81 research outputs found
Lenses in the forest: cross-correlation of the Lyman-alpha flux with CMB lensing
We present a theoretical estimate for a new observable: the cross-correlation
between the Lyman-alpha flux fluctuations in quasar (QSO) spectra and the
convergence of the cosmic microwave background (CMB) as measured along the same
line-of-sight. As a first step toward the assessment of its detectability, we
estimate the signal-to-noise ratio using linear theory. Although the
signal-to-noise is small for a single line-of-sight and peaks at somewhat
smaller redshifts than those probed by the Lyman-alpha forest, we estimate a
total signal-to-noise of 9 for cross-correlating QSO spectra of SDSS-III with
Planck and of 20 for cross-correlating with a future polarization based CMB
experiment. The detection of this effect would be a direct measure of the
neutral hydrogen-matter cross-correlation and could provide important
information on the growth of structures at large scales in a redshift range
which is still poorly probed by observations.Comment: 5 pages, 4 figures, matches published versio
Stirring the Embers: High Sensitivity VLBI Observations of GRB030329
We present high sensitivity Very Long Baseline Interferometry (VLBI)
observations 806 days after the gamma-ray burst of 2003 March 29 (GRB030329).
The angular diameter of the radio afterglow is measured to be 0.347 +- 0.09
mas, corresponding to 0.99 +- 0.26 pc at the redshift of GRB030329 (z =
0.1685). The evolution of the image size favors a uniform external density over
an R^-2 wind-like density profile (at distances of R >~10^18 cm from the
source), although the latter cannot be ruled out yet. The current apparent
expansion velocity of the image size is only mildly relativistic, suggesting a
non-relativistic transition time of t_NR ~ 1 yr. A rebrightening, or at least a
significant flattening in the flux decay, is expected within the next several
years as the counter-jet becomes visible (this has not yet been observed). An
upper limit of <1.9c is set on the proper motion of the flux centroid.Comment: 16 pages, 5 figures, accepted for publication in Astrophysical
Journa
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A kinetic approach to cosmic ray induced streaming instability at supernova shocks
We show here that a purely kinetic approach to the excitation of waves by cosmic rays in the vicinity of a shock front leads to predict the appearance of a non-alfvenic fastly growing mode which has the same dispersion relation as that previously found by Bell (2004) by treating the plasma in the MHD approximation. The kinetic approach allows us to investigate the dependence of the dispersion relation of these waves on the microphysics of the current which compensates the cosmic ray flow. We also show that a resonant and a non-resonant mode may appear at the same time and one of the two may become dominant on the other depending on the conditions in the acceleration region. We discuss the role of the unstable modes for magnetic field amplification and particle acceleration in supernova remnants at different stages of the remnant evolution
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On the escape of particles from cosmic ray modified shocks
The solution of the problem of particle acceleration in the non-linear regime, when the dynamical reaction of the accelerated particles cannot be neglected, shows strong shock modification.When stationarity is imposed by hand, the solution may show a prominent energy flux away from the shock towards upstream infinity. This feature is peculiar of cosmic ray modified shocks, while being energetically insignificant in the test particle regime. The escape flux appears also in situations in which it is physically impossible to have particle escape towards upstream infinity, thereby leading to question its interpretation.We show here that the appearance of an escape flux is due to the unphysical assumption of stationarity of the problem, and in a realistic situation it translates to an increase of the value of the maximum-momentum when the shock velocity is constant. On the other hand, when the shock velocity decreases (for instance during the Sedov-Taylor phase of a supernova explosion), escape to upstream infinity is possible for particles with momenta in a narrow range close to the maximum momentum
Constraining Emission Models of Luminous Blazar Sources
Many luminous blazars which are associated with quasar-type active galactic
nuclei display broad-band spectra characterized by a large luminosity ratio of
their high-energy (gamma-ray) and low-energy (synchrotron) spectral components.
This large ratio, reaching values up to 100, challenges the standard
synchrotron self-Compton models by means of substantial departures from the
minimum power condition. Luminous blazars have also typically very hard X-ray
spectra, and those in turn seem to challenge hadronic scenarios for the high
energy blazar emission. As shown in this paper, no such problems are faced by
the models which involve Comptonization of radiation provided by a broad
line-region, or dusty molecular torus. The lack or weakness of bulk Compton and
Klein-Nishina features indicated by the presently available data favors
production of gamma-rays via up-scattering of infrared photons from hot dust.
