86,606 research outputs found
New hyperthermal thermosetting heterocyclic polymers
Polyimidazopyrrolone polymers, formed by the condensation of aromatic dianhydrides with aromatic tetraamines in various solvents, form moldings that resist degradation in air and retain great strength at 400 to 700 degrees F. The resins have good insulating properties, are easy to mold, and make good protective coatings
Some winter characteristics of the northern high latitude ionosphere
Langmuir probe measurements of ionospheric electron density and temperature during winte
The Global Implications of the Hard X-ray Excess in Type 1 AGN
Recent evidence for a strong 'hard excess' of flux at energies > 20 keV in
some Suzaku observations of type 1 Active Galactic Nuclei (AGN) has motivated
an exploratory study of the phenomenon in the local type 1 AGN population. We
have selected all type 1 AGN in the Swift Burst Alert Telescope (BAT) 58-month
catalog and cross-correlated them with the holdings of the Suzaku public
archive. We find the hard excess phenomenon to be a ubiquitous property of type
1 AGN. Taken together, the spectral hardness and equivalent width of Fe K alpha
emission are consistent with reprocessing by an ensemble of Compton-thick
clouds that partially cover the continuum source. In the context of such a
model, ~ 80 % of the sample has a hardness ratio consistent with > 50% covering
of the continuum by low-ionization, Compton-thick gas. More detailed study of
the three hardest X-ray spectra in our sample reveal a sharp Fe K absorption
edge at ~ 7 keV in each of them, indicating that blurred reflection is not
responsible for the very hard spectral forms. Simple considerations place the
distribution of Compton-thick clouds at or within the optical broad line
region.Comment: Accepted for publication in Ap
Combining Physical galaxy models with radio observations to constrain the SFRs of high-z dusty star forming galaxies
We complement our previous analysis of a sample of z~1-2 luminous and
ultra-luminous infrared galaxies ((U)LIRGs), by adding deep VLA radio
observations at 1.4 GHz to a large data-set from the far-UV to the sub-mm,
including Spitzer and Herschel data. Given the relatively small number of
(U)LIRGs in our sample with high S/N radio data, and to extend our study to a
different family of galaxies, we also include 6 well sampled near IR-selected
BzK galaxies at z~1.5. From our analysis based on the radiative transfer
spectral synthesis code GRASIL, we find that, while the IR luminosity may be a
biased tracer of the star formation rate (SFR) depending on the age of stars
dominating the dust heating, the inclusion of the radio flux offers
significantly tighter constraints on SFR. Our predicted SFRs are in good
agreement with the estimates based on rest-frame radio luminosity and the Bell
(2003) calibration. The extensive spectro-photometric coverage of our sample
allows us to set important constraints on the SF history of individual objects.
For essentially all galaxies we find evidence for a rather continuous SFR and a
peak epoch of SF preceding that of the observation by a few Gyrs. This seems to
correspond to a formation redshift of z~5-6. We finally show that our physical
analysis may affect the interpretation of the SFR-M* diagram, by possibly
shifting, with respect to previous works, the position of the most dust
obscured objects to higher M* and lower SFRs.Comment: 26 pages, 15 figures, 3 tables, accepted for publication in MNRAS on
Dec. 4th, 201
The Variable X-ray Spectrum of Markarian 766 - II. Time-Resolved Spectroscopy
CONTEXT: The variable X-ray spectra of AGN systematically show steep
power-law high states and hard-spectrum low states. The hard low state has
previously been found to be a component with only weak variability. The origin
of this component and the relative importance of effects such as absorption and
relativistic blurring are currently not clear. AIMS: In a follow-up of previous
principal components analysis, we aim to determine the relative importance of
scattering and absorption effects on the time-varying X-ray spectrum of the
narrow-line Seyfert 1 galaxy Mrk~766. METHODS: Time-resolved spectroscopy,
slicing XMM and Suzaku data down to 25 ks elements, is used to investigate
whether absorption or scattering components dominate the spectral variations in
Mrk 766.Time-resolved spectroscopy confirms that spectral variability in Mrk
766 can be explained by either of two interpretations of principal components
analysis. Detailed investigation confirm rapid changes in the relative
strengths of scattered and direct emission or rapid changes in absorber
covering fraction provide good explanations of most of the spectral
variability. However, a strong correlation between the 6.97 keV absorption line
and the primary continuum together with rapid opacity changes show that
variations in a complex and multi-layered absorber, most likely a disk wind,
are the dominant source of spectral variability in Mrk 76
Role of the nuclear vector potential in deep inelastic scattering
We study the influence of the strong nuclear vector potential, treated using
the mean-field approximation, in deep inelastic scattering. A consistent
treatment of the electromagnetic current operator, combined with the use of the
operator product expansion is presented and discussed.Comment: 7 pages, 0 figur
X-ray variability analysis of a large series of XMM-Newton + NuSTAR observations of NGC 3227
We present a series of X-ray variability results from a long XMM-Newton +
NuSTAR campaign on the bright, variable AGN NGC 3227. We present an analysis of
the lightcurves, showing that the source displays typically
softer-when-brighter behaviour, although also undergoes significant spectral
hardening during one observation which we interpret as due to an occultation
event by a cloud of absorbing gas. We spectrally decompose the data and show
that the bulk of the variability is continuum-driven and, through rms
variability analysis, strongly enhanced in the soft band. We show that the
source largely conforms to linear rms-flux behaviour and we compute X-ray power
spectra, detecting moderate evidence for a bend in the power spectrum,
consistent with existing scaling relations. Additionally, we compute X-ray
Fourier time lags using both the XMM-Newton and - through maximum-likelihood
methods - NuSTAR data, revealing a strong low-frequency hard lag and evidence
for a soft lag at higher frequencies, which we discuss in terms of
reverberation models.Comment: Accepted for publication in MNRAS; 19 pages, 13 figures, 4 tables;
minor typographical errors corrected and reference list update
Dissipationless Anomalous Hall Current in the Ferromagnetic Spinel CuCrSeBr
In a ferromagnet, an applied electric field invariably produces an
anomalous Hall current that flows perpendicular to the plane
defined by and (the magnetization). For decades, the question
whether is dissipationless (independent of the scattering rate),
has been keenly debated without experimental resolution. In the ferromagnetic
spinel CuCrSeBr, the resistivity (at low temperature)
may be increased 1000 fold by varying (Br), without degrading the .
We show that (normalized per carrier, at 5 K) remains unchanged
throughout. In addition to resolving the controversy experimentally, our
finding has strong bearing on the generation and study of spin-Hall currents in
bulk samples.Comment: 7 pages, 6 figure
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