Extracting particle freeze-out phase-space densities and entropies from sources imaged in heavy-ion reactions

The space-averaged phase-space density and entropy per particle are both fundamental observables which can be extracted from the two-particle correlation functions measured in heavy-ion collisions. Two techniques have been proposed to extract the densities from correlation data: either by using the radius parameters from Gaussian fits to meson correlations or by using source imaging, which may be applied to any like pair correlation. We show that the imaging and Gaussian fits give the same result in the case of meson interferometry. We discuss the concept of an equivalent instantaneous source on which both techniques rely. We also discuss the phase-space occupancy and entropy per particle. Finally, we propose an improved formula for the phase-space occupancy that has a more controlled dependence on the uncertainty of the experimentally measured source functions.Comment: 14 pages, final version, to appear PRC. Fixed typos, added refs. for last section, added discussions of imaging and d/p ratio

Local Electronic Correlation at the Two-Particle Level

Electronic correlated systems are often well described by dynamical mean field theory (DMFT). While DMFT studies have mainly focused hitherto on one-particle properties, valuable information is also enclosed into local two-particle Green's functions and vertices. They represent the main ingredient to compute momentum-dependent response functions at the DMFT level and to treat non-local spatial correlations at all length scales by means of diagrammatic extensions of DMFT. The aim of this paper is to present a DMFT analysis of the local reducible and irreducible two-particle vertex functions for the Hubbard model in the context of an unified diagrammatic formalism. An interpretation of the observed frequency structures is also given in terms of perturbation theory, of the comparison with the atomic limit, and of the mapping onto the attractive Hubbard model.Comment: 29 pages, 26 Figures. Accepted for publication in Phys. Rev.

The neutron star soft X-ray transient 1H1905+000 in quiescence

In this Paper we report on our analysis of a ~25 ksec. Chandra X-ray observation of the neutron star soft X-ray transient (SXT) 1H1905+000 in quiescence. Furthermore, we discuss our findings of the analysis of optical photometric observations which we obtained using the Magellan telescope and photometric and spectroscopic observations which we obtained using the Very Large Telescope at Paranal. The X-ray counterpart of 1H1905+000 was not detected in our Chandra data, with a 95 per cent confidence limit to the source count rate of 1.2x10^-4 counts s^-1. For different spectral models this yields an upper limit on the luminosity of 1.8x10^31 erg s^-1 (for an upper limit on the distance of 10 kpc.) This luminosity limit makes 1H1905+000 the faintest neutron star SXT in quiescence observed to date. The neutron star luminosity is so low that it is similar to the lowest luminosities derived for black hole SXTs in quiescence. This low luminosity for a neutron star SXT challanges the hypothesis presented in the literature that black hole SXTs in quiescence have lower luminosities than neutron star SXTs as a result of the presence of a black hole event horizon. Furthermore, the limit on the neutron star luminosity obtained less than 20 years after the outburst has ceased, constrains the thermal conductivity of the neutron star crust. Finally, the neutron star core must be so cold that unless the time averaged mass accretion rate is lower than 2x10^-12 M_sun yr^-1, core cooling has to proceed via enhanced neutrino emission processes. We derive a limit on the absolute I-band magnitude of the quiescent counterpart of M_I>7.8 assuming the source is at 10 kpc. This is in line with 1H1905+000 being an ultra-compact X-ray binary, as has been proposed based on the low outburst V-band absolute magnitude.Comment: 10 pages, 5 figures, accepted for publication in MNRA

Black Hole Pair Creation and the Entropy Factor

It is shown that in the instanton approximation the rate of creation of black holes is always enhanced by a factor of the exponential of the black hole entropy relative to the rate of creation of compact matter distributions (stars). This result holds for any generally covariant theory of gravitational and matter fields that can be expressed in Hamiltonian form. It generalizes the result obtained previously for the pair creation of magnetically charged black holes by a magnetic field in Einstein--Maxwell theory. The particular example of pair creation of electrically charged black holes by an electric field in Einstein--Maxwell theory is discussed in detail.Comment: (12 pages, ReVTeX) Revised version of "Pair Creation of Electrically Charged Black Holes". New section shows that the BH pair creation rate is enhanced by a factor $\exp(BH entropy)$ for any Hamiltonian gravity + matter theor

