769 research outputs found
The radiative transfer equations for Compton scattering of polarized low frequency radiation on a hot electron gas
We deduce the equations that describe how polarized radiation is Comptonized
by a hot electron gas. Low frequencies are considered, and the equations are
expanded to second order in electron velocities. Induced scattering terms are
included and a Maxwellian velocity distribution for the electrons is assumed.
The special case of an axisymmetric radiation field is also considered, and the
corresponding radiative transfer equations are found. Our results correct
errors and misprints in previosly published transfer equations. The extension
to a moving electron gas is made, and the radiative transfer equations are
deduced to second order in gas velocity. We use the equations to study
polarization in the Sunyaev-Zeldovich effect.Comment: 9 pages, 2 figuers, MNRAS-LaTeX-style Submitted to the Monthly
Notices of the Royal astronomical Societ
Deep Imaging of the Double Quasar 0957+561: New Constraints on H_0
We present new results from extremely deep, high-resolution images of the
field around the double quasar QSO 0957+561. A possible gravitational arc
system near the double quasar has recently been reported, which, if real, would
set strong constraints on determinations of the Hubble constant from the time
delay in the double quasar. We find that both the morphology and the colors of
the claimed arc systems suggest that they are chance alignments of three and
two different objects, and not gravitationally lensed arcs. Hence, the
constraints on -determinations from the arcs are not valid. Also, a small
group of galaxies at near the line-of-sight which was required to have
a very large mass in the physically interesting arc models, is most likely
insignificant. From our deep images we are able to use weak lensing of faint
background galaxies in the field to map the gravitational potential in the main
cluster. This sets new constraints on determinations of . We find that the
Hubble constant is constrained to be less than 70km/(s Mpc), if the time delay
between the two images of the QSO is equal to or larger than 1.1 years.Comment: (uuencoded and compressed postscipt including 3 figures); 14 page
QSO environments at intermediate redshifts
We have made a survey of quasar environments at 0.5 < z < 0.8, using a sample
of both radio-loud and radio-quiet quasars matched in B-band luminosity. Our
observations include images of background control fields to provide a good
determination of the field galaxy counts. About 10 per cent of the quasars
appear to live in rich clusters, whereas approximately 45 per cent live in
environments similar to that of field galaxies.
The richness of galaxies within a 0.5 Mpc radius around the radio-quiet
quasars is found to be indistinguishable from the richness around the
radio-loud quasars, corresponding on average to groups or poorer clusters of
galaxies. Comparing the galaxy richness in the radio-loud quasar fields with
quasar fields in the literature, we find no evidence of an evolution in the
environment with epoch. Instead, a weak, but significant correlation between
quasar radio luminosity and environmental richness is present. It is thus
possible that the environments of quasars, at least the powerful ones, do not
evolve much between the present epoch and z \approx 0.8.Comment: 6 pages, 4 figures, to appear in the proc. of the workshop `QSO hosts
and their environments', IAA, Granada 10-12 Jan, 200
Radio-optical alignments in a low radio luminosity sample
We present an optically-based study of the alignment between the radio axes
and the optical major axes of eight z~0.7 radio galaxies in a 7C sample. The
radio galaxies in this sample are ~20-times less radio luminous than 3C
galaxies at the same redshift, and are significantly less radio-luminous than
any other well-defined samples studied to date. Using Nordic Optical Telescope
images taken in good seeing conditions at rest-frame wavelengths just longward
of the 4000A break, we find a statistically significant alignment effect in the
7C sample. Furthermore, in two cases where the aligned components are well
separated from the host we have been able to confirm spectroscopically that
they are indeed at the same redshift as the radio galaxy. However, a
quantitative analysis of the alignment in this sample and in a corresponding 3C
sample from HST archival data indicates that the percentage of aligned flux may
be lower and of smaller spatial scale in the 7C sample. Our study suggests that
alignments on the 50-kpc scale are probably closely related to the radio
luminosity, whereas those on the 15 kpc scale are not. We discuss these results
in the context of popular models for the alignment effect.Comment: 16 pages, 8 figures. Accepted by MNRA
Surface Superconductivity in Niobium for Superconducting RF Cavities
