3,757 research outputs found
Uncertainties in polarimetric 3D reconstructions of coronal mass ejections
This work is aimed at quantifying the uncertainties in the 3D reconstruction
of the location of coronal mass ejections (CMEs) obtained with the polarization
ratio technique. The method takes advantage of the different distributions
along the line of sight (LOS) of total (tB) and polarized (pB) brightnesses to
estimate the average location of the emitting plasma. To this end, we assumed
two simple electron density distributions along the LOS (a constant density and
Gaussian density profiles) for a plasma blob and synthesized the expected tB
and pB for different distances of the blob from the plane of the sky (POS)
and different projected altitudes . Reconstructed locations of the blob
along the LOS were thus compared with the real ones, allowing a precise
determination of uncertainties in the method. Independently of the analytical
density profile, when the blob is centered at a small distance from the POS
(i.e. for limb CMEs) the distance from the POS starts to be significantly
overestimated. Polarization ratio technique provides the LOS position of the
center of mass of what we call folded density distribution, given by reflecting
and summing in front of the POS the fraction of density profile located behind
that plane. On the other hand, when the blob is far from the POS, but with very
small projected altitudes (i.e. for halo CMEs, R), the
inferred distance from that plane is significantly underestimated. Better
determination of the real blob position along the LOS is given for intermediate
locations, and in particular when the blob is centered at an angle of
from the POS. These result have important consequences not only for
future 3D reconstruction of CMEs with polarization ratio technique, but also
for the design of future coronagraphs aimed at providing a continuous
monitoring of halo-CMEs for space weather prediction purposes
Short-Selling Bans around the World: Evidence from the 2007-09 Crisis
Most stock exchange regulators around the world reacted to the 2007-2009 crisis by imposing bans or regulatory constraints on short-selling. Short-selling restrictions were imposed and lifted at different dates in different countries, often applied to different sets of stocks and featured different degrees of stringency. We exploit this considerable variation in short-sales regimes to identify their effects with panel data techniques, and find that bans (i) were detrimental for liquidity, especially for stocks with small market capitalization, high volatility and no listed options; (ii) slowed down price discovery, especially in bear market phases, and (iii) failed to support stock prices, except possibly for U.S. financial stocks.short selling, ban, crisis, liquidity, price discovery.
When Did Cosmic Acceleration Start ?
A precise determination, and comparison, of the epoch of the onset of cosmic
acceleration, at redshift z_acc, and of dark energy domination, at z_eq,
provides an interesting measure with which to parameterize dark energy models.
By combining several cosmological datasets we place constraints on the redshift
and age of cosmological acceleration. For a Lambda-CDM model, we find the
constraint z_acc=0.76\pm0.10 at 95% c.l., occurring 6.7\pm0.4 Gyrs ago.
Allowing a constant equation of state but different from -1 changes the
constraints to z_acc=0.81\pm0.12 (6.9\pm0.5 Gyrs ago) and
z_eq=0.48\pm0.14(4.9\pm0.9 Gyrs ago), while dynamical models markedly increase
the error on the constraints with z_acc=0.81\pm0.30 (6.8\pm1.4 Gyrs ago) and
z_eq=0.44\pm0.20 (4.5\pm1.0 Gyrs ago). Unified dark energy models as Silent
Quartessence yield: z_acc=0.80\pm0.16 (6.8\pm0.6 Gyrs ago).Comment: 5 pages, 2 figure
Future capabilities of CME polarimetric 3D reconstructions with the METIS instrument: A numerical test
Understanding the 3D structure of coronal mass ejections (CMEs) is crucial
for understanding the nature and origin of solar eruptions. To derive
information on the 3D structure of CMEs from white-light (total and polarized
brightness) images, the polarization ratio technique is widely used. The
soon-to-be-launched METIS coronagraph on board Solar Orbiter will use this
technique to produce new polarimetric images. We determine the accuracy at
which the position of the centre of mass, direction and speed of propagation,
and the column density of the CME can be determined along the line of sight. We
perform a 3D MHD simulation of a flux rope ejection where a CME is produced.
From the simulation we (i) synthesize the corresponding METIS white-light
(total and polarized brightness) images and (ii) apply the polarization ratio
technique to these synthesized images and compare the results with the known
density distribution from the MHD simulation. We find that the polarization
ratio technique reproduces with high accuracy the position of the centre of
mass along the line of sight. However, some errors are inherently associated
with this determination. The polarization ratio technique also allows
information to be derived on the real 3D direction of propagation of the CME.
In addition, we find that the column density derived from white-light images is
accurate and we propose an improved technique where the combined use of the
polarization ratio technique and white-light images minimizes the error in the
estimation of column densities. Our method allows us to thoroughly test the
performance of the polarization ratio technique and allows a determination of
the errors associated with it, which means that it can be used to quantify the
results from the analysis of the forthcoming METIS observations in white light
(total and polarized brightness)
New constraints on primordial gravitational waves from Planck 2015
We show that the new precise measurements of Cosmic Microwave Background
(CMB) temperature and polarization anisotropies made by the Planck satellite
significantly improves previous constraints on the cosmic gravitational waves
background (CGWB) at frequencies Hz. On scales smaller than the
horizon at the time of decoupling, primordial gravitational waves contribute to
the total radiation content of the Universe. Considering adiabatic
perturbations, CGWB affects temperature and polarization CMB power spectra and
matter power spectrum in a manner identical to relativistic particles.
