264 research outputs found
Polyimide/silica hybrids via the sol-gel route: High performance materials for the new technological challenges
The present review article describes in detail the state-of-the-art of organic-inorganic hybrid materials based on polyimide/silica components. The article is divided in three parts. In the first the basic processing route for the preparation of these systems is described, i.e. the sol-gel technique, along with the strategies developed to control the final morphology. In the second part the curing characteristics, the dynamic-mechanical and the mechanical and fracture properties of hybrids with different morphologies are reviewed. Finally, the more technologically relevant applications devised for these high performance materials are discussed
Electrical and thermal properties of polyimide/silica nanocomposite
The electrical and thermal properties of thin films of polyimide/silica nanocomposites prepared via solâgel process were studied as a function of nanosilica particles content, temperature and applied field frequency. It was found that the dielectric constant and dielectric loss of the nanocomposites decrease with both the frequency and the nanosilica content, while increase with temperature. The AC-conductivity measured in frequency range 200âkHzâ1.5âMHz decreases with the filler concentration and increases with increasing temperature. For the (25âwt%) nanocomposite, it was found that the AC-conductivity increases with temperature, and the ColeâCole plots showed that the calculated activation energy and relaxation time decrease with temperature. The observed thermal conductivity increases gently with temperature. The empirical universal law was used to fit the observed electrical data under the measuring conditions
Time dynamics of background noise in geoelectrical and geochemical signals: an application in a seismic area of Southern Italy
We analyse geoelectrical and geochemical time series jointly measured by means of a multiparametric automatic station close to an anomalous fluid emission in Val dâAgri (Basilicata, Italy). The investigated area is located on Southern Apennine chain that in past and recent years was interested by destructive earthquakes. After a complete
pre-processing of time series, we analyse the fluctuations triggered by the seasonal cycles and focus our attention on the possible link between geoelectrical and geochemical signals. In order to extract quantitative dynamical information from experimental time series, we detect scaling laws in power spectra that are typical fingerprints of fractional Brownian processes. After this analysis, the problem of the identification of extreme events in the time series has been approached. We consider significant anomalous patterns only when more consecutive values are above/below a fixed threshold in almost two of the time series jointly measured. We give the first preliminary results about the comparison between anomalous patterns detected in geoelectrical and geochemical parameters and the local seismic activity and, finally, analyse the implications with the earthquake prediction problem
Optically variable active galactic nuclei in the 3 yr VST survey of the COSMOS field
The analysis of the variability of active galactic nuclei (AGNs) at different
wavelengths and the study of possible correlations among different spectral
windows are nowadays a major field of inquiry. Optical variability has been
largely used to identify AGNs in multivisit surveys. The strength of a
selection based on optical variability lies in the chance to analyze data from
surveys of large sky areas by ground-based telescopes. However the
effectiveness of optical variability selection, with respect to other
multiwavelength techniques, has been poorly studied down to the depth expected
from next generation surveys. Here we present the results of our r-band
analysis of a sample of 299 optically variable AGN candidates in the VST survey
of the COSMOS field, counting 54 visits spread over three observing seasons
spanning > 3 yr. This dataset is > 3 times larger in size than the one
presented in our previous analysis (De Cicco et al. 2015), and the observing
baseline is ~8 times longer. We push towards deeper magnitudes (r(AB) ~23.5
mag) compared to past studies; we make wide use of ancillary multiwavelength
catalogs in order to confirm the nature of our AGN candidates, and constrain
the accuracy of the method based on spectroscopic and photometric diagnostics.
