experimental data from independent collaborations are essential for the eventual verification of these medium effects.

nucl-th↗

arXiv1998

Evolution of Binary Compact Objects Which Merge

Beginning from massive binaries in the Galaxy we evolve black-hole, neutron-star binaries and binary neutron stars, such as the Hulse-Taylor system PSR 1913+16. The new point in our evolution is a quantitative calculation of the accretion of matter by a neutron star in common envelope evolution which sends it into a black hole. We calculate the mass of the latter to be $\sim 2.4\msun$. Our chief conclusion is that the production rate for black-hole, neutron-star binaries (in which the neutron star is unrecycled) is $\sim 10^{-4}$ per year per Galaxy, an order of magnitude greater than that of neutron star binaries. Not only should this result in a factor of $\sim$ 10 more mergings for gravitational wave detectors like LIGO, but the signal should be larger. We give some discussion of why black-hole, neutron-star binaries have not been observed, but conclude that they should be actively searched for.

astro-ph↗

arXiv1998

$K^-/K^+$ ratios in relativistic heavy-ion collisions

We study $K^-/K^+$ ratios as a function of centrality (participant nucleon number), transverse mass ($m_t$), and rapidity, in heavy-ion collisions at beam energies between 1A and 2A GeV. We use the relativistic transport model that includes expicitly the strangeness degrees of freedom and consider two scenarios for kaon properties in dense matter, one with and one without medium modifications of their properties. In both scenarios, The $K^-/K^+$ ratio does not change very much with the centrality, while the $K/π$ and ${\bar K}/π$ ratios increase with increasing centrality. Significant differences are predicted, both in magnitudes and shapes, for the $m_t$ spectra and rapidity distributions of $K^-/K^+$ ratio. Experimental measurement of these ratios, currently under investigation by the FOPI, KaoS, E866, and E895 collaborations, will be useful in revealing the kaon in-medium properties.

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arXiv1997

Electromagnetic Probes of Dense Matter in Relativistic Heavy-Ion Collisions

Dilepton and photon production in heavy-ion collisions at SPS energies is studied in the relativistic transport model that incorporates self-consistently the change of hadron masses in dense matter. It is found that the dilepton spectra in proton-nucleus reactions can be well described by the conventional mechanisms of Dalitz decay, primary vector meson decay, decay of charmed mesons, and the initial Drell-Yan processes. However, to provide a quantitative explanation of the observed dilepton spectra in central heavy-ion collisions requires contributions other than these direct decays and also various medium effects. Introducing a decrease of vector meson masses in hot dense medium, we find that the low-mass dilepton enhancement can be satisfactorily explained. Furthermore, to explain the intermediate-mass dilepton enhancement in heavy-ion collisions, secondary processes such as $πa_1\to l{\bar l}$ are found to be very important. Finally, the single photon spectra in our calculations with either free or in-medium meson masses do not exceed the upper limit measured by the WA80 Collaboration.

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arXiv1997

Photon Rates for Heavy-Ion Collisions from Hidden Local Symmetry

We study photon production from the hidden local symmetry approach that includes pions, rho and a1 mesons and compute the corresponding photon emission rates from a hadronic gas in thermal equilibrium. Together with experimental radiative decay widths of the background, these rates are used in a relativistic transport model to calculate single photon spectra in heavy-ion collisions at SPS energies. We then employ this effective theory to test three scenarios for the chiral phase transition in high-temperature nuclear matter including decreasing vector meson masses. Although all calculations respect the upper bound set by the WA80 Collaboration, we find the scenarios could be distinguished with more detailed data.

nucl-th↗

arXiv1997

Strange Particles in Dense Matter and Kaon Condensates

We discuss the role of strangeness in dense matter and especially in neutron stars. The early (in density) introduction of hyperons found in many calculations is probably delayed by the decrease in vector mean field acting on the neutron. The decrease results from both conventional many-body rescattering effects and from the movement towards asymptotic freedom at high densities. Subthreshold $K^-$-meson production by the KaoS collaboration at GSI shows that the $K^-$-mass must be substantially lowered, by $\gtrsim$ 200 MeV at $ρ\sim 2ρ_0$. It is shown that explicit chiral symmetry breaking through the kaon mass may be responsible for $Σ^-$-nucleon and $Ξ^-$-nucleon scalar attraction being weaker than obtained by simple quark scaling. The normal mode of the strangeness minus, charge $e^-$, excitation is constructed as a linear combination of $K^-$-meson and $Σ^-$, neutron-hole state. Except for zero momentum, where the terms are unmixed the "kaesobar" is a linear combination of these two components.

nucl-th↗

arXiv1997

Kaon production in heavy-ion collisions and maximum mass of neutron stars

We determine an `empirical' kaon dispersion relation by analysing and fitting recent experimental data on kaon production in heavy-ion collisions. We then investigate its effects on hadronic equation of state at high densities and on neutron star properties. We find that the maximum mass of neutron stars can be lowered by about 0.4$M_\odot$, once kaon condensation as constrained by our empirical dispersion relation is introduced. We emphasize the growing interplay between hadron physics, relativistic heavy-ion physics and the physics of compact objects in astrophysics.

nucl-th↗

arXiv1997

Kaon Production in Heavy-Ion Collisions and Kaon Condensation in Neutron Stars

The recent past witnesses the growing interdependence between the physics of hadrons, the physics of relativistic heavy-ion collisions, and the physics of compact objects in astrophysics. A notable example is the kaon which plays special roles in all the three fields. In this talk, we first review the various theoretical investigations of kaon properties in nuclear medium. We then present a detailed transport model study of kaon production and flow in heavy-ion collisions at SIS energies. Finally, We discuss the effects of the kaon in-medium properties extracted from heavy-ion data on neutron star properties, especially on the lowering of maximum mass of neutron stars with the onset of kaon condensation around $3ρ_0$.

nucl-th↗

arXiv1997

Photon production in heavy-ion collisions at SPS energies

Single photon spectra in heavy-ion collisions at SPS energies are studied in the relativistic transport model that incorporates self-consistently the change of hadron masses in dense matter. We separate the total photon spectrum into `background' arising from the radiative decays of $π^0$ and $η$ mesons, and the `themal' one from other sources. For the latter we include contributions from radiative decays of $ρ$, $ω$, $η'$, and $a_1$, radiative decays of baryon resonances, as well as two-body processes such as $ππ\to ργ$ and $πρ\to πγ$. It is found that more than 95% of all photons come from the decays of $π^0$ and $η$ mesons, while the thermal photons account for less than 5% of the total photon yield. The thermal photon spectra in our calculations with either free or in-medium meson masses do not exceed the upper bound set by the experimental measurment of the WA80 Collaboration.

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