A systematic study is presented on the superconductivity (sc) parameters of
the ultrapure niobium used for the fabrication of the nine-cell 1.3 GHz
cavities for the linear collider project TESLA. Cylindrical Nb samples have
been subjected to the same surface treatments that are applied to the TESLA
cavities: buffered chemical polishing (BCP), electrolytic polishing (EP),
low-temperature bakeout (LTB). The magnetization curves and the complex
magnetic susceptibility have been measured over a wide range of temperatures
and dc magnetic fields, and also for di erent frequencies of the applied ac
magnetic field. The bulk superconductivity parameters such as the critical
temperature Tc = 9.26 K and the upper critical field Bc2(0) = 410 mT are found
to be in good agreement with previous data. Evidence for surface
superconductivity at fields above Bc2 is found in all samples. The critical
surface field exceeds the Ginzburg-Landau field Bc3 = 1.695Bc2 by about 10% in
BCP-treated samples and increases even further if EP or LTB are applied. From
the field dependence of the susceptibility and a power-law analysis of the
complex ac conductivity and resistivity the existence of two different phases
of surface superconductivity can be established which resemble the Meissner and
Abrikosov phases in the bulk: (1) coherent surface superconductivity, allowing
sc shielding currents flowing around the entire cylindrical sample, for
external fields B in the range between Bc2 and Bcohc3, and (2) incoherent
surface superconductivity with disconnected sc domains between Bcohc3 and Bc3.
The coherent critical surface field separating the two phases is found to be
Bcoh c3 = 0.81Bc3 for all samples. The exponents in the power law analysis are
different for BCP and EP samples, pointing to different surface topologies.Comment: 15 pages, 21 figures, DESY-Report 2004-02
Asymmetries in the CMB anisotropy field
We report on the results from two independent but complementary statistical
analyses of the WMAP first-year data, based on the power spectrum and N-point
correlation functions. We focus on large and intermediate scales (larger than
about 3 degrees) and compare the observed data against Monte Carlo ensembles
with WMAP-like properties. In both analyses, we measure the amplitudes of the
large-scale fluctuations on opposing hemispheres and study the ratio of the two
amplitudes. The power-spectrum analysis shows that this ratio for WMAP, as
measured along the axis of maximum asymmetry, is high at the 95%-99% level
(depending on the particular multipole range included). The axis of maximum
asymmetry of the WMAP data is weakly dependent on the multipole range under
consideration but tends to lie close to the ecliptic axis. In the N-point
correlation function analysis we focus on the northern and southern hemispheres
defined in ecliptic coordinates, and we find that the ratio of the large-scale
fluctuation amplitudes is high at the 98%-99% level. Furthermore, the results
are stable with respect to choice of Galactic cut and also with respect to
frequency band. A similar asymmetry is found in the COBE-DMR map, and the axis
of maximum asymmetry is close to the one found in the WMAP data.Comment: 6 pages, 5 figures; version to appear in ApJ, textual improvements,
added reference
Weak Gravitational Lensing by a Sample of X-ray Luminous Clusters of Galaxies -- III. Serendipitous Weak Lensing Detections of Dark and Luminous Mass Concentrations
In the course of a weak gravitational lensing survey of 39 clusters of
galaxies,covering a total sky area of ~1 square degree, we have serendipitously
discovered mass concentrations in the fields of A1705 and A1722 which are most
probably not associated with the main cluster target. By combining weak lensing
information with two-color galaxy photometry in fields centered on our sample
clusters, we identify a new cluster candidate at z~0.5 in the field of A1705.
This cluster candidate also displays strong lensing in the form of a giant
luminous arc. The new mass concentration in the field of A1722 also seems to be
associated with an optically luminous cluster of galaxies at z~0.5, but in this
case there is some evidence for additional structures along the line of sight
that may contribute to the lensing signal. A third cluster, A959, has a dark
sub-clump which shows interesting morphological evidence in the mass map for
being associated with the main cluster. This is the first case where there is
any significant evidence for a physical association between a dark sub-clump
(discovered from weak lensing) and a normal cluster. Analysis of archival X-ray
data shows that the three new mass concentrations are not firmly detected in
X-rays and that they are X-ray underluminous.Comment: 14 pages, 10 figures, version accepted by ApJ. See
http://www.nordita.dk/~dahle/paper3.ps.gz for a version with high-resolution
figures and Fig.5 in colo
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