Considering the latest Planck results we constrain the CGWB energy density to
at 95\% CL. Combining CMB power
spectra with lensing, BAO and primordial Deuterium abundance observations, we
obtain at 95\% CL, improving previous
Planck bounds by a factor 3 and the recent direct upper limit from the LIGO and
VIRGO experiments a factor 2. A combined analysis of future satellite missions
as COrE and EUCLID could improve current bound by more than an order of
magnitude.Comment: 9 pages, 1 figure, matching the version published on PL
Constraints on Modified Gravity from ACT and SPT
The Atacama Cosmology Telescope (ACT) and the South Pole Telescope (SPT) have
recently provided new and precise measurements of the Cosmic Microwave
Background anisotropy damping tail. This region of the CMB angular spectra,
thanks to the angular distortions produced by gravitational lensing, can probe
the growth of matter perturbations and provide a test for general relativity.
Here we make use of the ACT and SPT power spectrum measurements (combined with
the recent WMAP9 data) to constrain f(R) gravity theories. Adopting a
parametrized approach, we obtain an upper limit on the lengthscale of the
theory of B_0 < 0.86 at 95% c.l. from ACT, while we get a significantly
stronger bound from SPT with B_0 < 0.14 at 95% c.l..Comment: 6 pages, 4 figures, some sentences correcte
Constraints on cosmological parameters from future cosmic microwave background experiments
The Planck satellite experiment will soon let cosmologists to determine most of the cosmological parameters with unprecedented accuracy. In particular a strong improvement is expected in many parameters of interest, including neutrino mass, the amount of relativistic particles at recombination, the primordial Helium abundance and the injection of extra ionizing photon by dark matter self-annihilation. Here we review the constraints achievable by future experiments and discuss the implications for fundamental physics. © 2010 IOP Publishing Ltd
Cosmological constraints on the neutron lifetime
We derive new constraints on the neutron lifetime based on the recent Planck
2015 observations of temperature and polarization anisotropies of the CMB.
Under the assumption of standard Big Bang Nucleosynthesis, we show that Planck
data constrains the neutron lifetime to at
c.l.. Moreover, by including the direct measurements of primordial
Helium abundance of Aver et al. (2015) and Izotov et al. (2014), we show that
cosmological data provide the stringent constraints and respectively. The latter
appears to be in tension with neutron lifetime value quoted by the Particle
Data Group (). Future CMB surveys as
COrE+, in combination with a weak lensing survey as EUCLID, could constrain the
neutron lifetime up to a precision.Comment: 13 pages, 3 figures. Matching JCAP accepted versio
Breaking Be: a sterile neutrino solution to the cosmological lithium problem
The possibility that the so-called "lithium problem", i.e. the disagreement
between the theoretical abundance predicted for primordial Li assuming
standard nucleosynthesis and the value inferred from astrophysical
measurements, can be solved through a non-thermal BBN mechanism has been
investigated by several authors. In particular, it has been shown that the
decay of a MeV-mass particle, like, e.g., a sterile neutrino, decaying after
BBN not only solves the lithium problem, but also satisfies cosmological and
laboratory bounds, making such a scenario worth to be investigated in further
detail. In this paper, we constrain the parameters of the model with the
combination of current data, including Planck 2015 measurements of temperature
and polarization anisotropies of the CMB, FIRAS limits on spectral distortions,
astrophysical measurements of primordial abundances and laboratory constraints.
We find that a sterile neutrino with mass (at
c.l.), a decay time (at
c.l.) and an initial density (at c.l.) in units of the number density of CMB photons,
perfectly accounts for the difference between predicted and observed Li
primordial abundance. This model also predicts an increase of the effective
number of relativistic degrees of freedom at the time of CMB decoupling at c.l..
The required abundance of sterile neutrinos is incompatible with the standard
thermal history of the Universe, but could be realized in a low reheating
temperature scenario. We provide forecasts for future experiments finding that
the combination of measurements from the COrE+ and PIXIE missions will allow to
significantly reduce the permitted region for the sterile lifetime and density.Comment: 28 pages, 13 figures, 4 tables, matching the published versio
Short-Selling Bans around the World: Evidence from the 2007-09 Crisis
Most stock exchange regulators around the world reacted to the 2007-2009 crisis byimposing bans or regulatory constraints on short-selling. Short-selling restrictions wereimposed and lifted at different dates in different countries, often applied to different sets ofstocks and featured different degrees of stringency. We exploit this considerable variationin short-sales regimes to identify their effects with panel data techniques, and find that bans(i) were detrimental for liquidity, especially for stocks with small market capitalization,high volatility and no listed options; (ii) slowed down price discovery, especially in bearmarket phases, and (iii) failed to support stock prices, except possibly for U.S. financialstocks
- …