2,932 research outputs found
First evidence for charge ordering in NaVO from Raman spectroscopy
We argue on the basis of symmetry selection rules and Raman scattering
spectra that NaVO undergoes a charge ordering phase transition at
T=34 K. Such a transition is characterized by the redistribution of the
charges at the phase transition and corresponds to the change of the vanadium
ions, from uniform V to two different V and V states. In
the low temperature phase the V ions are forming a "zig-zag" ladder
structure along the {\bf b}-axis, consistent with the symmetry of the P2/b
space group.Comment: to be published in solid state communication
Evidence of non-thermal X-ray emission from radio lobes of Cygnus A
Using deep Chandra ACIS observation data for Cygnus A, we report evidence of
non-thermal X-ray emission from radio lobes surrounded by a rich intra-cluster
medium (ICM). The diffuse X-ray emission, which are associated with the eastern
and western radio lobes, were observed in a 0.7--7 keV Chandra$ ACIS image. The
lobe spectra are reproduced with not only a single-temperature Mekal model,
such as that of the surrounding ICM component, but also an additional power-law
(PL) model. The X-ray flux densities of PL components for the eastern and
western lobes at 1 keV are derived as 77.7^{+28.9}_{-31.9} nJy and
52.4^{+42.9}_{-42.4} nJy, respectively, and the photon indices are
1.69^{+0.07}_{-0.13} and 1.84^{+2.90}_{-0.12}, respectively. The non-thermal
component is considered to be produced via the inverse Compton (IC) process, as
is often seen in the X-ray emission from radio lobes. From a re-analysis of
radio observation data, the multiwavelength spectra strongly suggest that the
seed photon source of the IC X-rays includes both cosmic microwave background
radiation and synchrotron radiation from the lobes. The derived parameters
indicate significant dominance of the electron energy density over the magnetic
field energy density in the Cygnus A lobes under the rich ICM environment.Comment: 8 pages, 5 figures, accepted for publication in Ap
Diffraction-limited Subaru imaging of M82: sharp mid-infrared view of the starburst core
We present new imaging at 12.81 and 11.7 microns of the central ~40"x30"
(~0.7x0.5 kpc) of the starburst galaxy M82. The observations were carried out
with the COMICS mid-infrared (mid-IR) imager on the 8.2m Subaru telescope, and
are diffraction-limited at an angular resolution of <0".4. The images show
extensive diffuse structures, including a 7"-long linear chimney-like feature
and another resembling the edges of a ruptured bubble. This is the clearest
view to date of the base of the kpc-scale dusty wind known in this galaxy.
These structures do not extrapolate to a single central point, implying
multiple ejection sites for the dust. In general, the distribution of dust
probed in the mid-IR anticorrelates with the locations of massive star clusters
that appear in the near-infrared. The 10-21 micron mid-IR emission,
spatially-integrated over the field of view, may be represented by hot dust
with temperature of ~160 K. Most discrete sources are found to have extended
morphologies. Several radio HII regions are identified for the first time in
the mid-IR. The only potential radio supernova remnant to have a mid-IR
counterpart is a source which has previously also been suggested to be a weak
active galactic nucleus. This source has an X-ray counterpart in Chandra data
which appears prominently above 3 keV and is best described as a hot (~2.6 keV)
absorbed thermal plasma with a 6.7 keV Fe K emission line, in addition to a
weaker and cooler thermal component. The mid-IR detection is consistent with
the presence of strong [NeII]12.81um line emission. The broad-band source
properties are complex, but the X-ray spectra do not support the active
galactic nucleus hypothesis. We discuss possible interpretations regarding the
nature of this source.Comment: Accepted for publication in PASJ Subaru special issue. High
resolution version available temporarily at
http://www.astro.isas.jaxa.jp/~pgandhi/pgandhi_m82.pd
Anomalous thermal conductivity of NaV2O5 as compared to conventional spin-Peierls system CuGeO3
A huge increase of thermal conductivity k is observed at the phase transition
in stoichiometric NaV2O5. This anomaly decreases and gradually disappears with
deviation from stoichiometry in Na(1-x}V2O5 (x = 0.01, 0.02, 0.03, and 0.04).
This behavior is compared with that of pure and Zn-doped CuGeO3 where only
modest kinks in the k(T) curves are observed at the spin-Peierls transition.
The change of k at critical temperature Tc could be partially attributed to the
opening of an energy gap in the magnetic excitation spectrum excluding the
scattering of thermal phonons on spin fluctuations. However, the reason for
such a strong anomaly in the k(T) may lie not only in the different energy
scales of CuGeO3 and NaV2O5, but also in the different character of the phase
transition in NaV2O5 which can have largely a structural origin, e.g. connected
with the charge ordering.Comment: PostScript 4 pages, 4 PostScript pictures. Submitted to Physical
Review Letter
Pressure-induced changes in the magnetic and valence state of EuFe2As2
We present the results of electrical resistivity, ac specific heat, magnetic
susceptibility, X-ray absorption spectroscopy (XAS) and X-ray magnetic circular
dichroism (XMCD) of the ternary iron arsenide EuFe2As2 single crystal under
pressure. Applying pressure leads to a continuous suppression of the
antiferromagnetism associated with Fe moments and the antiferromagnetic
transition temperature becomes zero in the vicinity of a critical pressure Pc
~2.5-2.7 GPa. Pressure-induced re-entrant superconductivity, which is highly
sensitive to the homogeneity of the pressure, only appears in the narrow
pressure region in the vicinity of Pc due to the competition between
superconductivity and the antiferromagnetic ordering of Eu2+ moments. The
antiferromagnetic state of Eu2+ moments changes to the ferromagnetic state
above 6 GPa. We also found that the ferromagnetic order is suppressed with
further increasing pressure, which is connected with a valence change of Eu
ions.Comment: 7 pages, 7 figures, accepted for publication in Phys. Rev.
First evidence for spectral state transitions in the ESO243-49 hyper luminous X-ray source HLX-1
The brightest Ultra-Luminous X-ray source (ULX), ESO 243-49 HLX-1, with a 0.2
- 10 keV X-ray luminosity of up to 10^42 erg s^-1, provides the strongest
evidence to date for the existence of intermediate mass black holes. Although
small scale X-ray spectral variability has already been demonstrated, we have
initiated a monitoring campaign with the X-ray Telescope onboard the Swift
satellite to search for luminosity-related spectral changes and to compare its
behavior with the better studied stellar mass black holes. In this paper, we
report a drop in the XRT count rate by a factor of ~8 which occurred
simultaneously with a hardening of the X-ray spectrum. A second observation
found that the source had re-brightened by a factor of ~21 which occurred
simultaneously with a softening of the X-ray spectrum. This may be the first
evidence for a transition between the low/hard and high/soft states.Comment: Accepted by ApJ Letter, 2 figure
N(HI) and jet power/emission in AGNs
Neutral hydrogen (HI) 21 cm absorption has been detected against more and
more powerful radio jets. In this work, based on the Guppta et al. 2006a
sample, we present our preliminary study of the correlations between the HI
column density N(HI) and the jet power, N(HI) versus the low frequency
luminosity at 408MHz, and N(HI) versus the radio luminosity at 1400MHz.Comment: 3 pages, 1 figures, Multiwavelength Variability of Blazars Guangzhou,
China, Sept. 22-24, 2010; will be published by JA
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