988 research outputs found
Dielectric anomalies and spiral magnetic order in CoCr2O4
We have investigated the structural, magnetic, thermodynamic, and dielectric
properties of polycrystalline CoCrO, an insulating spinel exhibiting
both ferrimagnetic and spiral magnetic structures. Below = 94 K the
sample develops long-range ferrimagnetic order, and we attribute a sharp phase
transition at 25 K with the onset of long-range spiral magnetic
order. Neutron measurements confirm that while the structure remains cubic at
80 K and at 11 K; there is complex magnetic ordering by 11 K. Density
functional theory supports the view of a ferrimagnetic semiconductor with
magnetic interactions consistent with non-collinear ordering. Capacitance
measurements on CoCrO, show a sharp decrease in the dielectric constant
at , but also an anomaly showing thermal hysteresis falling between
approximately = 50 K and = 57 K. We tentatively attribute the
appearance of this higher temperature dielectric anomaly to the development of
\textit{short-range} spiral magnetic order, and discuss these results in the
context of utilizing dielectric spectroscopy to investigate non-collinear
short-range magnetic structures.Comment: & Figure
Thermodynamics of black holes: an analogy with glasses
The present equilibrium formulation of thermodynamics for black holes has
several drawbacks, such as assuming the same temperature for black hole and
heat bath. Recently the author formulated non-equilibrium thermodynamics for
glassy systems. This approach is applied to black holes, with the cosmic
background temperature being the bath temperature, and the Hawking temperature
the internal temperature. Both Hawking evaporation and absorption of background
radiation are taken into account.
It is argued that black holes did not form in the very early universe.Comment: 4 pages revtex; submitted to Phys. Rev. Let
The XMM-SSC survey of hard-spectrum XMM-Newton sources 1: optically bright sources
We present optical and X-ray data for a sample of serendipitous XMM-Newton
sources that are selected to have 0.5-2 keV vs 2-4.5 keV X-ray hardness ratios
which are harder than the X-ray background. The sources have 2-4.5 keV X-ray
flux >= 10^-14 cgs, and in this paper we examine a subsample of 42 optically
bright (r < 21) sources; this subsample is 100 per cent spectroscopically
identified. All but one of the optical counterparts are extragalactic, and we
argue that the single exception, a Galactic M star, is probably a coincidental
association. The X-ray spectra are consistent with heavily absorbed power laws
(21.8 < log NH < 23.4), and all of them appear to be absorbed AGN. The majority
of the sources show only narrow emission lines in their optical spectra,
implying that they are type-2 AGN. Only a small fraction of the sources (7/42)
show broad optical emission lines, and all of these have NH < 10^23 cm^-2. This
implies that ratios of X-ray absorption to optical/UV extinction equivalent to
> 100 times the Galactic gas-to-dust ratio are rare in AGN absorbers (at most a
few percent of the population), and may be restricted to broad absorption-line
QSOs. Seven objects appear to have an additional soft X-ray component in
addition to the heavily absorbed power law. We consider the implications of our
results in the light of the AGN unified scheme. We find that the soft
components in narrow-line objects are consistent with the unified scheme
provided that > 4 per cent of broad-line AGN have ionised absorbers that
attenuate their soft X-ray flux by >50 per cent. In at least one of the X-ray
absorbed, broad-line AGN in our sample the X-ray spectrum requires an ionised
absorber, consistent with this picture.Comment: accepted for publication in MNRA
Anisotropic multi-gap superfluid states in nuclear matter
It is shown that under changing density or temperature a nucleon Fermi
superfluid can undergo a phase transition to an anisotropic superfluid state,
characterized by nonvanishing gaps in pairing channels with singlet-singlet
(SS) and triplet-singlet (TS) pairing of nucleons (in spin and isospin spaces).
