345 research outputs found
From planes to spheres: About gravitational lens magnifications
We discuss the classic theorem according to which a gravitational lens always
produces a total magnification greater than unity. This theorem seems to
contradict the conservation of total flux from a lensed source. The standard
solution to this paradox is based on the exact definition of the reference
'unlensed' situation.
We calculate magnifications and amplifications for general lensing scenarios
not limited to regions close to the optical axis. In this way the formalism is
naturally extended from tangential planes for the source and lensed images to
complete spheres. We derive the lensing potential theory on the sphere and find
that the Poisson equation is modified by an additional source term that is
related to the mean density and to the Newtonian potential at the positions of
observer and source. This new term generally reduces the magnification, to
below unity far from the optical axis, and ensures conservation of the total
photon number received on a sphere around the source.
This discussion does not affect the validity of the 'focusing theorem', in
which the unlensed situation is defined to have an unchanged affine distance
between source and observer. The focusing theorem does not contradict flux
conservation, because the mean total magnification directly corresponds to
different areas of the source sphere in the lensed and unlensed situation. We
argue that a constant affine distance does not define an astronomically
meaningful reference.
By exchanging source and observer, we confirm that magnification and
amplification differ according to Etherington's reciprocity law, so that
surface brightness is no longer strictly conserved. [ abridged ]Comment: MNRAS accepted. 15 pages, 6 figure
Warming and elevated CO2 promote rapid incorporation and degradation of plant-derived organic matter in an ombrotrophic peatland
Rising temperatures have the potential to directly affect carbon cycling in peatlands by enhancing organic matter (OM) decomposition, contributing to the release of CO2 and CH4 to the atmosphere. In turn, increasing atmospheric CO2 concentration may stimulate photosynthesis, potentially increasing plant litter inputs belowground and transferring carbon from the atmosphere into terrestrial ecosystems. Key questions remain about the magnitude and rate of these interacting and opposing environmental change drivers. Here, we assess the incorporation and degradation of plant- and microbe-derived OM in an ombrotrophic peatland after 4 years of whole-ecosystem warming (+0, +2.25, +4.5, +6.75 and +9°C) and two years of elevated CO2 manipulation (500 ppm above ambient). We show that OM molecular composition was substantially altered in the aerobic acrotelm, highlighting the sensitivity of acrotelm carbon to rising temperatures and atmospheric CO2 concentration. While warming accelerated OM decomposition under ambient CO2, new carbon incorporation into peat increased in warming × elevated CO2 treatments for both plant- and microbe-derived OM. Using the isotopic signature of the applied CO2 enrichment as a label for recently photosynthesized OM, our data demonstrate that new plant inputs have been rapidly incorporated into peat carbon. Our results suggest that under current hydrological conditions, rising temperatures and atmospheric CO2 levels will likely offset each other in boreal peatlands
Boson Stars as Gravitational Lenses
We discuss boson stars as possible gravitational lenses and study the lensing
effect by these objects made of scalar particles. The mass and the size of a
boson star may vary from an individual Newtonian object similar to the Sun to
the general relativistic size and mass of a galaxy close to its Schwarzschild
radius. We assume boson stars to be transparent which allows the light to pass
through them though the light is gravitationally deflected. We assume boson
stars of the mass to be on non-cosmological distance from
the observer. We discuss the lens equation for these stars as well as the
details of magnification. We find that there are typically three images of a
star but the deflection angles may vary from arcseconds to even degrees. There
is one tangential critical curve (Einstein ring) and one radial critical curve
for tangential and radial magnification, respectively. Moreover, the deflection
angles for the light passing in the gravitational field of boson stars can be
very large (even of the order of degrees) which reflects the fact they are very
strong relativistic objects. We also propose a suitable formula for the lens
equation for such large deflection angles, and with the reservation that large
deflection angle images are highly demagnified but in the area of the
tangential critical curve, their existence may help in observational detection
of suitable lenses possessing characteristic features of boson stars which
could also serve as a direct evidence for scalar fields in the universe.Comment: accepted by Astrophys. J., 31 pages, AASTeX, 6 figure
Dermanyssus gallinae in layer farms in Kosovo: a high risk for salmonella prevalence
Background
The poultry red mite (PRM), Dermanyssus gallinae (D.g.) is a serious ectoparasitic pest of poultry and potential pathogen vector. The prevalence of D. g. and the prevalence of Salmonella spp. within mites on infested laying poultry farms were investigated in Kosovo.
