Effects of Stress Evolution Process on the Thermal Stability of Thin Accretion Discs

The stress evolution process is taken into account in the linear stability analysis of standard thin accretion discs. We find that the growth rate of thermally unstable modes can decrease significantly owing to the stress delay, which may help to understand the quasi-periodic variability of GRS 1915+105. We also discuss possible application of stress evolution to the stability of Shapiro-Lightman-Eardley disc.Comment: 5 pages, 2 figures, accepted by MNRA

New thought experiment to test the generalized second law of thermodynamics

We propose an extension of the original thought experiment proposed by Geroch, which sparked much of the actual debate and interest on black hole thermodynamics, and show that the generalized second law of thermodynamics is in compliance with it.Comment: 4 pages (revtex), 3 figure

On the Determination of the Gluon Density of the Proton from Heavy-Flavour Production at HERA

Using a recent next-to-leading-order calculation of the photoproduction double differential cross section for heavy quarks, we study the possibility of extracting the gluon density of the proton from heavy-quark photoproduction data. We discuss the theoretical uncertainties connected with this method, and we conclude that they are well under control in a wide $x$ domain.Comment: CERN-TH.6864/93, GeF-TH-12/93. Latex, 5 topdrawer figures appended at the en

Static Observers in Curved Spaces and Non-inertial Frames in Minkowski Spacetime

Static observers in curved spacetimes may interpret their proper acceleration as the opposite of a local gravitational field (in the Newtonian sense). Based on this interpretation and motivated by the equivalence principle, we are led to investigate congruences of timelike curves in Minkowski spacetime whose acceleration field coincides with the acceleration field of static observers of curved spaces. The congruences give rise to non-inertial frames that are examined. Specifically we find, based on the locality principle, the embedding of simultaneity hypersurfaces adapted to the non-inertial frame in an explicit form for arbitrary acceleration fields. We also determine, from the Einstein equations, a covariant field equation that regulates the behavior of the proper acceleration of static observers in curved spacetimes. It corresponds to an exact relativistic version of the Newtonian gravitational field equation. In the specific case in which the level surfaces of the norm of the acceleration field of the static observers are maximally symmetric two-dimensional spaces, the energy-momentum tensor of the source is analyzed.Comment: 28 pages, 4 figures

Relativistic Astrophysics Explorer

The great success of the Rossi X-Ray Timing Explorer (RXTE) has shown that X-ray timing is an excellent tool for the study of strong gravitational fields and the measurement of fundamental physical properties of black holes and neutron stars. Here, we describe a next-generation X-ray timing mission, the Relativistic Astrophysics Explorer (RAE), designed to fit within the envelope of a medium-sized mission. The instruments will be a narrow-field X-ray detector array with an area of 6 m^2 equal to ten times that of RXTE and a wide-field X-ray monitor. We describe the science made possible with this mission, the design of the instruments, and results on prototype large-area X-ray detectors.Comment: to appear in Advances in Space Research, 6 pages with one color figure (low-res

The influence of outflows on the 1/f-like luminosity fluctuations

In accretion systems, outflows may have significant influence on the luminosity fluctuations. In this paper, following the Lyubarskii's general scheme, we revisit the power spectral density of luminosity fluctuations by taking into account the role of outflows. Our analysis is based on the assumption that the coupling between the local outflow and inflow is weak on the accretion rate fluctuations. We find that, for the inflow mass accretion rate $\dot M \propto r^{s}$, the power spectrum of flicker noise component will present a power-law distribution $p(f) \propto f^{-(1+4s/3)}$ for advection-dominated flows. We also obtain descriptions of $p(f)$ for both standard thin discs and neutrino-cooled discs, which show that the power-law index of a neutrino-cooled disc is generally larger than that of a photon-cooled disc. Furthermore, the obtained relationship between $p(f)$ and $s$ indicates the possibility of evaluating the strength of outflows by the power spectrum in X-ray binaries and gamma-ray bursts. In addition, we discuss the possible influence of the outflow-inflow coupling on our results.Comment: 6 pages, 1 figure, accepted for publication in MNRA

Simultaneous Extraction of the Fermi constant and PMNS matrix elements in the presence of a fourth generation

