548 research outputs found
Optical Hall Effect in the Integer Quantum Hall Regime
Optical Hall conductivity is measured from the Faraday
rotation for a GaAs/AlGaAs heterojunction quantum Hall system in the terahertz
frequency regime. The Faraday rotation angle ( fine structure constant
mrad) is found to significantly deviate from the Drude-like behavior to
exhibit a plateau-like structure around the Landau-level filling . The
result, which fits with the behavior expected from the carrier localization
effect in the ac regime, indicates that the plateau structure, although not
quantized, still exists in the terahertz regime.Comment: 4 pages, 4 figure
UV observations of the galaxy cluster Abell 1795 with the optical monitor on XMM-Newton
We present the results of an analysis of broad band UV observations of the
central regions of Abell 1795 observed with the optical monitor on XMM-Newton.
As have been found with other UV observations of the central regions of
clusters of galaxies, we find evidence for star formation. However, we also
find evidence for absorption in the cD galaxy on a more extended scale than has
been seen with optical imaging. We also report the first UV observation of part
of the filamentary structure seen in H, X-rays and very deep U band
imaging. The part of the filament we see is very blue with UV colours
consistent with a very early (O/B) stellar population. This is the first direct
evidence of a dominant population of early type stars at the centre of Abell
1795 and implies very recent star formation at the centre of this clusterComment: 6 pages, 3 figures accepted by A&A Letter
Electrically driven spin excitation in a ferroelectric magnet DyMnO_3
Temperature (5--250 K) and magnetic field (0--70 kOe) variations of the
low-energy (1--10 meV) electrodynamics of spin excitations have been
investigated for a complete set of light-polarization configurations for a
ferroelectric magnet DyMnO by using terahertz time-domain spectroscopy. We
identify the pronounced absorption continuum (1--8 meV) with a peak feature
around 2 meV, which is electric-dipole active only for the light -vector
along the a-axis. This absorption band grows in intensity with lowering
temperature from the spin-collinear paraelectric phase above the ferroelectric
transition, but is independent of the orientation of spiral spin plane ( or
), as shown on the original (ferroelectric polarization)
phase as well as the magnetic field induced phase. The possible origin of this electric-dipole active band is argued in
terms of the large fluctuations of spins and spin-current.Comment: New version, 11 pages including colored 8 figure
X-ray Spectroscopy of the Cluster of Galaxies Abell 1795 with XMM-Newton
The initial results from XMM-Newton observations of the rich cluster of
galaxies Abell 1795 are presented. The spatially-resolved X-ray spectra taken
by the European Photon Imaging Cameras (EPIC) show a temperature drop at a
radius of kpc from the cluster center, indicating that the ICM is
cooling. Both the EPIC and the Reflection Grating Spectrometers (RGS) spectra
extracted from the cluster center can be described by an isothermal model with
a temperature of keV. The volume emission measure of any cool
component ( keV) is less than a few % of the hot component at the cluster
center. A strong OVIII Lyman-alpha line was detected with the RGS from the
cluster core. The O abundance and its ratio to Fe at the cluster center is
0.2--0.5 and 0.5--1.5 times the solar value, respectively.Comment: Accepted: A&A Letters, 2001, 6 page
Detection of an X-Ray Hot Region in the Virgo Cluster of Galaxies with ASCA
Based on mapping observations with ASCA, an unusual hot region with a spatial
extent of 1 square degree was discovered between M87 and M49 at a center
coordinate of R. A. = 12h 27m 36s and Dec. = (J2000). The X-ray
emission from the region has a 2-10 keV flux of ergs
s cm and a temperature of keV, which is
significantly higher than that in the surrounding medium of keV. The
internal thermal energy in the hot region is estimated to be ergs with a gas density of cm. A power-law
spectrum with a photon index is also allowed by the data. The hot
region suggests there is an energy input due to a shock which is probably
caused by the motion of the gas associated with M49, infalling toward the M87
cluster with a velocity km s.Comment: 12 pages, 3 figures, accepted to ApJ
Discovery of the Central Excess Brightness in Hard X-rays in the Cluster of Galaxies Abell 1795
Using the X-ray data from \ASCA, spectral and spatial properties of the
intra-cluster medium (ICM) of the cD cluster Abell 1795 are studied, up to a
radial distance of ( kpc). The ICM
temperature and abundance are spatially rather constant, although the cool
emission component is reconfirmed in the central region. The azimuthally-
averaged radial X-ray surface brightness profiles are very similar between soft
(0.7--3 keV) and hard (3--10 keV) energy bands, and neither can be fitted with
a single- model due to a strong data excess within of the
cluster center. In contrast, double- models can successfully reproduce
the overall brightness profiles both in the soft and hard energy bands, as well
as that derived with the \ROSAT PSPC. Properties of the central excess
brightness are very similar over the 0.2--10 keV energy range spanned by \ROSAT
and \ASCA. Thus, the excess X-ray emission from the core region of this cluster
is confirmed for the first time in hard X-rays above 3 keV. This indicates that
the shape of the gravitational potential becomes deeper than the King-type one
towards the cluster center. Radial profiles of the total gravitating matter,
calculated using the double- model, reveal an excess mass of within kpc of the cluster
center. This suggests a hierarchy in the gravitational potential corresponding
to the cD galaxy and the entire cluster.Comment: 27 pages, 8 figures; to appear ApJ 500 (June 20, 1998
Measuring the Cluster Magnetic Field Power Spectra from Faraday Rotation Maps of Abell 400, Abell 2634 and Hydra A
We apply a novel technique of Faraday Rotation measure (RM) map analysis to
three galaxy clusters, Abell 400, Abell 2634 and Hydra A, in order to estimate
cluster magnetic field strengths, length scales and power spectra. This
analysis is based on the assumption that the magnetic fields are statistically
isotropically distributed across the Faraday screen. We investigate the
difficulties involved in the application of the analysis to observational data.