This implies that the blazar emission zone is located at parsec-scale distances
from the nucleus, and as such is possibly associated with the extended,
quasi-stationary reconfinement shocks formed in relativistic outflows. This
scenario predicts characteristic timescales for flux changes in luminous
blazars to be days/weeks, consistent with the variability patterns observed in
such systems at infrared, optical and gamma-ray frequencies. We also propose
that the parsec-scale blazar activity can be occasionally accompanied by
dissipative events taking place at sub-parsec distances and powered by internal
shocks and/or reconnection of magnetic fields. These could account for the
multiwavelength intra-day flares occasionally observed in powerful blazars
sources.Comment: 34 pages, accepted for publication in the Astrophysical Journa
All Weather Calibration of Wide Field Optical and NIR Surveys
The science goals for ground-based large-area surveys, such as the Dark
Energy Survey, Pan-STARRS, and the Large Synoptic Survey Telescope, require
calibration of broadband photometry that is stable in time and uniform over the
sky to precisions of a per cent or better. This performance will need to be
achieved with data taken over the course of many years, and often in less than
ideal conditions. This paper describes a strategy to achieve precise internal
calibration of imaging survey data taken in less than photometric conditions,
and reports results of an observational study of the techniques needed to
implement this strategy. We find that images of celestial fields used in this
case study with stellar densities of order one per arcmin-squared and taken
through cloudless skies can be calibrated with relative precision of 0.5 per
cent (reproducibility). We report measurements of spatial structure functions
of cloud absorption observed over a range of atmospheric conditions, and find
it possible to achieve photometric measurements that are reproducible to 1 per
cent in images that were taken through cloud layers that transmit as little as
25 per cent of the incident optical flux (1.5 magnitudes of extinction). We
find, however, that photometric precision below 1 per cent is impeded by the
thinnest detectable cloud layers. We comment on implications of these results
for the observing strategies of future surveys.Comment: Accepted for publication in The Astronomical Journal (AJ
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An Automated Method for Characterizing the Relaxedness of Galaxy Clusters
Relaxed galaxy clusters are useful tools for probing cosmological parameters like the gas mass fraction of the universe. Selecting relaxed clusters for this purpose can be a time-consuming and subjective task, so we present methods to automate parts of the process. We fit elliptical isophotes to a diverse sample of Chandra cluster data and summarize other methods for quantifying relaxedness which will be included in future work. Analysis of the results of tests from isophote fitting, combined with numerical simulations of cluster structures and comparison to previous classifications will allow us to formulate criteria for selection of relaxed clusters. We find that they tend to have core radii less than approximately 60 kpc from King model fits, shifts in isophote centroids of less than 25 kpc over a range in semi-major axes of several hundred kpc, and significantly greater surface brightness profile gradients within 30 kpc of their cores than unrelaxed clusters. These criteria will help with future cosmological work as larger amounts of cluster data are taken and need objective classification
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Dynamical Feedback of Self-generated Magnetic Fields in Cosmic Rays Modified Shocks
We present a semi-analytical kinetic calculation of the process of non-linear diffusive shock acceleration (NLDSA) which includes magnetic field amplification due to cosmic ray induced streaming instability, the dynamical reaction of the amplified magnetic field and the possible effects of turbulent heating. This kinetic calculation allows us to show that the net effect of the amplified magnetic field is to enhance the maximum momentum of accelerated particles while reducing the concavity of the spectra, with respect to the standard predictions of NLDSA. This is mainly due to the dynamical reaction of the amplified field on the shock, which smoothens the shock precursor. The total compression factors which are obtained for parameters typical of supernova remnants are R{sub tot} {approx} 7-10, in good agreement with the values inferred from observations. The strength of the magnetic field produced through excitation of streaming instability is found in good agreement with the values inferred for several remnants if the thickness of the X-ray rims are interpreted as due to severe synchrotron losses of high energy electrons. We also discuss the relative role of turbulent heating and magnetic dynamical reaction in smoothening the shock precursor
Unveiling the nature of the unidentified gamma-ray sources IV: the catalog of potential X-ray counterparts
A significant fraction ( %) of the high-energy gamma-ray sources
listed in the second LAT (2FGL) catalog are still of unknown
origin, being not yet associated with counterparts at lower energies. In order
to investigate the nature of these enigmatic sources, we present here an
extensive search of X-ray sources lying in the positional uncertainty region of
a selected sample of these Unidentified Gamma-ray Sources (UGSs) that makes use
of all available observations performed by the X-ray Telescope
before March 31, 2013, available for 205 UGSs. To detect the fainter sources,
we merged all the observations covering the LAT positional
uncertainty region at 95 % level of confidence of each UGSs. This yields a
catalog of 357 X-ray sources, finding {candidate} X-ray counterparts for % of the selected sample. In particular, 25 % of the UGSs feature a single
X-ray source within their positional uncertainty region while 45 % have
multiple X-ray sources. For each X-ray source we also looked in the
corresponding UVOT merged images for optical and ultraviolet
counterparts, also performing source photometry. We found ultraviolet-optical
correspondences for % of the X-ray sources. We searched several major
radio, infrared, optical and ultraviolet surveys for possible counterparts
within the positional error of the sources in the X-ray catalog to obtain
additional information on their nature. Applying the kernel density estimator
technique to infrared colors of WISE counterparts of our X-ray sources we
select 6 -ray blazar candidates. In addition, comparing our results
with previous analyses, we select 11 additional -ray blazar candidates.Comment: 19 pages, 10 figures, 6 tables. Accepted for publication on Ap
A search for the most massive galaxies: Double Trouble?
We describe the results of a search for galaxies with large (> 350 km/s)
velocity dispersions. The largest systems we have found appear to be the
extremes of the early-type galaxy population: compared to other galaxies with
similar luminosities, they have the largest velocity dispersions and the
smallest sizes. However, they are not distant outliers from the Fundamental
Plane and mass-to-light scaling relations defined by the bulk of the early-type
galaxy population. They may host the most massive black holes in the Universe,
and their abundance and properties can be used to constrain galaxy formation
models. Clear outliers from the scaling relations tend to be objects in
superposition (angular separations smaller than 1 arcsec), evidence for which
comes sometimes from the spectra, sometimes from the images, and sometimes from
both. The statistical properties of the superposed pairs, e.g., the
distribution of pair separations and velocity dispersions, can be used to
provide useful information about the expected distribution of image
multiplicities, separations and flux ratios due to gravitational lensing by
multiple lenses, and may also constrain models of their interaction rates.Comment: 20 pages, 8 figures. Accepted by AJ. The full set of figures in
Appendix B is available at
http://www.physics.upenn.edu/~bernardm/PAPERS/BIGEtypes/bernardi.FIG-B.ps.gz
Figure 8 did not show the set of galaxies described in the text of the
appendix. This has now been correcte
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