Several recent studies performed on constraints of a fourth generation of quarks and leptons suffer from the ad-hoc assumption that 3 x 3 unitarity holds for the first three generations in the neutrino sector. Only under this assumption one is able to determine the Fermi constant G_F from the muon lifetime measurement with the claimed precision of G_F = 1.16637 (1) x 10^-5 GeV^-2. We study how well G_F can be extracted within the framework of four generations from leptonic and radiative mu and tau decays, as well as from K_l3 decays and leptonic decays of charged pions, and we discuss the role of lepton universality tests in this context. We emphasize that constraints on a fourth generation from quark and lepton flavour observables and from electroweak precision observables can only be obtained in a consistent way if these three sectors are considered simultaneously. In the combined fit to leptonic and radiative mu and tau decays, K_l3 decays and leptonic decays of charged pions we find a p-value of 2.6% for the fourth generation matrix element |U_{e 4}|=0 of the neutrino mixing matrix.Comment: 19 pages, 3 figures with 16 subfigures, references and text added refering to earlier related work, figures and text in discussion section added, results and conclusions unchange

Complete Next to Leading Order QCD Corrections to the Photon Structure Functions F2γ(x,Q2)F^\gamma_2(x,Q^2) and FLγ(x,Q2)F_L^\gamma(x,Q^2)

We present the complete NLO QCD analysis of the photon structure functions $F_2^\gamma(x,Q^2)$ and $F_L^\gamma(x,Q^2)$ for a real photon target. In particular we study the heavy flavor content of the structure functions which is due to two different production mechanisms, namely collisions of a virtual photon with a real photon, and with a parton. We observe that the charm contributions are noticeable for $F_2^\gamma(x,Q^2)$ as well as $F_L^\gamma(x,Q^2)$ in the x-region studied.Comment: Latex 34 pages, 24 figures, uuencoded, attached at end, ITP-SB-93-46, FERMILAB-Pub-93/240-T, SMU HEP 93-1

Accreting Neutron Stars in Low-Mass X-Ray Binary Systems

Using the Rossi X-ray Timing Explorer (RossiXTE), astronomers have discovered that disk-accreting neutron stars with weak magnetic fields produce three distinct types of high-frequency X-ray oscillations. These oscillations are powered by release of the binding energy of matter falling into the strong gravitational field of the star or by the sudden nuclear burning of matter that has accumulated in the outermost layers of the star. The frequencies of the oscillations reflect the orbital frequencies of gas deep in the gravitational field of the star and/or the spin frequency of the star. These oscillations can therefore be used to explore fundamental physics, such as strong-field gravity and the properties of matter under extreme conditions, and important astrophysical questions, such as the formation and evolution of millisecond pulsars. Observations using RossiXTE have shown that some two dozen neutron stars in low-mass X-ray binary systems have the spin rates and magnetic fields required to become millisecond radio-emitting pulsars when accretion ceases, but that few have spin rates above about 600 Hz. The properties of these stars show that the paucity of spin rates greater than 600 Hz is due in part to the magnetic braking component of the accretion torque and to the limited amount of angular momentum that can be accreted in such systems. Further study will show whether braking by gravitational radiation is also a factor. Analysis of the kilohertz oscillations has provided the first evidence for the existence of the innermost stable circular orbit around dense relativistic stars that is predicted by strong-field general relativity. It has also greatly narrowed the possible descriptions of ultradense matter.Comment: 22 pages, 7 figures, updated list of sources and references, to appear in "Short-period Binary Stars: Observation, Analyses, and Results", eds. E.F. Milone, D.A. Leahy, and D. Hobill (Dordrecht: Springer, http://www.springerlink.com

New Parton Distribution Functions for the Photon

We present new improved parton distributions for the photon. We fit {\bf all} available data on the photon structure function, $F^{\gamma}_{2}(x,Q^2)$, with $Q^2\ge 3$ GeV$^2$, in order to determine the quark distributions. We also pay particular attention to the gluon distribution in the photon, $g^{\gamma}(x,Q^2)$, which has been poorly constrained in earlier analyses which only include structure function data. We use large $p_T$ jet production in $\gamma \gamma$ collisions from TRISTAN to constrain $g^\gamma$. We also see what information can be gleaned from $\gamma p$ collisions at HERA on $g^{\gamma}$ and on the quark distributions at large $x$, where there are no structure function data. We review future prospects of elucidating the parton distributions of the photon.Comment: 33 pages, 8 figures, uses eps