Thermonuclear Burning on the Accreting X-Ray Pulsar GRO J1744-28

We investigate the thermal stability of nuclear burning on the accreting X-ray pulsar GRO J1744-28. The neutron star's dipolar magnetic field is <3\times 10^{11} G if persistent spin-up implies that the magnetospheric radius is less than the co-rotation radius. After inferring the properties of the neutron star, we study the thermal stability of hydrogen/helium burning and show that thermonuclear instabilities are unlikely causes of the hourly bursts seen at very high accretion rates. We then discuss how the stability of the thermonuclear burning depends on both the global accretion rate and the neutron star's magnetic field strength. We emphasize that the appearance of the instability (i.e., whether it looks like a Type I X-ray burst or a flare lasting a few minutes) will yield crucial information on the neutron star's surface magnetic field and the role of magnetic fields in convection. We suggest that a thermal instability in the accretion disk is the origin of the long (~300 days) outburst and that the recurrence time of these outbursts is >50 years. We also discuss the nature of the binary and point out that a velocity measurement of the stellar companion (most likely a Roche-lobe filling giant with m_K>17) will constrain the neutron star mass.Comment: 19 pages, 3 PostScript figures, uses aaspp4.sty and epsfig.sty, to appear in the Astrophysical Journa

Applying machine learning to the problem of choosing a heuristic to select the variable ordering for cylindrical algebraic decomposition

Cylindrical algebraic decomposition(CAD) is a key tool in computational algebraic geometry, particularly for quantifier elimination over real-closed fields. When using CAD, there is often a choice for the ordering placed on the variables. This can be important, with some problems infeasible with one variable ordering but easy with another. Machine learning is the process of fitting a computer model to a complex function based on properties learned from measured data. In this paper we use machine learning (specifically a support vector machine) to select between heuristics for choosing a variable ordering, outperforming each of the separate heuristics.Comment: 16 page

3D gravity and non-linear cosmology

By the inclusion of an additional term, non-linear in the scalar curvature $R$, it is tested if dark energy could rise as a geometrical effect in 3D gravitational formulations. We investigate a cosmological fluid obeying a non-polytropic equation of state (the van der Waals equation) that is used to construct the energy-momentum tensor of the sources, representing the hypothetical inflaton in gravitational interaction with a matter contribution. Following the evolution in time of the scale factor, its acceleration, and the energy densities of constituents it is possible to construct the description of an inflationary 3D universe, followed by a matter dominated era. For later times it is verified that, under certain conditions, the non-linear term in $R$ can generate the old 3D universe in accelerated expansion, where the ordinary matter is represented by the barotropic limit of the van der Waals constituent.Comment: 7 pages, to appear in Mod. Phys. Let

Towards an optical potential for rare-earths through coupled channels

The coupled-channel theory is a natural way of treating nonelastic channels, in particular those arising from collective excitations, defined by nuclear deformations. Proper treatment of such excitations is often essential to the accurate description of reaction experimental data. Previous works have applied different models to specific nuclei with the purpose of determining angular-integrated cross sections. In this work, we present an extensive study of the effects of collective couplings and nuclear deformations on integrated cross sections as well as on angular distributions in a consistent manner for neutron-induced reactions on nuclei in the rare-earth region. This specific subset of the nuclide chart was chosen precisely because of a clear static deformation pattern. We analyze the convergence of the coupled-channel calculations regarding the number of states being explicitly coupled. Inspired by the work done by Dietrich \emph{et al.}, a model for deforming the spherical Koning-Delaroche optical potential as function of quadrupole and hexadecupole deformations is also proposed. We demonstrate that the obtained results of calculations for total, elastic and inelastic cross sections, as well as elastic and inelastic angular distributions correspond to a remarkably good agreement with experimental data for scattering energies above around a few MeV.Comment: 7 pages, 6 figures. Submitted to the proceedings of the XXXVI Reuni\~ao de Trabalho de F\'{\i}sica Nuclear no Brasil (XXXVI Brazilian Workshop on Nuclear Physics), held in Maresias, S\~ao Paulo, Brazil in September 2013, which should be published on AIP Conference Proceeding Series. arXiv admin note: substantial text overlap with arXiv:1311.1115, arXiv:1311.042