We also perform tests aimed at assessing the relevance of dense sampling in
view of future wide-field surveys. We demonstrate that the method allows the
selection of high-purity (> 86%) samples. We take advantage of the longer
observing baseline to achieve great improvement in the completeness of our
sample with respect to X-ray and spectroscopically confirmed samples of AGNs
(59%, vs. ~15% in our previous work), as well as in the completeness of
unobscured and obscured AGNs. The effectiveness of the method confirms the
importance to develop future, more refined techniques for the automated
analysis of larger datasets.Comment: 21 pages, 10 figures; accepted for publication in A&
Variability and transient search in the SUDAREâVOICE field: A new method to extract the light curves
The VLT Survey Telescope (VST) Optical Imaging of the CDFS and ES1 Fields Survey, in
synergy with the SUDARE survey, is a deep optical ugri imaging of the CDFS and ES1 fields
using the VST. The observations for the CDFS field comprise about 4.38 deg2 down to r âŒ
26 mag. The total on-sky time spans over 4 yr in this field, distributed over four adjacent subfields. In this paper, we use the multiepoch r-band imaging data to measure the variability of
the detected objects and search for transients. We perform careful astrometric and photometric
calibrations and point spread function modelling. A new method, referring to as differential
running-average photometry, is proposed to measure the light curves of the detected objects.
With the method, the difference of PSFs between different epochs can be reduced, and the
background fluctuations are also suppressed
Differing myocardial response to a single session of hemodialysis in end-stage renal disease with and without type 2 diabetes mellitus and coronary artery disease
BACKGROUND: Though hemodialysis (HD) acutely improves cardiac function, the impact of background diseases like coronary artery disease (CAD) and Type 2 diabetes (DM) in the setting of end-stage renal disease (ESRD) is not known. Tissue velocity echocardiography (TVE) offers a fast choice to follow changes in myocardial function after HD in ESRD with concomitant DM and /or CAD. METHODS: 46 subjects (17 with ESRD, Group 1; 15 with DM, Group 2; 14 with DM+CAD, Group 3) underwent standard and TVE prior to and shortly after HD. Besides standard Doppler variables, regional myocardial systolic and diastolic velocities, as well as systolic strain rate were post processed. RESULTS: Compared with pre-HD, post-HD body weight (kg) significantly decreased in all the three groups (51 ± 9 vs. 48 ± 8, 62 ± 10 vs.59 ± 10, and 61 ± 9 vs. 58 ± 9 respectively; all p < 0.01). Left ventricular end diastolic dimensions (mm) also decreased post- HD (46 ± 5 vs. 42 ± 7, 53 ± 7 vs. 50 ± 7, 51 ± 7 vs. 47 ± 8 respectively; all p < 0.01). Regional longitudinal peak systolic velocity in septum (cm/s) significantly increased post-HD in Group 1(5.7 ± 1.6 vs. 7.2 ± 2.3; p < 0.001) while remained unchanged in the other two groups. Similar trends were noted in other left ventricular walls. When the myocardial velocities (cm/s) were computed globally, the improvement was seen only in Group 1 (6.3 ± 1.5 vs. 7.9 ± 2.0; p < 0.001). Global early regional diastolic velocity (cm/s) improved in Group 1, remained unchanged in Group 2, while significantly decreased in Group 3(-5.9 ± 1.3 vs. -4.1 ± 1.8; p < 0.01). Global systolic strain rate (1/sec) increased in the first 2 Groups but remained unchanged (-0.87 ± 0.4 vs. -0.94 ± 0.3; p = ns) in Group 3. CONCLUSION: A single HD session improves LV function only in ESRD without coexistent DM and/or CAD. The present data suggest that not only dialysis-dependent changes in loading conditions but also co-existent background diseases determine the myocardial response to HD
A Sensitive Search for Supernova Emission Associated with the Extremely Energetic and Nearby GRB 221009A
We report observations of the optical counterpart of the long gamma-ray burst
(LGRB) GRB 221009A. Due to the extreme rarity of being both nearby () and highly energetic ( erg), GRB
221009A offers a unique opportunity to probe the connection between massive
star core collapse and relativistic jet formation across a very broad range of
-ray properties. Adopting a phenomenological power-law model for the
afterglow and host galaxy estimates from high-resolution Hubble Space Telescope
imaging, we use Bayesian model comparison techniques to determine the
likelihood of an associated SN contributing excess flux to the optical light
curve. Though not conclusive, we find moderate evidence
() for the presence of an additional component arising
