86 research outputs found
Validation of the Exoplanet Kepler-21b using PAVO/CHARA Long-Baseline Interferometry
We present long-baseline interferometry of the Kepler exoplanet host star
HD179070 (Kepler-21) using the PAVO beam combiner at the CHARA Array. The
visibility data are consistent with a single star and exclude stellar
companions at separations ~1-1000 mas (~ 0.1-113 AU) and contrasts < 3.5
magnitudes. This result supports the validation of the 1.6 R_{earth} exoplanet
Kepler-21b by Howell et al. (2012) and complements the constraints set by
adaptive optics imaging, speckle interferometry, and radial velocity
observations to rule out false-positives due to stellar companions. We conclude
that long-baseline interferometry has strong potential to validate transiting
extrasolar planets, particularly for future projects aimed at brighter stars
and for host stars where radial velocity follow-up is not available.Comment: 5 pages, 3 figures, accepted for publication in MNRAS Letters; v2:
minor changes added in proo
Spectral Formation in X-Ray Pulsars: Bulk Comptonization in the Accretion Shock
Accretion-powered X-ray pulsars are among the most luminous X-ray sources in
the Galaxy. However, despite decades of theoretical and observational work
since their discovery, no satisfactory model for the formation of the observed
X-ray spectra has emerged. In particular, the previously available theories are
unable to reproduce the power-law variation observed at high energies in many
sources. In this paper, we present the first self-consistent calculation of the
spectrum emerging from a pulsar accretion column that includes an explicit
treatment of the energization occurring in the shock. Using a rigorous
eigenfunction expansion method based on the exact dynamical solution for the
velocity profile in the column, we obtain a closed-form expression for the
Green's function describing the upscattering of radiation injected into the
column from a monochromatic source located at the top of the thermal mound,
near the base of the flow. The Green's function is convolved with a Planck
distribution to calculate the radiation spectrum resulting from the
reprocessing of blackbody photons emitted by the thermal mound. We demonstrate
that the energization of the photons in the shock naturally produces an X-ray
spectrum with a power-law shape at high energies and a blackbody shape at low
energies, in agreement with many observations of accreting X-ray pulsars.Comment: Accepted for publication in Ap
Exact Expressions for the Critical Mach Numbers in the Two-Fluid Model of Cosmic-Ray Modified Shocks
The acceleration of relativistic particles due to repeated scattering across
a shock wave remains the most attractive model for the production of energetic
cosmic rays. This process has been analyzed extensively during the past two
decades using the ``two-fluid'' model of diffusive shock acceleration. It is
well known that 1, 2, or 3 distinct solutions for the flow structure can be
found depending on the upstream parameters. The precise nature of the critical
conditions delineating the number and character of shock transitions has
remained unclear, mainly due to the inappropriate choice of parameters used in
the determination of the upstream boundary conditions. We derive the exact
critical conditions by reformulating the upstream boundary conditions in terms
of two individual Mach numbers defined with respect to the cosmic-ray and gas
sound speeds, respectively. The gas and cosmic-ray adiabatic indices are
assumed to remain constant throughout the flow, although they may have
arbitrary, independent values. Our results provide for the first time a
complete, analytical classification of the parameter space of shock transitions
in the two-fluid model. When multiple solutions are possible, we propose using
the associated entropy distributions as a means for indentifying the most
stable configuration.Comment: Accepted for publication in ApJ; corrected a few typos; added journal
re
Spectrum and ionization rate of low energy Galactic cosmic rays
We consider the rate of ionization of diffuse and molecular clouds in the
interstellar medium by Galactic cosmic rays (GCR) in order to constrain its low
energy spectrum. We extrapolate the GCR spectrum obtained from PAMELA at high
energies ( GeV/ nucleon) and a recently derived GCR proton flux at
GeV from observations of gamma rays from molecular clouds, and
find that the observed average Galactic ionization rate can be reconciled with
this GCR spectrum if there is a low energy cutoff for protons at
MeV. We also identify the flattening below a few GeV as being
due to (a) decrease of the diffusion coefficient and dominance of convective
loss at low energy and (b) the expected break in energy spectrum for a constant
spectral index in momentum. We show that the inferred CR proton spectrum of
for few GeV, is consistent
with a power-law spectrum in momentum , which we identify as
the spectrum at source. Diffusion loss at higher energies then introduces a
steepening by with , making it consistent with
high energy measurements.Comment: 5 pages, 3 figures, to appear in MNRAS Letter
Testing gravitational-wave searches with numerical relativity waveforms: Results from the first Numerical INJection Analysis (NINJA) project
The Numerical INJection Analysis (NINJA) project is a collaborative effort
between members of the numerical relativity and gravitational-wave data
analysis communities. The purpose of NINJA is to study the sensitivity of
existing gravitational-wave search algorithms using numerically generated
waveforms and to foster closer collaboration between the numerical relativity
and data analysis communities. We describe the results of the first NINJA
analysis which focused on gravitational waveforms from binary black hole
coalescence. Ten numerical relativity groups contributed numerical data which
were used to generate a set of gravitational-wave signals. These signals were
injected into a simulated data set, designed to mimic the response of the
Initial LIGO and Virgo gravitational-wave detectors. Nine groups analysed this
data using search and parameter-estimation pipelines. Matched filter
algorithms, un-modelled-burst searches and Bayesian parameter-estimation and
model-selection algorithms were applied to the data. We report the efficiency
of these search methods in detecting the numerical waveforms and measuring
their parameters. We describe preliminary comparisons between the different
search methods and suggest improvements for future NINJA analyses.Comment: 56 pages, 25 figures; various clarifications; accepted to CQ
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