144 research outputs found
A Tidal Flare Candidate in Abell 1795
As part of our ongoing archival X-ray survey of galaxy clusters for tidal
flares, we present evidence of an X-ray transient source within 1 arcmin of the
core of Abell 1795. The extreme variability (a factor of nearly 50), luminosity
(> 2 x 10^42 erg s^{-1}), long duration (> 5 years) and supersoft X-ray
spectrum (< 0.1 keV) are characteristic signatures of a stellar tidal
disruption event according to theoretical predictions and to existing X-ray
observations, implying a massive >~10^5 M_sun black hole at the centre of that
galaxy. The large number of X-ray source counts (~700) and long temporal
baseline (~12 years with Chandra and XMM-Newton) make this one of the
best-sampled examples of any tidal flare candidate to date. The transient may
be the same EUV source originally found contaminating the diffuse ICM
observations of Bowyer et al. (1999), which would make it the only tidal flare
candidate with reported EUV observations and implies an early source luminosity
1-2 orders of magnitude greater. If the host galaxy is a cluster member then it
must be a dwarf galaxy, an order of magnitude less massive than the quiescent
galaxy Henize 2-10 which hosts a massive black hole that is difficult to
reconcile with its low mass. The unusual faintness of the host galaxy may be
explained by tidal stripping in the cluster core.Comment: Accepted by MNRAS 2013 July 23. 27 pages, 10 figure
Structural and insulator-to-metal phase transition at 50 GPa in GdMnO3
We present a study of the effect of very high pressure on the orthorhombic
perovskite GdMnO3 by Raman spectroscopy and synchrotron x-ray diffraction up to
53.2 GPa. The experimental results yield a structural and insulator-to-metal
phase transition close to 50 GPa, from an orthorhombic to a metrically cubic
structure. The phase transition is of first order with a pressure hysteresis of
about 6 GPa. The observed behavior under very high pressure might well be a
general feature in rare-earth manganites.Comment: 4 pages, 3 figures and 2 table
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High-contrast imaging of 180° ferroelectric domains by optical microscopy using ferroelectric liquid crystals
Ferroelectric liquid crystals (FLCs) couple the direction of their
spontaneous electric polarization to the direction of tilt of their optic axis.
Consequently, reversal of the electric polarization by an electric field gives
rise to an immediate and lasting optical response when an appropriately aligned
FLC is observed between crossed polarizers, with one field direction yielding a
dark image, and the opposite direction yielding a bright image. Here this
peculiar electro-optic response is used to image, with high optical contrast,
180{\deg} ferroelectric domains in a crystalline substrate of magnesium-doped
lithium niobate. The lithium niobate substrate contains a few domains with
upwards electric polarization surrounded by regions with downward electric
polarization. In contrast to a reference non-chiral liquid crystal that is
unable to show ferroelectric behavior due to its high symmetry, the FLC, which
is used as a thin film confined between the lithium niobate substrate and an
inert aligning substrate, reveals ferroelectric domains as well as their
boundaries, with strong black and white contrast. The results show that FLCs
can be used for non-destructive read-out of domains in underlying
ferroelectrics, with potential applications in e.g. photonic devices and
non-volatile ferroelectric memories.Royal Society.
Royal Commission for the Exhibition of 1851
The XXL Survey VIII: MUSE characterisation of intracluster light in a z0.53 cluster of galaxies
Within a cluster, gravitational effects can lead to the removal of stars from
their parent galaxies. Gas hydrodynamical effects can additionally strip gas
and dust from galaxies. The properties of the ICL can therefore help constrain
the physical processes at work in clusters by serving as a fossil record of the
interaction history. The present study is designed to characterise this ICL in
a ~10^14 M_odot and z~0.53 cluster of galaxies from imaging and spectroscopic
points of view. By applying a wavelet-based method to CFHT Megacam and WIRCAM
images, we detect significant quantities of diffuse light. These sources were
then spectroscopically characterised with MUSE. MUSE data were also used to
compute redshifts of 24 cluster galaxies and search for cluster substructures.