In the SS pairing channel nucleons are paired with nonzero orbital angular
momentum. Such two-gap states can arise as a result of branching from the
one-gap solution of the self-consistent equations, describing SS or TS pairing
of nucleons, that depends on the relationship between SS and TS coupling
constants at the branching point. The density/temperature dependence of the
order parameters and the critical temperature for transition to the anisotropic
two-gap state are determined in a model with the SkP effective interaction. It
is shown that the anisotropic SS-TS superfluid phase corresponds to a
metastable state in nuclear matter.Comment: Prepared with RevTeX4, 7p., 5 fi
Peripheral tolerance to alloantigen results from altered regulation of the interleukin-2 pathway
Tolerance to alloantigen may be induced in rats by administration of blood followed by transplantation of a renal allograft. The mechanism of this tolerance was investigated by directly analyzing the functional activity of graft-infiltrating cells. We have previously shown cytotoxic T lymphocyte infiltration of, and major histocompatibility complex induction on, grafts of tolerant animals. We now report that cells isolated from the grafts of tolerant rats show a reduced expression of the p55 interleukin 2 receptor (IL-2R) chain on the cell surface compared with that seen on the cells of untreated animals. Scatchard analysis further reveals low expression of high affinity IL-2R. This is due to reduced transcription of both IL-2R alpha and beta chain mRNAs and results in a reduced ability of cells to proliferate in response to IL-2. Cells isolated from tolerant animals are unable to make biologically active IL-2 in culture, whereas cells from untreated animals make high levels. This is not reflected at the mRNA level as the IL-2 gene is induced in both tolerant and untreated animals to similar levels. The induction of tolerance is abrogated by administration of recombinant IL-2 to animals at the time of transplantation. Thus, we conclude that an altered regulation of the IL-2 pathway results in tolerance in these alloantigen-treated and transplanted animals
Detection of an X-ray periodicity in the Narrow-line Seyfert 1 Galaxy Mrk 766 with XMM-Newton
We have analyzed the timing properties of the Narrow-line Seyfert 1 galaxy
Mrk 766 observed with XMM-Newton during the PV phase. The source intensity
changes by a factor of 1.3 over the 29,000 second observation. If the soft
excess is modeled by a black body component, as indicated by the EPIC pn data,
the luminosity of the black body component scales with its temperature
according to L ~ T^4. This requires a lower limit 'black body size` of about
1.3*10^25 cm^2. In addition, we report the detection of a strong periodic
signal with 2.4*10^-4 Hz. Simulations of light curves with the observed time
sequence and phase randomized for a red noise spectrum clearly indicate that
the periodicity peak is intrinsic to the distant AGN. Furthermore, its
existence is confirmed by the EPIC MOS and RGS data. The spectral fitting
results show that the black body temperature and the absorption by neutral
hydrogen remain constant during the periodic oscillations. This observational
fact tends to rule out models in which the intensity changes are due to hot
spots orbiting the central black hole. Precession according to the
Bardeen-Petterson effect or instabilities in the inner accretion disk may
provide explanations for the periodic signal.Comment: 6 pages, 8 Figures, accepted for publication in the special A&A
Letters issue for XMM-Newton; corrections in Section 2.2 require
Silicate dust in the environment of RS Ophiuchi following the 2006 eruption
We present further Spitzer Space Telescope observations of the recurrent nova
RS Ophiuchi, obtained over the period 208-430 days after the 2006 eruption. The
later Spitzer IRS data show that the line emission and free-free continuum
emission reported earlier is declining, revealing incontrovertible evidence for
the presence of silicate emission features at 9.7 and 18microns. We conclude
that the silicate dust survives the hard radiation impulse and shock blast wave
from the eruption. The existence of the extant dust may have significant
implications for understanding the propagation of shocks through the red giant
wind and likely wind geometry.Comment: 12 pages, 4 figures, accepted for publication in ApJ (Letters
Multi-wavelength observations of the energetic GRB 080810: detailed mapping of the broadband spectral evolution
GRB 080810 was one of the first bursts to trigger both Swift and the Fermi
Gamma-ray Space Telescope. It was subsequently monitored over the X-ray and
UV/optical bands by Swift, in the optical by ROTSE and a host of other
telescopes and was detected in the radio by the VLA. The redshift of z= 3.355
+/- 0.005 was determined by Keck/HIRES and confirmed by RTT150 and NOT. The
prompt gamma/X-ray emission, detected over 0.3-10^3 keV, systematically softens
over time, with E_peak moving from ~600 keV at the start to ~40 keV around 100
s after the trigger; alternatively, this spectral evolution could be identified
with the blackbody temperature of a quasithermal model shifting from ~60 keV to
~3 keV over the same time interval. The first optical detection was made at 38
s, but the smooth, featureless profile of the full optical coverage implies
that this originated from the afterglow component, not the pulsed/flaring
prompt emission.
