19,203 research outputs found
Investigation of the shear-mechanical and dielectric relaxation processes in two mono-alcohols close to the glass transition
Shear-mechanical and dielectric measurements on the two monohydroxy
(mono-alcohol) molecular glass formers 2-ethyl-1-hexanol and 2-butanol close to
the glass transition temperature are presented. The shear-mechanical data are
obtained using the piezoelectric shear-modulus gauge method covering
frequencies from 1mHz to 10kHz. The shear-mechanical relaxation spectra show
two processes, which follow the typical scenario of a structural (alpha)
relaxation and an additional (Johari-Goldstein) beta relaxation. The dielectric
relaxation spectra are dominated by a Debye-type peak with an additional
non-Debye peak visible. This Debye-type relaxation is a common feature peculiar
to mono-alcohols. The time scale of the non-Debye dielectric relaxation process
is shown to correspond to the mechanical structural (alpha) relaxation.
Glass-transition temperatures and fragilities are reported based on the
mechanical alpha relaxation and the dielectric Debye-type process, showing that
the two glass-transition temperatures differ by approximately 10K and that the
fragility based on the Debye-type process is a factor of two smaller than the
structural fragility. If a mechanical signature of the Debye-type relaxation
exists in these liquids, its relaxation strength is at most 1% and 3% of the
full relaxation strength of 2-butanol and 2-ethyl-1-hexanol respectively. These
findings support the notion that it is the non-Debye dielectric relaxation
process that corresponds to the structural alpha relaxation in the liquid.Comment: 8 pages, 6 figures. Minor corrections, updated figures, more
dielectric data show
Observations of MCG-5-23-16 with Suzaku, XMM-Newton and NuSTAR: Disk tomography and Compton hump reverberation
MCG-5-23-16 is one of the first AGN where relativistic reverberation in the
iron K line originating in the vicinity of the supermassive black hole was
found, based on a short XMM-Newton observation. In this work, we present the
results from long X-ray observations using Suzaku, XMM-Newton and NuSTAR
designed to map the emission region using X-ray reverberation. A relativistic
iron line is detected in the lag spectra on three different time-scales,
allowing the emission from different regions around the black hole to be
separated. Using NuSTAR coverage of energies above 10 keV reveals a lag between
these energies and the primary continuum, which is detected for the first time
in an AGN. This lag is a result of the Compton reflection hump responding to
changes in the primary source in a manner similar to the response of the
relativistic iron K line.Comment: Accepted for Publication in Ap
The Compton hump and variable blue wing in the extreme low-flux NuSTAR observations of 1H0707-495
The Narrow-line Seyfert I galaxy, 1H0707-495, has been well observed in the
0.3-10 keV band, revealing a dramatic drop in flux in the iron K alpha band, a
strong soft excess, and short timescale reverberation lags associated with
these spectral features. In this paper, we present the first results of a deep
250 ks NuSTAR observation of 1H0707-495, which includes the first sensitive
observations above 10 keV. Even though the NuSTAR observations caught the
source in an extreme low-flux state, the Compton hump is still significantly
detected. NuSTAR, with its high effective area above 7 keV, clearly detects the
drop in flux in the iron K alpha band, and by comparing these observations with
archival XMM-Newton observations, we find that the energy of this drop
increases with increasing flux. We discuss possible explanations for this, the
most likely of which is that the drop in flux is the blue wing of the
relativistically broadened iron K alpha emission line. When the flux is low,
the coronal source height is low, thus enhancing the most gravitationally
redshifted emission.Comment: Submitted to MNRAS, comments are welcome. 9 pages, 5 figure
Spin liquid in a single crystal of the frustrated diamond lattice antiferromagnet CoAl2O4
We study spin liquid in the frustrated diamond lattice antiferromagnet
CoAl2O4 by means of single crystal neutron scattering in zero and applied
magnetic field. The magnetically ordered phase appearing below TN=8 K remains
nonconventional down to 1.5 K. The magnetic Bragg peaks at the q=0 positions
remain broad and their profiles have strong Lorentzian contribution.
