362 research outputs found
Systematic study of Coulomb distortion effects in exclusive (e,e'p) reactions
A technique to deal with Coulomb electron distortions in the analysis of
(e,e'p) reactions is presented. Thereby, no approximations are made. The
suggested technique relies on a partial-wave expansion of the electron wave
functions and a multipole decomposition of the electron and nuclear current in
momentum space. In that way, we succeed in keeping the computational times
within reasonable limits. This theoretical framework is used to calculate the
quasielastic (e,e'p) reduced cross sections for proton knockout from the
valence shells in O, Ca, Zr and Pb. The
final-state interaction of the ejected proton with the residual nucleus is
treated within an optical potential model. The role of electron distortion on
the extracted spectroscopic factors is discussed.Comment: 45 pages, 10 encapsulated postscript figures, Revtex, uses epsfig.sty
and fancybox.sty, to be published in Physical Review
Collisional Hardening of Compact Binaries in Globular Clusters
We consider essential mechanisms for orbit-shrinkage or "hardening" of
compact binaries in globular clusters to the point of Roche-lobe contact and
X-ray emission phase, focussing on the process of collisional hardening due to
encounters between binaries and single stars in the cluster core. The interplay
between this kind of hardening and that due to emission of gravitational
radiation produces a characteristic scaling of the orbit-shrinkage time with
the single-star binary encounter rate in the cluster which we
introduce, clarify, and explore. We investigate possible effects of this
scaling on populations of X-ray binaries in globular clusters within the
framework of a simple "toy" scheme for describing the evolution of pre-X-ray
binaries in globular clusters. We find the expected qualitative trends
sufficiently supported by data on X-ray binaries in galactic globular clusters
to encourage us toward a more quantitative study.Comment: 8 pages, 4 figures. Accepted for publication in MNRA
Stability of helium accretion discs in ultracompact binaries
Stellar companions of accreting neutron stars in ultra compact X-ray binaries
(UCXBs) are hydrogen-deficient. Their helium or C/O accretion discs are
strongly X-ray irradiated. Both the chemical composition and irradiation
determine the disc stability with respect to thermal and viscous perturbations.
At shorter periods, UCXBs are persistent, whereas longer-period systems are
mostly transient. To understand this behaviour one has to derive the stability
criteria for X-ray irradiated hydrogen-poor accretion discs. We use a modified
and updated version of the Dubus et al. code describing time-dependent
irradiated accretion discs around compact objects. We obtained the relevant
stability criteria and compared the results to observed properties of UCXBs.
Although the general trend in the stability behaviour of UCXBs is consistent
with the prediction of the disc instability model, in a few cases the
inconsistency of theoretical predictions with the system observed properties is
weak enough to be attributed to observational and/or theoretical uncertainties.
Two systems might require the presence of some amount of hydrogen in the donor
star.Comment: Astronomy & Astrophysics, in pres
Meson exchange currents in electromagnetic one-nucleon emission
The role of meson exchange currents (MEC) in electron- and photon-induced
one-nucleon emission processes is studied in a nonrelativistic model including
correlations and final state interactions. The nuclear current is the sum of a
one-body and of a two-body part. The two-body current includes pion seagull,
pion-in-flight and the isobar current contributions. Numerical results are
presented for the exclusive 16O(e,e'p)15N and 16O(\gamma,p)15N reactions. MEC
effects are in general rather small in (e,e'p), while in (\gamma,p) they are
always large and important to obtain a consistent description of (e,e'p) and
(\gamma,p) data, with the same spectroscopic factors. The calculated (\gamma,p)
cross sections are sensitive to short-range correlations at high values of the
recoil momentum, where MEC effects are larger and overwhelm the contribution of
correlations.Comment: 9 pages, 6 figure
Detection Strategies for Extreme Mass Ratio Inspirals
The capture of compact stellar remnants by galactic black holes provides a
unique laboratory for exploring the near horizon geometry of the Kerr
spacetime, or possible departures from general relativity if the central cores
prove not to be black holes. The gravitational radiation produced by these
Extreme Mass Ratio Inspirals (EMRIs) encodes a detailed map of the black hole
geometry, and the detection and characterization of these signals is a major
scientific goal for the LISA mission. The waveforms produced are very complex,
and the signals need to be coherently tracked for hundreds to thousands of
cycles to produce a detection, making EMRI signals one of the most challenging
data analysis problems in all of gravitational wave astronomy. Estimates for
the number of templates required to perform an exhaustive grid-based
matched-filter search for these signals are astronomically large, and far out
of reach of current computational resources. Here I describe an alternative
approach that employs a hybrid between Genetic Algorithms and Markov Chain
Monte Carlo techniques, along with several time saving techniques for computing
the likelihood function. This approach has proven effective at the blind
extraction of relatively weak EMRI signals from simulated LISA data sets.Comment: 10 pages, 4 figures, Updated for LISA 8 Symposium Proceeding
A Measurement of the Interference Structure Function, R_LT, for the 12C(e,e'p) reaction in the Quasielastic Region
The coincidence cross-section and the interference structure function, R_LT,
were measured for the 12C(e,e'p) 11B reaction at quasielastic kinematics and
central momentum transfer of q=400 MeV/c. The measurement was at an opening
angle of theta_pq=11 degrees, covering a range in missing energy of E_m = 0 to
65 MeV. The R_LT structure function is found to be consistent with zero for E_m
> 50 MeV, confirming an earlier study which indicated that R_L vanishes in this
region. The integrated strengths of the p- and s-shell are compared with a
Distorted Wave Impulse Approximation calculation. The s-shell strength and
shape are compared with a Hartree Fock-Random Phase Approximation calculation.
