38 research outputs found
A Lens Mapping Algorithm for Weak Lensing
We develop an algorithm for the reconstruction of the two-dimensional mass
distribution of a gravitational lens from the observable distortion of
background galaxies. From the measured reduced shear, the lens mapping is
obtained, from which a mass distribution is derived. This is unlike other
methods where the convergence ("kappa") is directly obtained. We show that this
method works best for sub-critical lenses, but can be applied to a critical
lens away from the critical lines. For finite fields the usual mass-sheet
degeneracy is shown to exist in this method as well. We show that the algorithm
reproduces the mass distribution within acceptable limits when applied to
simulated noisy data.Comment: 4 pages, 6 figures, uses emulateapj5.sty (included); substantially
revised; a slightly shorter version (fewer figures) will appear in
Ap.J.Letter
Non-Gaussianity and direction dependent systematics in HST key project data
Two new statistics, namely and , based on
extreme value theory, were derived in \cite{gupta08,gupta10}. We use these
statistics to study direction dependence in the HST key project data which
provides the most precise measurement of the Hubble constant. We also study the
non-Gaussianity in this data set using these statistics. Our results for
show that the significance of direction dependent systematics
is restricted to well below one confidence limit, however, presence of
non-Gaussian features is subtle. On the other hand statistic,
which is more sensitive to direction dependence, shows direction dependence
systematics to be at slightly higher confidence level, and the presence of
non-Gaussian features at a level similar to the statistic.Comment: 6 pages, 4 figures; accepted for publication in MNRA
Modal analysis of gravitational instabilities in nearly Keplerian, counter-rotating collisionless discs
We present a modal analysis of instabilities of counter-rotating,
self-gravitating collisionless stellar discs, using the recently introduced
modified WKB formulation of spiral density waves for collisionless systems
(Gulati \& Saini). The discs are assumed to be axisymmetric and in coplanar
orbits around a massive object at the common center of the discs. The mass in
both discs is assumed to be much smaller than the mass of the central object.
For each disc, the disc particles are assumed to be in near circular orbits.
The two discs are coupled to each other gravitationally. The perturbed dynamics
of the discs evolves on the order of the precession time scale of the discs,
which is much longer than the Keplerian time scale. We present results for the
azimuthal wave number and , for the full range of disc mass ratio
between the prograde and retrograde discs. The eigenspectra are in general
complex, therefore all eigenmodes are unstable. Eigenfunctions are radially
more compact for as compared to . Pattern speed of eigenmodes is
always prograde with respect to the more massive disc. The growth rate of
unstable modes increases with increasing mass fraction in the retrograde disc,
and decreases with ; therefore instability is likely to play the
dominant role in the dynamics of such systems.Comment: 24 pages, 8 figures, 1 tabl
Slow pressure modes in thin accretion discs
Thin accretion discs around massive compact objects can support slow pressure
modes of oscillations in the linear regime that have azimuthal wavenumber
. We consider finite, flat discs composed of barotropic fluid for various
surface density profiles and demonstrate--through WKB analysis and numerical
solution of the eigenvalue problem--that these modes are stable and have
spatial scales comparable to the size of the disc. We show that the eigenvalue
equation can be mapped to a Schr\"odinger-like equation. Analysis of this
equation shows that all eigenmodes have discrete spectra. We find that all the
models we have considered support negative frequency eigenmodes; however, the
positive eigenfrequency modes are only present in power law discs, albeit for
physically uninteresting values of the power law index and barotropic
index .Comment: 9 pages, 7 figures, 1 table, accepted in MNRAS for pulicatio
Reconstructing the Properties of Dark Energy using Standard Sirens
Future space-based gravity wave experiments such as the Big Bang Observatory
(BBO), with their excellent projected, one sigma angular resolution, will
measure the luminosity distance to a large number of gravity wave (GW) sources
to high precision, and the redshift of the single galaxies in the narrow solid
angles towards the sources will provide the redshifts of the gravity wave
sources. One sigma BBO beams contain the actual source only in 68 per cent
cases; the beams that do not contain the source may contain a spurious single
galaxy, leading to misidentification. To increase the probability of the source
falling within the beam, larger beams have to be considered, decreasing the
chances of finding single galaxies in the beams. Saini, Sethi and Sahni (2010)
argued, largely analytically, that identifying even a small number of GW source
galaxies furnishes a rough distance-redshift relation, which could be used to
further resolve sources that have multiple objects in the angular beam. In this
work we further develop this idea by introducing a self-calibrating iterative
scheme which works in conjunction with Monte-Carlo simulations to determine the
luminosity distance to GW sources with progressively greater accuracy. This
iterative scheme allows one to determine the equation of state of dark energy
to within an accuracy of a few percent for a gravity wave experiment possessing
a beam width an order of magnitude larger than BBO (and therefore having a far
poorer angular resolution). This is achieved with no prior information about
the nature of dark energy from other data sets such as SN Ia, BAO, CMB etc.Comment: 12 pages, 10 figures. Expanded discussion, additional references.
Main results unchanged. Matches published versio
Decoding X-ray observations from centres of galaxy clusters using MCMC
We correct for the use of electron densities instead of total gas density in
the pressure fits which were used to derive the local free-fall times (t) in the original paper
Possible use of self-calibration to reduce systematic uncertainties in determining distance-redshift relation via gravitational radiation from merging binaries
By observing mergers of compact objects, future gravity wave experiments
would measure the luminosity distance to a large number of sources to a high
precision but not their redshifts. Given the directional sensitivity of an
experiment, a fraction of such sources (gold plated -- GP) can be identified
optically as single objects in the direction of the source. We show that if an
approximate distance-redshift relation is known then it is possible to
statistically resolve those sources that have multiple galaxies in the beam. We
study the feasibility of using gold plated sources to iteratively resolve the
unresolved sources, obtain the self-calibrated best possible distance-redshift
relation and provide an analytical expression for the accuracy achievable. We
derive lower limit on the total number of sources that is needed to achieve
this accuracy through self-calibration. We show that this limit depends
exponentially on the beam width and give estimates for various experimental
parameters representative of future gravitational wave experiments DECIGO and
BBO.Comment: 6 pages, 2 figures, accepted for publication in PR
Estimation of Cosmological Parameters from HI Observations of Post-reionization Epoch
The emission from neutral hydrogen (HI) clouds in the post-reionization era
(z < 6), too faint to be individually detected, is present as a diffuse
background in all low frequency radio observations below 1420 MHz. The angular
and frequency fluctuations of this radiation (~ 1 mK) is an important future
probe of the large scale structures in the Universe. We show that such
observations are a very effective probe of the background cosmological model
and the perturbed Universe. In our study we focus on the possibility of
determining the redshift space distortion parameter, coordinate distance and
its derivative with redshift. Using reasonable estimates for the observational
uncertainties and configurations representative of the ongoing and upcoming
radio interferometers, we predict parameter estimation at a precision
comparable with supernova Ia observations and galaxy redshift surveys, across a
wide range in redshift that is only partially accessed by other probes. Future
HI observations of the post-reionization era present a new technique,
complementing several existing one, to probe the expansion history and to
elucidate the nature of the dark energy.Comment: 11 pages, 5 figure