Findings
In total, 14 populated layer farms located in the Southern Kosovo were assessed for D. g. presence. Another two farms in this region were investigated 6 months after depopulation. Investigated flocks were all maintained in cages, a common housing system in Kosovo. A total of eight farms were found to be infested with D. g. (50%) at varying levels, including the two depopulated farms. The detection of Salmonella spp. from D. g. was carried out using PCR. Out of the eight layer farms infested with D. g., Salmonella spp. was present in mites on three farms (37.5%).
Conclusions
This study confirms the high prevalence of D. g. in layer flocks in Kosovo and demonstrates the link between this mite and the presence of Salmonella spp. on infested farms
Climate warming and elevated CO2 alter peatland soil carbon sources and stability
Peatlands are an important carbon (C) reservoir storing one-third of global soil organic carbon (SOC), but little is known about the fate of these C stocks under climate change. Here, we examine the impact of warming and elevated atmospheric CO concentration (eCO) on the molecular composition of SOC to infer SOC sources (microbe-, plant- and fire-derived) and stability in a boreal peatland. We show that while warming alone decreased plant- and microbe-derived SOC due to enhanced decomposition, warming combined with eCO increased plant-derived SOC compounds. We further observed increasing root-derived inputs (suberin) and declining leaf/needle-derived inputs (cutin) into SOC under warming and eCO. The decline in SOC compounds with warming and gains from new root-derived C under eCO, suggest that warming and eCO may shift peatland C budget towards pools with faster turnover. Together, our results indicate that climate change may increase inputs and enhance decomposition of SOC potentially destabilising C storage in peatlands
The HRX-BL Lac sample - evolution of BL Lac objects
The unification of X-ray and radio selected BL Lacs has been an outstanding
problem in the blazar research in the past years. Recent investigations have
shown that the gap between the two classes can be filled with intermediate
objects and that apparently all differences can be explained by mutual shifts
of the peak frequencies of the synchrotron and inverse Compton component of the
emission. We study the consequences of this scheme using a new sample of X-ray
selected BL Lac objects comprising 104 objects with z<0.9 and a mean redshift
z=0.34. 77 BL Lacs, of which the redshift could be determined for 64 (83%)
objects, form a complete sample. The new data could not confirm our earlier
result, drawn from a subsample, that the negative evolution vanishes below a
synchrotron peak frequency log (peak-frequency) = 16.5. The complete sample
shows negative evolution at the 2 sigma level ( = 0.42 +- 0.04). We
conclude that the observed properties of the HRX BL Lac sample show typical
behaviour for X-ray selected BL Lacs. They support an evolutionary model, in
which flat-spectrum radio quasars (FSRQ) with high energetic jets evolve
towards low frequency peaked (mostly radio-selected) BL Lac objects and later
on to high frequency peaked (mostly X-ray selected) BL Lacs.Comment: 24 pages, 35 figures, accepted by A&
The WIRCAM Deep Infrared Cluster Survey I: Groups and Clusters at z > 1.1
We use CFHTLS deep optical data, WIRCam Deep Survey (WIRDS) NIR data and XMM
data to identify z>1.1 clusters in the CFHTLS D1 and D4 fields. Counterparts to
such clusters can not be identified without deep NIR data and as such the total
of =1deg2 of J , H & Ks band imaging provided by WIRDS is an indispensable tool
in such work. Using public XMM X-ray data, we identify extended X-ray sources
in the two fields. The resulting catalogue of extended X-ray sources was
analyzed for optical/NIR counterparts, using a red-sequence algorithm.