Broadband optical and X-ray coverage of the afterglow at the start of the
final X-ray decay (~8 ks) reveals a spectral break between the optical and
X-ray bands in the range 10^15 - 2x10^16 Hz. The decay profiles of the X-ray
and optical bands show that this break initially migrates blueward to this
frequency and then subsequently drifts redward to below the optical band by
~3x10^5 s. GRB 080810 was very energetic, with an isotropic energy output for
the prompt component of 3x10^53 erg and 1.6x10^52 erg for the afterglow; there
is no evidence for a jet break in the afterglow up to six days following the
burst.Comment: 15 pages, 9 figures, 4 in colour. Accepted for publication in MNRA
A multi-wavelength survey of AGN in the XMM-LSS field: I. Quasar selection via the KX technique
AIMS: We present a sample of candidate quasars selected using the
KX-technique. The data cover 0.68 deg^2 of the X-ray Multi-Mirror (XMM)
Large-Scale Structure (LSS) survey area where overlapping multi-wavelength
imaging data permits an investigation of the physical nature of selected
sources. METHODS: The KX method identifies quasars on the basis of their
optical (R and z') to near-infrared (Ks) photometry and point-like morphology.
We combine these data with optical (u*,g'r',i',z') and mid-infrared (3.6-24
micron) wavebands to reconstruct the spectral energy distributions (SEDs) of
candidate quasars. RESULTS: Of 93 sources selected as candidate quasars by the
KX method, 25 are classified as quasars by the subsequent SED analysis.
Spectroscopic observations are available for 12/25 of these sources and confirm
the quasar hypothesis in each case. Even more, 90% of the SED-classified
quasars show X-ray emission, a property not shared by any of the false
candidates in the KX-selected sample. Applying a photometric redshift analysis
to the sources without spectroscopy indicates that the 25 sources classified as
quasars occupy the interval 0.7 < z < 2.5. The remaining 68/93 sources are
classified as stars and unresolved galaxies.Comment: 13 pages, 9 figures, A&A 494, p. 579-589. Replaced with published
version. Fig. 9 in first astro-ph submission has been update
Systems-level analyses identify extensive coupling among gene expression machines
Here, we develop computational methods to assess and consolidate large, diverse protein interaction data sets, with the objective of identifying proteins involved in the coupling of multicomponent complexes within the yeast gene expression pathway. From among ∼43 000 total interactions and 2100 proteins, our methods identify known structural complexes, such as the spliceosome and SAGA, and functional modules, such as the DEAD-box helicases, within the interaction network of proteins involved in gene expression. Our process identifies and ranks instances of three distinct, biologically motivated motifs, or patterns of coupling among distinct machineries involved in different subprocesses of gene expression. Our results confirm known coupling among transcription, RNA processing, and export, and predict further coupling with translation and nonsense-mediated decay. We systematically corroborate our analysis with two independent, comprehensive experimental data sets. The methods presented here may be generalized to other biological processes and organisms to generate principled, systems-level network models that provide experimentally testable hypotheses for coupling among biological machines
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