Additionally, they are connected by weak diffuse streaks along the
directions. These observations are explained within the spiral spin liquid
model as short-range magnetic correlations of spirals populated at these finite
temperatures, as the energy minimum around q=0 is flat and the energy of
excited states with q=(111) is low. The agreement is only qualitative, leading
us to suspect that microstructure effects are also important. Magnetic field
significantly perturbs spin correlations. The 1.5 K static magnetic moment
increases from 1.58 mB/Co at zero field to 2.08 mB/Co at 10 T, while the
magnetic peaks, being still broad, acquire almost Gaussian profile. Spin
excitations are rather conventional spin waves at zero field, resulting in the
exchange parameters J1=0.92(1) meV, J2=0.101(2) meV and the anisotropy term
D=-0.0089(2) meV for CoAl2O4. The application of a magnetic field leads to a
pronounced broadening of the excitations at the zone center, which at 10 T
appear gapless and nearly featureless
The merging/AGN connection: A case for 3D spectroscopy
We discuss an ongoing study of the connection between galaxy
merging/interaction and AGN activity, based on integral field spectroscopy. We
focus on the search for AGN ionization in the central regions of mergers,
previously not classified as AGNs. We present here the science case, the
current status of the project, and plans for future observations.Comment: 4 pages, 3 figure, Euro3D Science Workshop, Cambridge, May 2003, AN,
accepte
NEAR-SURFACE EFFECTS IN MODELLING OSCILLATIONS OF ETA BOO
Following the report of solar-like oscillations in the G0 V star eta Boo
(Kjeldsen et al. 1995, AJ 109, 1313), a first attempt to model the observed
frequencies was made by Christensen-Dalsgaard et al. (1995, ApJ Letters, in
press). This attempt succeeded in reproducing the observed frequency
separations, although there remained a difference of about 10 microHz between
observed and computed frequencies. In those models, the near-surface region of
the star was treated rather crudely. Here we consider more sophisticated models
that include non-local mixing-length theory, turbulent pressure and
nonadiabatic oscillations.Comment: uuencoded and compressed Postscript (2 pages, including figure); To
appear in Proceedings of IAU Colloquium 155, "Astrophysical Applications of
Stellar Pulsation", Cape Town, South Afric
IC 751: a new changing-look AGN discovered by NuSTAR
We present the results of five NuSTAR observations of the type 2 active
galactic nucleus (AGN) in IC 751, three of which were performed simultaneously
with XMM-Newton or Swift/XRT. We find that the nuclear X-ray source underwent a
clear transition from a Compton-thick () to a Compton-thin () state on timescales of months, which makes
IC 751 the first changing-look AGN discovered by NuSTAR. Changes of the
line-of-sight column density at a level are also found on a
time-scale of hours ().
From the lack of spectral variability on timescales of ks we infer
that the varying absorber is located beyond the emission-weighted average
radius of the broad-line region, and could therefore be related either to the
external part of the broad-line region or a clumpy molecular torus. By adopting
a physical torus X-ray spectral model, we are able to disentangle the column
density of the non-varying absorber () from that of the varying clouds
[], and to constrain that of
the material responsible for the reprocessed X-ray radiation (). We find evidence of significant intrinsic X-ray
variability, with the flux varying by a factor of five on timescales of a few
months in the 2-10 and 10-50 keV band.Comment: Accepted for publication in ApJ, 11 pages, 6 figure
Dark matter line emission constraints from NuSTAR observations of the Bullet Cluster
Line emission from dark matter is well motivated for some candidates e.g.
sterile neutrinos. We present the first search for dark matter line emission in
the 3-80keV range in a pointed observation of the Bullet Cluster with NuSTAR.
We do not detect any significant line emission and instead we derive upper
limits (95% CL) on the flux, and interpret these constraints in the context of
sterile neutrinos and more generic dark matter candidates. NuSTAR does not have
the sensitivity to constrain the recently claimed line detection at 3.5keV, but
improves on the constraints for energies of 10-25keV.Comment: 7 pages, 5 figures, submitted to Ap
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