859 research outputs found
Hard X-ray lags in GRO J1719-24
We have used the Fourier cross spectra of GRO J1719-24, as obtained with
BATSE, to estimate the phase lags between the X-ray flux variations in the
20--50 and 50--100 keV energy bands as a function of Fourier frequency in the
interval 0.002--0.488 Hz. Our analysis covers the entire ~80 day X-ray outburst
of this black-hole candidate, following the first X-ray detection on 1993
September 25. The X-ray variations in the 50--100 keV band lag those in the
20--50 keV energy band by an approximately constant phase difference of 0.072
+/- 0.010 rad in the frequency interval 0.02--0.20 Hz. The peak phase lags in
the interval 0.02--0.20 Hz are about twice those of Cyg X-1 and GRO
J0422+32.These results are consistent with models for Comptonization regions
composed of extended non-uniform clouds around the central source.Comment: 10 pages, including 4 postscript figures, AASTEX. Accepted for
publication by Ap
Probing spacetime foam with extragalactic sources
Due to quantum fluctuations, spacetime is probably ``foamy'' on very small
scales. We propose to detect this texture of spacetime foam by looking for
core-halo structures in the images of distant quasars. We find that the Very
Large Telescope interferometer will be on the verge of being able to probe the
fabric of spacetime when it reaches its design performance. Our method also
allows us to use spacetime foam physics and physics of computation to infer the
existence of dark energy/matter, independent of the evidence from recent
cosmological observations.Comment: LaTeX, 11 pages, 1 figure; version submitted to PRL; several
references added; very useful comments and suggestions by Eric Perlman
incorporate
Hard X-ray variability of the black-hole candidate GRO J0422+32 during its 1992 outburst
We have studied the hard X-ray variability of the soft X-ray transient GRO
J0422+32 with BATSE in the 20-100 keV energy band. Our analysis covers 180 days
following the first X-ray detection of the source on 1992 August 5, fully
covering its primary and secondary X-ray outburst. We computed power density
spectra (PDSs) in the 20-50, 50-100, and 20-100 keV energy bands, in the
frequency interval 0.002-0.488 Hz. The PDSs of GRO J0422+32 are approximately
flat up to a break frequency, and decay as a power law above, with index ~1.
The canonical anticorrelation between the break frequency and the power density
at the break, observed in Cyg X-1 and other BHCs in the low state, is not
observed in the PDSs of GRO J0422+32. We compare our results with those of
similar variability studies of Cyg X-1. The relation between the spectral slope
and the amplitude of the X-ray variations of GRO J0422+32 is similar to that of
Cyg X-1; however, the relation between the hard X-ray flux and the amplitude of
its variation is opposite to what has been found in Cyg X-1. Phase lags between
the X-ray flux variations of GRO J0422+32 at high and low photon energies,
could only be derived during the first 30 days of its outburst. During this
period, the variations in the 50-100 keV lag those in the 20-50 keV energy band
by an approximately constant phase difference of 0.039(3) rad in the frequency
interval 0.02-0.20 Hz.Comment: 33 pages, including 14 postscript figures, AASTEX. To appear in ApJ
1999, March 1, vol. 513 #
Probing for Instanton Quarks with epsilon-Cooling
We use epsilon-cooling, adjusting at will the order a^2 corrections to the
lattice action, to study the parameter space of instantons in the background of
non-trivial holonomy and to determine the presence and nature of constituents
with fractional topological charge at finite and zero temperature for SU(2). As
an additional tool, zero temperature configurations were generated from those
at finite temperature with well-separated constituents. This is achieved by
"adiabatically" adjusting the anisotropic coupling used to implement finite
temperature on a symmetric lattice. The action and topological charge density,
as well as the Polyakov loop and chiral zero-modes are used to analyse these
configurations. We also show how cooling histories themselves can reveal the
presence of constituents with fractional topological charge. We comment on the
interpretation of recent fermion zero-mode studies for thermalized ensembles at
small temperatures.Comment: 26 pages, 14 figures in 33 part
Infinities within graviton scattering amplitudes
We present unitarity as a method for determining the infinities present in
graviton scattering amplitudes. The infinities are a combination of IR and UV.