from an associated supernova, SN 2022xiw, and find that it must be
substantially fainter ( 67% as bright at the 99% confidence interval) than
SN 1998bw. Given the large and uncertain line-of-sight extinction, we attempt
to constrain the supernova parameters (, ,
and ) under several different assumptions with respect to the
host galaxy's extinction. We find properties that are broadly consistent with
previous GRB-associated SNe: - ,
- , and - . We note that these properties are weakly
constrained due to the faintness of the supernova with respect to the afterglow
and host emission, but we do find a robust upper limit on the of
. Given the tremendous range in isotropic
gamma-ray energy release exhibited by GRBs (7 orders of magnitude), the SN
emission appears to be decoupled from the central engine in these systems.Comment: 18 pages, accepted to ApJL, 4 tables, 5 figures. Updated abstract in
Previe
Extending the variability selection of active galactic nuclei in the W-CDF-S and SERVS/SWIRE region
Variability has proven to be a powerful tool to detect active galactic nuclei (AGN) in multi-epoch surveys. The new-generation facilities expected to become operational in the next few years will mark a new era in time-domain astronomy and their wide-field multi-epoch campaigns will favor extensive variability studies.We present our analysis of AGN variability in the second half of the VST survey of the Wide Chandra Deep Field South, performed in the r band and covering a 2 sq. deg area. The analysis complements a previous work, in which the first half of the area was investigated. We provide a reliable catalog of variable AGN candidates, which will be critical targets in future variability studies
SN 2022jli: a type Ic supernova with periodic modulation of its light curve and an unusually long rise
We present multi-wavelength photometry and spectroscopy of SN 2022jli, an
unprecedented Type Ic supernova discovered in the galaxy NGC 157 at a distance
of 23 Mpc. The multi-band light curves reveal many remarkable
characteristics. Peaking at a magnitude of , the high-cadence
photometry reveals 12.5day periodic undulations superimposed on the
200 day supernova decline. This periodicity is observed in the light curves
from nine separate filter and instrument configurations with peak-to-peak
amplitudes of 0.1 mag. This is the first time that repeated periodic
oscillations, over many cycles, have been detected in a supernova light curve.
SN 2022jli also displays an extreme early excess which fades over 25
days followed by a rise to a peak luminosity of erg
s. Although the exact explosion epoch is not constrained by data, the
time from explosion to maximum light is 59 days. The luminosity can
be explained by a large ejecta mass (M)
powered by Ni but we find difficulty in quantitatively modelling the
early excess with circumstellar interaction and cooling. Collision between the
supernova ejecta and a binary companion is a possible source of this emission.
We discuss the origin of the periodic variability in the light curve, including
interaction of the SN ejecta with nested shells of circumstellar matter and
neutron stars colliding with binary companions.Comment: Accepted in ApJ
Multiwavelength observations of the extraordinary accretion event AT2021lwx
We present observations from X-ray to mid-infrared wavelengths of the most
energetic non-quasar transient ever observed, AT2021lwx. Our data show a single
optical brightening by a factor to a luminosity of erg
s, and a total radiated energy of erg, both greater
than any known optical transient. The decline is smooth and exponential and the
ultra-violet - optical spectral energy distribution resembles a black body with
temperature K. Tentative X-ray detections indicate a secondary
mode of emission, while a delayed mid-infrared flare points to the presence of
dust surrounding the transient. The spectra are similar to recently discovered
optical flares in known active galactic nuclei but lack some characteristic
features. The lack of emission for the previous seven years is inconsistent
with the short-term, stochastic variability observed in quasars, while the
extreme luminosity and long timescale of the transient disfavour the disruption
of a single solar-mass star. The luminosity could be generated by the
disruption of a much more massive star, but the likelihood of such an event
occurring is small. A plausible scenario is the accretion of a giant molecular
cloud by a dormant black hole of solar masses. AT2021lwx thus
represents an extreme extension of the known scenarios of black hole accretion.Comment: 11 pages, 5 figures, Accepted for publication in MNRA
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