Redshifts of candidate groups and clusters were estimated using the median
photometric redshifts of detected counterparts and where available
spectroscopic data. Additionally, we surveyed X-ray point sources for potential
group systems at the limit of our detection range in the X-ray data. A
catalogue of z > 1.1 cluster candidates in the two fields has been compiled and
cluster masses, radii and temperatures have been estimated using the scaling
relations. The catalogue consists of 15 z > 1.1 candidates. Three of the
detections are previously published extended X-ray sources. Of note is JKSC 041
for which we identify possible structures at z = 0.8, z = 0.96, z = 1.13 and z
= 1.49. We also make an independent detection of the massive cluster, XMMXCS
J2215.9-1738. We use the z > 1.1 catalogue to compare the cluster number counts
in these fields with models based on WMAP 7-year cosmology and find that the
models slightly over-predict the observations, whilst at z>1.5 we do not detect
any clusters. We note that cluster number counts at z > 1.1 are highly
sensitive to the cosmological model, however a significant reduction in present
statistical (due to available survey area) and systematic (due to cluster
scaling relations) uncertainties is required in order to confidently constrain
cosmological parameters using cluster number counts at high redshift.Comment: 22 pages, 22 figures, Accepted 4 August 201
Multifocal peliosis hepatis: MR and diffusion-weighted MR-imaging findings of an atypical case
Peliosis is a rare benign disorder that is characterized by the presence of diffuse blood-filled cystic spaces and can occur in the liver, spleen, bone-marrow, and lungs. We present a 10-year-old boy with Fanconi anemia who presented with peliosis hepatis due to androgen treatment. Magnetic resonance (MR) imaging revealed multiple non-enhancing masses. Some of the lesions revealed fluid-fluid levels and extrahepatic extension on MR images. Diffusion-weighted (DW) imaging showed restricted diffusion. Fluid-fluid levels and extrahepatic extensions are unusual findings for hepatic peliotic lesions. In addition, DW imaging findings of peliosis hepatis have not been reported previously
XMM-Newton and Suzaku analysis of the Fe K complex in the Seyfert 1 galaxy Mrk 509
We report on partially overlapping XMM-Newton (~260 ks) and Suzaku (~100 ks)
observations of the iron K band in the nearby, bright Seyfert 1 galaxy Mrk 509.
The source shows a resolved neutral Fe K line, most probably produced in the
outer part of the accretion disc. Moreover, the source shows further emission
blue-ward of the 6.4 keV line due to ionized material. This emission is well
reproduced by a broad line produced in the accretion disc, while it cannot be
easily described by scattering or emission from photo-ionized gas at rest. The
summed spectrum of all XMM-Newton observations shows the presence of a narrow
absorption line at 7.3 keV produced by highly ionized outflowing material. A
spectral variability study of the XMM-Newton data shows an indication for an
excess of variability at 6.6-6.7 keV. These variations may be produced in the
red wing of the broad ionized line or by variation of a further absorption
structure. The Suzaku data indicate that the neutral Fe Kalpha line intensity
is consistent with being constant on long timescales (of a few years) and they
also confirm as most likely the interpretation of the excess blueshifted
emission in terms of a broad ionized Fe line. The average Suzaku spectrum
differs from the XMM-Newton one for the disappearance of the 7.3 keV absorption
line and around 6.7 keV, where the XMM-Newton data alone suggested variability.Comment: MNRAS in pres
The VIMOS VLT Deep Survey: the faint type-1 AGN sample
We present the type-1 active galactic nuclei (AGN) sample extracted from the
VIMOS VLT Deep Survey first observations of 21000 spectra in 1.75 square
degree. This sample, which is purely magnitude limited, free of morphological
or color selection biases, contains 130 broad line AGN (BLAGN) spectra with
redshift up to 5. Our data are divided into a wide (Iab < 22.5) and a deep (Iab
< 24) subsample containing 56 and 74 objects respectively. Because of its depth
and selection criteria, this sample is uniquely suited to study the population
of faint type-1 AGN. Our measured surface density (~ 472 +- 48 BLAGN per square
degree with Iab < 24) is significantly higher than that of any other optically
selected sample of BLAGN with spectroscopic confirmation. By applying a
morphological and color analysis to our AGN sample we find that: (1)~23% of the
AGN brighter than Iab=22.5 are classified as extended; this percentage
increases to ~42% for those with z < 1.6; (2) a non-negligible fraction of our
BLAGN are lying close to the color space area occupied by stars in u*-g' versus
g'-r' color-color diagram. This leads us to the conclusion that classical
optical ultraviolet preselection technique, if employed at such deep magnitudes
(Iab=22.5) in conjuction with a preselection of point-like sources, can miss
miss up to ~35% of the AGN population. Finally, we present a composite spectrum
of our sample of objects. While the continuum shape is very similar to that of
the SDSS composite at short wavelengths, it is much redder than it at lambda >
3000 A. We interpret this as due to significant contamination from emission of
the host galaxies, as expected from the faint absolute magnitudes sampled by
our survey.Comment: Accepted to A&A, 18 pages, 14 figure
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