arXiv subjects

G. E. Brown

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

At least 55 records · Page 3Linked to original sources

arXiv2002

Low Momentum Nucleon-Nucleon Interaction and Fermi Liquid Theory

We use the induced interaction of Babu and Brown to derive two novel relations between the quasiparticle interaction in nuclear matter and the unique low momentum nucleon-nucleon interaction V_low k in vacuum. These relations provide two independent constraints on the Fermi liquid parameters of nuclear matter. We derive the full renormalization group equations in the particle-hole channels from the induced interaction. The new constraints, together with the Pauli principle sum rules, define four combinations of Fermi liquid parameters that are invariant under the renormalization group flow. Using empirical values for the spin-independent Fermi liquid parameters, we are able to compute the major spin-dependent ones by imposing the new constraints and the Pauli principle sum rules. The effects of tensor forces are discussed.

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arXiv2001

Renormalization Group Equation for Low Momentum Effective Nuclear Interactions

We consider two nonperturbative methods originally used to derive shell model effective interactions in nuclei. These methods have been applied to the two nucleon sector to obtain an energy independent effective interaction V_{low k}, which preserves the low momentum half-on-shell T matrix and the deuteron pole, with a sharp cutoff imposed on all intermediate state momenta. We show that V_{low k} scales with the cutoff precisely as one expects from renormalization group arguments. This result is a step towards reformulating traditional model space many-body calculations in the language of effective field theories and the renormalization group. The numerical scaling properties of V_{low k} are observed to be in excellent agreement with our exact renormalization group equation.

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arXiv2001

Broad and shifted iron-group emission lines in gamma-ray bursts as tests of the hypernova scenario

In the hypernova/collapsar model of gamma-ray bursts, it is natural that radiation is emitted by the inner engine for some time after the burst. This has been discussed as a possible source of the X-ray line emission observed in some afterglows. We show here that the natural geometry of a hypernova --a source of radiation at the bottom of a deep funnel-- has very significant consequences for the shape and central energy of the observed emission lines: the lines acquire a very broad scattering wing on the low-energy side and a characteristic second peak one Compton wavelength away from the initial energy, due to the once- and twice-scattered photons. We suggest that this explains the large width of the observed lines. Furthermore, the downscattering lowers the central line energy (by up to 1 keV in the source rest frame, before the lines become unrecognizable), so that the observed line energies can become consistent with originating from cobalt and nickel, as expected in few-day-old supernova ejecta.

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arXiv2001

Effective Nucleon-Nucleon Interaction and Fermi Liquid Theory

We present two novel relations between the quasiparticle interaction in nuclear matter and the unique low momentum nucleon-nucleon interaction in vacuum. These relations provide two independent constraints on the Fermi liquid parameters of nuclear matter. Moreover, the new constraints define two combinations of Fermi liquid parameters, which are invariant under the renormalization group flow in the particle-hole channels. Using empirical values for the spin-independent Fermi liquid parameters, we are able to compute the major spin-dependent ones by imposing the new constraints as well as the Pauli principle sum rules.

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arXiv2001

On the Manifestation of Chiral Symmetry in Nuclei and Dense Nuclear Matter

This article reviews our view on how chiral symmetry, its pattern of breaking and restoration under extreme conditions manifest themselves in the nucleon, nuclei, nuclear matter and dense hadronic matter. Topics treated are nucleon structure in terms of chiral symmetry, "first-principle" (QCD) calculations of the properties of finite nuclei effectuated by embedding the ``standard nuclear physics approach" into the framework of effective field theories of nuclei with predictions for certain astrophysical processes, a reinterpretation of the Brown-Rho (BR) scaling that implements chiral symmetry property of baryon-rich medium à la "vector manifestation" of hidden local symmetry, evidences for BR scaling in nuclear processes at normal nuclear matter density and at higher density, the notion of "broadband equilibration" in heavy-ion processes, and the role of strangeness in the formation of compact stars and their collapse into black-holes. We revisit the "Cheshire-Cat phenomenon" recently revived in the form of "quark-hadron continuity" in mapping low-density structure of hadrons to high-density structure of quarks and gluons and suggest how color-flavor locking in terms of QCD variables and hidden local symmetry in terms of hadronic variables may be connected and how BR scaling could fit into this "continuity" scheme.

