217 research outputs found
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
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Explaining the Trust in Rochefoucauld v Boustead
In Rochefoucauld v Boustead, the Court of Appeal enforced a trust of land despite an absence of written evidence to satisfy s 7 of the Statute of Frauds. The case has attracted intense academic attention, not because this precise point arises often for decision, but rather as part of wider attempt by academics to explain and rationalise the recent growth of constructive trusts. Some scholars claim Rochefoucauld v Boustead as a constructive trust to draw from it a wide principle under which other wayward and orphan constructive trusts might be sheltered. Others argue that Rochefoucauld v Boustead concerned an express trust. This article argues that Rochefoucauld v Boustead is properly understood as an express trust. It will consider the nature of the fraud, the way in which the Statute of Frauds and the fraud must be pleaded, the arguments against a constructive trust and the detailed facts of Rochefoucauld v Boustead as it was pleaded and argued
What Can Gamma Ray Bursts Teach Us About Dark Energy?
It has been suggested that Gamma Ray Bursts (GRB) may enable the expansion
rate of our Universe to be measured out to very high redshifts (z \gsim 5)
just as type Ia supernovae have done at 1--1.5. We explore this
possibility here, and find that GRB have the potential to detect dark energy at
high statistical significance, but they are unlikely to be competitive with
future supernovae missions, such as SNAP, in measuring the properties of the
dark energy. The exception to this conclusion is if there is appreciable dark
energy at early times, in which case the information from GRB's will provide an
excellent complement to the information from supernovae.Comment: 5 pages, 9 figure
On The Origin Of The Mass-Metallicity Relation For GRB Host Galaxies
We investigate the nature of the mass-metallicity (M-Z) relation for long
gamma-ray burst (LGRB) host galaxies. Recent studies suggest that the M-Z
relation for local LGRB host galaxies may be systematically offset towards
lower metallicities relative to the M-Z relation defined by the general star
forming galaxy (SDSS) population. The nature of this offset is consistent with
suggestions that low metallicity environments may be required to produce high
mass progenitors, although the detection of several GRBs in high-mass,
high-metallicity galaxies challenges the notion of a strict metallicity cut-off
for host galaxies that are capable of producing GRBs. We show that the nature
of this reported offset may be explained by a recently proposed
anti-correlation between the star formation rate (SFR) and the metallicity of
star forming galaxies. If low metallicity galaxies produce more stars than
their equally massive, high-metallicity counterparts, then transient events
that closely trace the SFR in a galaxy would be more likely to be found in
these low metallicity, low mass galaxies. Therefore, the offset between the GRB
and SDSS defined M-Z relations may be the result of the different methods used
to select their respective galaxy populations, with GRBs being biased towards
low metallicity, high SFR, galaxies. We predict that such an offset should not
be expected of transient events that do not closely follow the star formation
history of their host galaxies, such as short duration GRBs and SN Ia, but
should be evident in core collapse SNe found through upcoming untargeted
surveys.Comment: 6 pages, 4 figures, submitted to ApJ
Can Cosmic Structure form without Dark Matter?
One of the prime pieces of evidence for dark matter is the observation of
large overdense regions in the universe. Since we know from the cosmic
microwave background that the regions that contained the most baryons when the
universe was ~400,000 years old were overdense by only one part in ten
thousand, perturbations had to have grown since then by a factor greater than
where is the epoch of recombination. This enhanced
growth does not happen in general relativity, so dark matter is needed in the
standard theory. We show here that enhanced growth can occur in alternatives to
general relativity, in particular in Bekenstein's relativistic version of
MOdified Newtonian Dynamics (MOND). The vector field introduced in that theory
for a completely different reason plays a key role in generating the
instability that produces large cosmic structures today.Comment: 5 pages, 3 figure
Primordial non-Gaussianity and Dark Energy constraints from Cluster Surveys
Galaxy cluster surveys will be a powerful probe of dark energy. At the same
time, cluster abundance is sensitive to any non-Gaussianity of the primordial
density field. It is therefore possible that non-Gaussian initial conditions
might be misinterpreted as a sign of dark energy or at least degrade the
expected constraints on dark energy parameters. To address this issue, we
perform a likelihood analysis of an ideal cluster survey similar in size and
depth to the upcoming South Pole Telescope/Dark Energy Survey (SPT-DES). We
analyze a model in which the strength of the non-Gaussianity is parameterized
by the constant fNL; this model has been used extensively to derive Cosmic
Microwave Background (CMB) anisotropy constraints on non-Gaussianity, allowing
us to make contact with those works. We find that the constraining power of the
cluster survey on dark energy observables is not significantly diminished by
non-Gaussianity provided that cluster redshift information is included in the
analysis. We also find that even an ideal cluster survey is unlikely to improve
significantly current and future CMB constraints on non-Gaussianity. However,
when all systematics are under control, it could constitute a valuable cross
check to CMB observations.Comment: 10 pages, 4 figures. Corrected a minor discrepancy between our
earlier definition of fNL and CMB constraints. References adde
The relation between accretion rate and jet power in X-ray luminous elliptical galaxies
Using Chandra X-ray observations of 9 nearby, X-ray luminous ellipticals with
good optical velocity dispersion measurements, we show that a tight correlation
exists between the Bondi accretion rates calculated from the X-ray data and
estimated black hole masses, and the power emerging from these systems in
relativistic jets. The jet powers, inferred from the energies and timescales
required to inflate the cavities observed in the surrounding X-ray emitting
gas, can be related to the accretion rates by a power law model. A significant
fraction (2.2^{+1.0}_{-0.7} per cent, for P_jet=10^{43} erg/s) of the energy
associated with the rest mass of material entering the accretion radius
eventually emerges in the jets. The data also hint that this fraction may rise
slightly with increasing jet power. Our results have significant implications
for studies of accretion, jet formation and galaxy formation. The tight
correlation between P_Bondi and P_jet suggests that the Bondi formulae provide
a reasonable description of the accretion process, despite the likely presence
of magnetic pressure and angular momentum in the accreting gas, and that the
accretion flows are approximately stable over timescales of a few million
years. Our results show that the black hole `engines' at the hearts of large
elliptical galaxies and groups can feed back sufficient energy to stem cooling
and star formation, leading naturally to the observed exponential cut off at
the bright end of the galaxy luminosity function.Comment: Accepted for publication in MNRAS. 10 pages, 4 figures. Includes an
enhanced statistical analysis and some additional data. Conclusions unchange
A Bayesian analysis of regularised source inversions in gravitational lensing
Strong gravitational lens systems with extended sources are of special
interest because they provide additional constraints on the models of the lens
systems. To use a gravitational lens system for measuring the Hubble constant,
one would need to determine the lens potential and the source intensity
distribution simultaneously. A linear inversion method to reconstruct a
pixellated source brightness distribution of a given lens potential model was
introduced by Warren & Dye. In the inversion process, a regularisation on the
source intensity is often needed to ensure a successful inversion with a
faithful resulting source. In this paper, we use Bayesian analysis to determine
the optimal regularisation constant (strength of regularisation) of a given
form of regularisation and to objectively choose the optimal form of
regularisation given a selection of regularisations. We consider and compare
quantitatively three different forms of regularisation previously described in
the literature for source inversions in gravitational lensing: zeroth-order,
gradient and curvature. We use simulated data with the exact lens potential to
demonstrate the method. We find that the preferred form of regularisation
depends on the nature of the source distribution.Comment: 18 pages, 10 figures; Revisions based on referee's comments after
initial submission to MNRA
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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