An atypically large amount of ICL has been detected in this cluster. Part of
the detected diffuse light has a very weak optical stellar component and
apparently consists mainly of gas emission, while other diffuse light sources
are clearly dominated by old stars. Furthermore, emission lines were detected
in several places of diffuse light. Our spectral analysis shows that this
emission likely originates from low-excitation parameter gas. The stellar
contribution to the ICL is about 2.3x10^9 yrs old even though the ICL is not
currently forming a large number of stars. On the other hand, the contribution
of the gas emission to the ICL in the optical is much greater than the stellar
contribution in some regions, but the gas density is likely too low to form
stars. These observations favour ram pressure stripping, turbulent viscous
stripping, or supernovae winds as the origin of the large amount of
intracluster light. Since the cluster appears not to be in a major merging
phase, we conclude that ram pressure stripping is the most plausible process
that generates the observed ICL sources.Comment: Accepted in A&A, english enhanced, figure location different than in
the A&A version due to different style files, shortened abstrac
Dynamic and structural properties of orthorhombic rare-earth manganites under high pressure
We report a high-pressure study of orthorhombic rare-earth manganites AMnO3
using Raman scattering (for A = Pr, Nd, Sm, Eu, Tb and Dy) and synchrotron
X-ray diffraction (for A = Pr, Sm, Eu, and Dy). In all cases, a structural and
insulator-to-metal transition was evidenced, with a critical pressure that
depends on the A-cation size. We analyze the compression mechanisms at work in
the different manganites via the pressure dependence of the lattice parameters,
the shear strain in the a-c plane, and the Raman bands associated with
out-of-phase MnO6 rotations and in-plane O2 symmetric stretching modes. Our
data show a crossover across the rare-earth series between two different kinds
of behavior. For the smallest A-cations, the compression is nearly isotropic in
the ac plane, with presumably only very slight changes of tilt angles and
Jahn-Teller distortion. As the radius of the A-cation increases, the
pressure-induced reduction of Jahn-Teller distortion becomes more pronounced
and increasingly significant as a compression mechanism, while the
pressure-induced bending of octahedra chains becomes conversely less
pronounced. We finally discuss our results in the light of the notion of
chemical pressure, and show that the analogy with hydrostatic pressure works
quite well for manganites with small A-cations but can be misleading with large
A-cations.Comment: 23 pages, 11 figure
Piezoelectric properties of twinned ferroelectric perovskites with head-to-head and tail-to-tail domain walls
Longitudinal piezoelectric coefficient of a twinned ferroelectric perovskite material with an array of partially compensated head-to-head and tail-to-tail 90-degree domain walls has been studied by phase-field simulations in the framework of the Ginzburg-Landau-Devonshire model of BaTiO3 ferroelectric. In particular, it is shown that the magnitude of the build-in extrinsic charge at the domain wall and the nanoscale domain size can both promote rotation of the static polarization vector within the body of adjacent domains. This polarization rotation drives the domain closer to an orthorhombic state, and the proximity to this ferroelectric-ferroelectric phase transition is directly responsible for the enhancement of the properties. Our simulations and the theory also suggest that the same system with nominally overcompensated charged walls may show a negative effective longitudinal piezoelectric coefficient. The obtained results can be used for quantitative estimates of piezoelectric properties of domain-engineered crystals
Multiple strain-induced phase transitions in LaNiO3 thin films
Strain effects on epitaxial thin films of LaNiO3 grown on different single
crystalline substrates are studied by Raman scattering and first-principles
simulation. New Raman modes, not present in bulk or fully-relaxed films, appear
under both compressive and tensile strains, indicating symmetry reductions.
Interestingly, the Raman spectra and the underlying crystal symmetry for
tensile and compressively strained films are different. Extensive mapping of
LaNiO3 phase stability is addressed by simulations, showing that a variety of
crystalline phases are indeed stabilized under strain which may impact the
electronic orbital hierarchy. The calculated Raman frequencies reproduce the
principal features of the experimental spectra, supporting the validity of the
multiple strain-driven structural transitions predicted by the simulations.Comment: 18 pages, 7 figure
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