hep-ph↗

arXiv2001

Formation of High Mass X-ray Black Hole Binaries

The discrepancy in the past years of many more black-hole soft X-ray transients (SXTs), of which a dozen have now been identified, had challenged accepted wisdom in black hole evolution. The sheer number of inferred SXTs requires that many, if not most, stars of ZAMS masses $20-35\msun$ end up as black holes (Portegies Zwart et al. 1997; Ergma & van den Heuvel 1998). In this paper we show that this can be understood by challenging the accepted wisdom that the result of helium core burning in a massive star is independent of whether the core is covered by a hydrogen envelope, or "naked" while it burns. The latter case occurs in binaries when the envelope of the more massive star is transferred to the companion by Roche Lobe overflow while in either main sequence or red giant stage. For solar metallicity, whereas the helium cores which burn while naked essentially never go into high-mass black holes, those that burn while clothed do so, beginning at ZAMS mass $\sim 20\msun$, the precise mass depending on the $^{12}C(α,γ)^{16}$O rate as we outline. In this way the SXTs can be evolved, provided that the H envelope of the massive star is removed only following the He core burning. We also show that in order to evolve a black hole of mass $\gsim 10\msun$ such as observed in Cyg X-1, even employing extremely massive progenitors of ZAMS mass $\gsim 60\msun$ for the black hole, the core must be covered by hydrogen during a substantial fraction of the core burning. In other words, the progenitor must be a WNL star. We evolve Cyg X-1 in an analogous way to which the SXTs are evolved, the difference being that the companion in Cyg X-1 is more massive than those in the SXTs, so that Cyg X-1 shines continuously.

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arXiv2001

The Fate of Hadron Masses in Dense Matter: Hidden Local Symmetry and Color Flavor Locking

The notion that hadron masses scale according to the scaling of the quark condensate in hadronic matter, partly supported by a number of observations in finite nuclei, can be interpreted in terms of Harada-Yamawaki's ``vector manifestation" (VM) of chiral symmetry. In this scenario, near chiral restoration, the vector meson masses drop to zero {\it in the chiral limit} with vanishing widths. This scenario appears to differ from the standard linear sigma model scenario. We exploit a link between the VM and color-flavor locking inferred by us from lattice data on quark number susceptibility (QNS) measured as a function of temperature to suggest that local flavor symmetry gets mapped to color gauge symmetry at the chiral phase transition.

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arXiv2001

Formation and Evolution of Black Hole X-ray Transient Systems

We study the formation of low-mass black hole X-ray binaries with main sequence companions that have formed through case C mass transfer (mass transfer following the helium core burning phase of the black hole progenitor). We identify these objects with the observed soft X-ray transients. Although this scenario requires a set of fine tuned conditions, we are able to produce a current Galactic population of about 2000 objects, in agreement with estimates based on observations. The narrow interval in initial separations leading to case C mass transfer, combined with the allowed narrow range of separations after the common envelope evolution, constrains the common envelope efficiency in this scenario: $λα_{ce} \approx 0.2-0.5$.

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arXiv2001

The Evolution of Relativistic Binary Progenitor Systems

Relativistic binary pulsars, such as B1534+12 and B1913+16 are characterized by having close orbits with a binary separation of ~ 3 R_\sun. The progenitor of such a system is a neutron star, helium star binary. The helium star, with a strong stellar wind, is able to spin up its compact companion via accretion. The neutron star's magnetic field is then lowered to observed values of about 10^{10} Gauss. As the pulsar lifetime is inversely proportional to its magnetic field, the possibility of observing such a system is, thus, enhanced by this type of evolution. We will show that a nascent (Crab-like) pulsar in such a system can, through accretion-braking torques (i.e. the "propeller effect") and wind-induced spin-up rates, reach equilibrium periods that are close to observed values. Such processes occur within the relatively short helium star lifetimes. Additionally, we find that the final outcome of such evolutionary scenarios depends strongly on initial parameters, particularly the initial binary separation and helium star mass. It is, indeed, determined that the majority of such systems end up in the pulsar "graveyard", and only a small fraction are strongly recycled. This fact might help to reconcile theoretically expected birth rates with limited observations of relativistic binary pulsars.