The DWIA calculation overestimates the cross sections for p- and s-shell proton
knockout as expected, but surprisingly agrees with the extracted R_LT value for
both shells. The HF-RPA calculation describes the data more consistently, which
may be due to the inclusion of 2-body currents in this calculation.Comment: 8 Pages LaTex, 5 postscript figures. Submitted to Phys. Rev.
Analysis of Meson Exchange and Isobar Currents in (e,e'p) Reactions from O-16
An analysis of the effects of meson exchange and isobar currents in exclusive
(e,e'p) processes from O-16 under quasi-free kinematics is presented. A model
that has probed its feasibility for inclusive quasi-elastic (e,e') processes is
considered. Sensitivity to final state interactions between the outgoing proton
and the residual nucleus is discussed by comparing the results obtained with
phenomenological optical potentials and a continuum nuclear shell-model
calculation. The contribution of the meson-exchange and isobar currents to the
response functions is evaluated and compared to previous calculations, which
differ notably from our results. These two-body contributions cannot solve the
puzzle of the simultaneous description of the different responses
experimentally separated. Copyright 1999 by The American Physical SocietyComment: 5 pages, plus 3 PS figures. To be published in Phys. Rev. C Updated
figure
Enhancing the capabilities of LIGO time-frequency plane searches through clustering
One class of gravitational wave signals LIGO is searching for consists of
short duration bursts of unknown waveforms. Potential sources include core
collapse supernovae, gamma ray burst progenitors, and mergers of binary black
holes or neutron stars. We present a density-based clustering algorithm to
improve the performance of time-frequency searches for such gravitational-wave
bursts when they are extended in time and/or frequency, and not sufficiently
well known to permit matched filtering. We have implemented this algorithm as
an extension to the QPipeline, a gravitational-wave data analysis pipeline for
the detection of bursts, which currently determines the statistical
significance of events based solely on the peak significance observed in
minimum uncertainty regions of the time-frequency plane. Density based
clustering improves the performance of such a search by considering the
aggregate significance of arbitrarily shaped regions in the time-frequency
plane and rejecting the isolated minimum uncertainty features expected from the
background detector noise. In this paper, we present test results for simulated
signals and demonstrate that density based clustering improves the performance
of the QPipeline for signals extended in time and/or frequency.Comment: 17 pages, 6 figures. Submitted to CQG on Dec 12, 2008; accepted on
June 18, 200
Mean-field calculations of quasi-elastic responses in 4He
We present calculations of the quasi-elastic responses functions in 4He based
upon a mean-field model used to perform analogous calculations in heavier
nuclei. The meson exchange current contribution is small if compared with the
results of calculations where short-range correlations are explicitly
considered. It is argued that the presence of these correlations in the
description of the nuclear wave functions is crucial to make meson exchange
current effects appreciable.Comment: uuencoded file containing 7 LaTex peges plus 3 ps figures. To be
published in Physical Review
Compact Binary Coalescences in the Band of Ground-based Gravitational-Wave Detectors
As the ground-based gravitational-wave telescopes LIGO, Virgo, and GEO 600
approach the era of first detections, we review the current knowledge of the
coalescence rates and the mass and spin distributions of merging neutron-star
and black-hole binaries. We emphasize the bi-directional connection between
gravitational-wave astronomy and conventional astrophysics. Astrophysical input
will make possible informed decisions about optimal detector configurations and
search techniques. Meanwhile, rate upper limits, detected merger rates, and the
distribution of masses and spins measured by gravitational-wave searches will
constrain astrophysical parameters through comparisons with astrophysical
models. Future developments necessary to the success of gravitational-wave
astronomy are discussed.Comment: Replaced with version accepted by CQG
- …