We derive magnetic power spectra for the three clusters and discuss influences
on their shapes. We show that magnetic fluctuations are probed on length scales
ranging over at least one order of magnitude. Using this range for the
determination of central cluster magnetic field strength yields 3 muG in Abell
2634, 6 muG in Abell 400 and 12 muG in Hydra A. The magnetic field
autocorrelation length was determined to be 4.9 kpc for Abell 2634, 3.6 kpc for
Abell 400 and 0.9 kpc for Hydra A. We show that the RM autocorrelation length
is larger than the magnetic field autocorrelation length. We investigate in a
response analysis if it is possible to determine spectral slopes of the power
spectra. We find that integrated numbers can be determined from this analysis
but differential parameters such as spectral slopes have to be treated
differently. Our response analysis results in spectral slopes of the power
spectra of spectral indices alpha = 1.6 to 2.0 suggesting that Kolmogorov
spectra are possible but flatter spectral slopes than alpha = 1.3 can be
excluded.Comment: 14 pages, 9 figures, accepted by A&
The XMMâNEWTON ⊠Project: I. The X-ray luminosity â temperature relation at z>0.4
We describe XMM-Newton Guaranteed Time observations of a sample of eight high redshift (0.45 < z < rvirial) bolometric luminosities, performed ÎČ-model fits to the radial surface profiles and made spectral fits to a single temperature isothermal model. We describe data analysis techniques that pay particular attention to background mitigation. We have also estimated temperatures and luminosities for two known clusters (Abell 2246 and RXJ1325.0-3814), and one new high redshift cluste r candidate (XMMU J084701.8 +345117), that were detected o ff-axis. Characterizing the L x â Tx relation as L x = L 6 ( T 6keV ) α , we find L 6 = 15 . 9 + 7 . 6 â 5 . 2 Ă 1044erg s â 1 and α =2.7 ±0.4 for an ⊠Π= 0 . 0 , ⊠M = 1 .0, H0 = 50 km s â 1 Mpc â 1 cosmology at a typical redshift z ⌠0 .55. Comparing with the low redshift study by Markevitch, 1998, we find α to be in agreement, and assuming L x â Tx to evolve as (1 + z ) A , we find A =0.68 ±0.26 for the same cosmology and A = 1 .52 + 0 .26 â 0 .27 for an ⊠Π= 0 . 7 , ⊠M = 0 . 3 cosmology. Our A values are very similar to those found previously by Vikhlinin et al., 2002 using a compilation of Chandra observations of 0 .39 < z < 1 .26 clusters. We conclude that there is now evidence from both XMM-Newton and Chandra for an evolutionary trend in the L x â Tx relation. This evolution is significantly below the level expected from the predictions of the self-similar model for an ⊠Π= 0 . 0 , ⊠M = 1 .0, cosmology, but consistent with self-similar model in an ⊠Π= 0 . 7 , ⊠M = 0 . 3 cosmology. Our observations lend support to the robustness and completeness of the SHARC and 160SD surveys
Spatially-resolved X-ray spectroscopy of the core of the Centaurus cluster
We present Chandra data from a 31.7 ks observation of the Centaurus cluster,
using the ACIS-S detector. Images of the X-ray emission show a plume-like
feature at the centre of the cluster, of extent 60 arcsec (20 kpc in
projection). The feature has the same metallicity as gas at a similar radius,
but is cooler. Using adaptive binning, we generate temperature, abundance and
absorption maps of the cluster core. The radial abundance profile shows that
the previously known, steep abundance gradient peaks with a metallicity of
1.3-1.8 Zsolar at a radius of about 45 arcsec (15 kpc), before falling back to
0.4 Zsolar at the centre of the cluster. A radial temperature profile shows
that the temperature decreases inwards. We determine the spatial distributions
of each of two temperature components, where applicable. The radiative cooling
time of the cooler component within the inner 10 arcsec (3 kpc) is less than
2x10^7 yr. X-ray holes in the image coincident with the radio lobes are seen,
as well as two outer sharp temperature drops, or cold fronts. The origin of the
plume is unclear. The existence of the strong abundance gradient is a strong
constraint on extensive convection or gas motion driven by a central radio
source.Comment: 11 pages, 14 figures (3 colour), accepted by MNRAS, high res. version
at http://www-xray.ast.cam.ac.uk/papers/cen1_accptd.pdf . Updated version
includes a section considering a non-thermal componen
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