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arXiv2000

Strangeness Equilibration at GSI Energies

We develop the notion of "broad-band equilibration" in heavy-ion processes involving dense medium. Given density-dependent \Km-masses we show that the equilibration at GSI energies claimed to hold in previous treatments down to $\sim ρ_0/4$, can be replaced by a broad-band equilibration in which the \Km-meson and hyperons are produced in an essentially constant ratio independent of density. There are experimental indications that this also holds for AGS energies. We then proceed to argue that {\it both} $K^+$ and $K^-$ must get lighter in dense medium at some density $ρ>ρ_0$ due to the decoupling of the vector mesons. As a consequence, kaon condensation in compact stars could take place {\it before} chiral restoration since the sum of bare quark masses in the kaon should lie below $μ_e$. Another consequence of the decoupling vector interactions is that the quasi-particle picture involving (quasi)quarks, presumably ineffective at low densities, becomes more appropriate at higher densities as chiral restoration is approached.

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arXiv2000

Evolution of Neutron-Star, Carbon-Oxygen White-Dwarf Binaries

At least one, but more likely two or more, eccentric neutron-star, carbon-oxygen white-dwarf binaries with an unrecycled pulsar have been observed. According to the standard scenario for evolving neutron stars which are recycled in common envelope evolution we expect to observe $\gsim 50$ such circular neutron star-carbon oxygen white dwarf binaries, since their formation rate is roughly equal to that of the eccentric binaries and the time over which they can be observed is two orders of magnitude longer, as we shall outline. We observe at most one or two such circular binaries and from that we conclude that the standard scenario must be revised. Introducing hypercritical accretion into common envelope evolution removes the discrepancy by converting the neutron star into a black hole which does not emit radio waves, and therefore would not be observed.

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arXiv2000

A Theory of Gamma-Ray Bursts

We present a specific scenario for the link between GRB and hypernovae, based on Blandford-Znajek extraction of black-hole spin energy. Such a mechanism requires a high angular momentum in the progenitor object. The observed association of gamma-ray bursts with type Ibc supernovae leads us to consider massive helium stars that form black holes at the end of their lives as progenitors. We combine the numerical work of MacFadyen & Woosley with analytic calculations, to show that about 1E53 erg each are available to drive the fast GRB ejecta and the supernova. The GRB ejecta are driven by the power output through the open field lines, whereas the supernova is powered by closed filed lines and jet shocks. We also present a much simplified approximate derivation of these energetics. Helium stars that leave massive black-hole remnants in special ways, namely via soft X-ray transients or very massive WNL stars. Since binaries naturally have high angular momentum, we propose a link between black-hole transients and gamma-ray bursts. Recent observations of one such transient, GRO J1655-40/Nova Scorpii 1994, explicitly support this connection: its high space velocity indicates that substantial mass was ejected in the formation of the black hole, and the overabundance of alpha-nuclei, especially sulphur, indicates that the explosion energy was extreme, as in SN 1998bw/GRB 980425. (abstract shortened)

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arXiv2000

Nucleon Resonance Transition Couplings to Vector Mesons

Recent heavy ion experiments indicate modifications of the $ρ-$meson mass in medium. In the CERES experiments $ρ-$mesons are produced at $\sim$ normal nuclear matter density, where hadrons are more appropriate constituents than quarks. A collective "nuclear $ρ$", in which every nucleon is excited to the N(1520) state, with equal amplitude, enters in this description. At the higher densities reached by future experiments constituent quarks become the appropriate variables. Here the $ρ$ and $ω$ transition couplings to the nucleon resonances up to 1700 MeV, including the N(1520), are derived by means of the chiral quark model. The relevant coupling constants are expressed in terms of the corresponding vector coupling constants to nucleons. The quality of the model relations is tested by a calculation of the corresponding pion-resonance coupling constants, which are known from the empirical pion decay widths.