By understanding the soft singularities we may extract the UV infinities and
relate these to counter-terms in the effective action. As an demonstration of
this method we rederive the UV infinities present at one-loop when gravity is
coupled to matter.Comment: revised versio
Renormalization of gauge fields using Hopf algebras
We describe the Hopf algebraic structure of Feynman graphs for non-abelian
gauge theories, and prove compatibility of the so-called Slavnov-Taylor
identities with the coproduct. When these identities are taken into account,
the coproduct closes on the Green's functions, which thus generate a Hopf
subalgebra.Comment: 16 pages, 1 figure; uses feynmp. To appear in "Recent Developments in
Quantum Field Theory". Eds. B. Fauser, J. Tolksdorf and E. Zeidler.
Birkhauser Verlag, Basel 200
Correlation Between BATSE Hard X-ray Spectral and Timing Properties of Cygnus X-1
We have analyzed approximately 1100 days of Cygnus X-1 hard X-ray data
obtained with BATSE to study its rapid variability. We find for the first time
correlations between the slope of the spectrum and the hard X-ray intensity,
and between the spectral slope and the amplitude of the rapid variations of the
hard X-ray flux. We compare our results with expectations from current theories
of accretion onto black holes.Comment: 17 pages, 3 Postscript figures, uses aasms4.sty. Accepted for
publication in Astrophysical Journal Letter
Formation of the Black Hole in Nova Scorpii
Israelian et al. (1999) showed that the stellar companion of the black-hole
binary Nova Sco is polluted with material ejected in the supernova that
accompanied the formation of the black-hole primary. Here we systematically
investigate the implications of these observations for the black-hole formation
process. Using a variety of supernova models, including both standard as well
as hypernova models (for different helium-star masses, explosion energies, and
explosion geometries) and a simple model for the evolution of the binary and
the pollution of the secondary, we show that most of the observed abundance
anomalies can be explained for a large range of model parameters (apart from
the abundance of Ti). The best models are obtained for He star masses of 10 to
16 Msun, where spherical hypernova models are generally favoured over standard
supernova ones. Aspherical hypernova models also produce acceptable fits,
provided there is extensive lateral mixing. All models require substantial
fallback and that the fallback material either reached the orbit of the
secondary or was mixed efficiently with material that escaped. The black hole
therefore formed in a two-step process, where the initial mass of the collapsed
remnant was increased substantially by matter that fell back after the initial
collapse. This may help to explain the high observed space velocity of Nova Sco
either because of a neutrino-induced kick (if a neutron star was formed first)
or by asymmetric mass ejection in an asymmetric supernova explosion.Comment: 16 pages, 3 Figures, 4 Tables. submitted to Ap
Confinement in Covariant Gauges
We examine the weak coupling limit of Euclidean SU(n) gauge theory in
covariant gauges. Following an earlier suggestion, an equivariant
BRST-construction is used to define the continuum theory on a finite torus. The
equivariant gauge fixing introduces constant ghost fields as moduli of the
model. We study the parameter- and moduli- space perturbatively. For quark flavors, the moduli flow to a non-trivial fixed point in certain
critical covariant gauges and the one-loop effective potential indicates that
the global SU(n) color symmetry of the gauge fixed model is spontaneously
broken to . Ward identities and renormalization group arguments
imply that the longitudinal gauge boson propagator at long range is dominated
by Goldstone bosons in these critical covariant gauges. In the large
limit, we derive a nonlinear integral equation for the expectation value of
large Wilson loops assuming that the exchange of Goldstone bosons dominates the
interaction at long range in critical covariant gauges. We find numerically
that the expectation value of large circular Wilson loops decreases
exponentially with the enclosed area in the absence of dynamical fermions. The
gauge invariance of this mechanism for confinement in critical covariant gauges
is discussed.Comment: 45 pages, Latex, uses psfig.sty and epsfig.sty to include
postscript-figure
Strong Coupling Constant with Flavour Thresholds at Four Loops in the MS-bar Scheme
We present in analytic form the matching conditions for the strong coupling
constant alpha_s^(n_f)(mu) at the flavour thresholds to three loops in the
modified minimal-subtraction scheme. Taking into account the recently
calculated coefficient beta_3 of the Callan-Symanzik beta function of quantum
chromodynamics, we thus derive a four-loop formula for alpha_s^(n_f)(mu)
together with appropriate relationships between the asymptotic scale parameters
Lambda^(n_f) for different numbers of flavours n_f.Comment: 10 pages (Latex), 3 figures (Postscript
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