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arXiv2000

Hypercritical Advection Dominated Accretion Flow

In this note we study the accretion disc that arises in hypercritical accretion of $\dot M\sim 10^8 M_{\rm Edd}$ onto a neutron star while it is in common envelope evolution with a massive companion. In order to raise the temperature high enough that the disc might cool by neutrino emission, Chevalier found a small value of the $α$-parameter, where the kinematic coefficient of shear viscosity is $ν=αc_s H$, with $c_s$ the velocity of sound and $H$ the disc height; namely, $α\sim 10^{-6}$ was necessary for gas pressure to dominate. He also considered results with higher values of $α$, pointing out that radiation pressure would then predominate. With these larger $α$'s, the temperatures of the accreting material are much lower, $\lsim 0.35$ MeV. The result is that neutrino cooling during the flow is negligible, satisfying very well the advection dominating conditions. The low temperature of the accreting material means that it cannot get rid of its energy rapidly by neutrino emission, so it piles up, pushing its way through the accretion disc. An accretion shock is formed, far beyond the neutron star, at a radius $\gsim 10^8$ cm, much as in the earlier spherically symmetric calculation, but in rotation. Two-dimensional numerical simulation shows that an accretion disc is reformed inside of the accretion shock, allowing matter to accrete onto the neutron star with pressure high enough so that neutrinos can carry off the energy.

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arXiv2000

A Schematic Model for $ρ$-$a_1$ Mixing at Finite Density and In-Medium Effective Lagrangian

Based on schematic two-level models extended to $a_1$-meson degrees of freedom, we investigate possible mechanisms of chiral restoration in the vector/axialvector channels in cold nuclear matter. In the first part of this article we employ the massive Yang-Mills framework to construct an effective chiral Lagrangian based on low-energy mesonic modes at finite density. The latter are identified through nuclear collective excitations of `meson'-sobar type such as $π\leftrightarrow [Δ(1232)N^{-1}]\equiv\hatπ$, $ρ\leftrightarrow [N^* (1520)N^{-1}]\equiv\hatρ$, etc.. In a mean-field type treatment the in-medium gauge coupling $\hat g$, the (axial-) vector meson masses and $\hat f_π$ are found to decrease with density indicating the approach towards chiral restoration in the language of in-medium effective fields. In the second part of our analysis we evaluate the (first) in-medium Weinberg sum rule which relates vector and axialvector correlators to the pion decay constant. Using in-medium $ρ$/$a_1$ spectral functions (computed in the two-level model) also leads to a substantial reduction of the pion decay constant with increasing density.

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arXiv2000

Issues in the Blandford-Znajek Process for GRB Inner Engine

Several issues regarding the Blandford-Znajek process are discussed to demonstrate that it can be an effective mechanism for powering the gamma ray bursts. Using a simple circuit analysis it is argued that the disk power increases the effective power of the black hole-accretion disk system, although a part of disk power can be dissipated into black hole entropy. Within the framework of the force-free magnetosphere with the strong magnetic field, the magnetically dominated MHD flow is found to support the Blandford-Znajek process and it is demonstrated that the possible magnetic repulsion by the rotating black hole will not affect the efficiency substantially.

astro-ph↗

arXiv1999

Is Nova Sco 1994 (GRO 1655-40) a Relic of a GRB ?

We suggest Nova Sco 1994 (GRO 1655-40) as a possible relic of a Gamma Ray Burster (GRB) and Type Ib supernova (SN) explosion, showing that there is evidence both that the black hole was spun up by accretion and that there was a supernova explosion. We use the disc energy delivered from the rotational energy of the black hole to power the SN, and give arguments that roughly equal energy goes into the GRB and into the accretion disc to power the supernova.

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arXiv1999

Evolution of Black Holes in the Galaxy

In this article we consider the formation and evolution of black holes, especially those in binary stars where radiation from the matter falling on them can be seen. We consider a number of effects introduced by some of us, which are not traditionally included in binary evolution of massive stars. These are (i) hypercritical accretion, which allows neutron stars to accrete enough matter to collapse to a black hole during their spiral-in into another star. (ii) the strong mass loss of helium stars, which causes their evolution to differ from that of the helium core of a massive star. (iii) The direct formation of low-mass black holes ($M\sim2\msun$) from single stars, a consequence of a significant strange-matter content of the nuclear-matter equation of state at high density. We discuss these processes here, and then review how they affect various populations of binaries with black holes and neutron stars.

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