21,008 research outputs found
A comparative study of optical/ultraviolet variability of narrow-line Seyfert 1 and broad-line Seyfert 1 active galactic nuclei
The ensemble optical/ultraviolet variability of narrow-line Seyfert 1 (NLS1)
type active galactic nuclei (AGNs) is investigated, based on a sample selected
from the Sloan Digital Sky Survey (SDSS) Stripe-82 region with multi-epoch
photometric scanning data. As a comparison a control sample of broad-line
Seyfert 1 (BLS1) type AGNs is also incorporated. To quantify properly the
intrinsic variation amplitudes and their uncertainties, a novel method of
parametric maximum-likelihood is introduced, that has, as we argued, certain
virtues over previously used methods. The majority of NLS1-type AGNs exhibit
significant variability on timescales from about ten days to a few years with,
however, on average smaller amplitudes compared to BLS1-type AGNs. About 20
NLS1- type AGNs showing relatively large variations are presented, that may
deserve future monitoring observations, for instance, reverberation mapping.
The averaged structure functions of variability, constructed using the same
maximumlikelihood method, show remarkable similarity in shape for the two types
of AGNs on timescales longer than about 10 days, which can be approximated by a
power-law or an exponential function. This, along with other similar
properties, such as the wavelength-dependent variability, are indicative of a
common dominant mechanism responsible for the long-term optical/UV variability
of both NLS1- and BLS1-type AGNs. Towards the short timescales, however, there
is tentative evidence that the structure function of NLS1-type AGNs continues
declining, whereas that of BLS1-type AGNs flattens with some residual
variability on timescales of days. If this can be confirmed, it may suggest
that an alternative mechanism, such as X-ray reprocessing, starts to become
dominating in BLS1-type AGNs, but not in NLS1-, on such timescales.Comment: 53 pages, 13 figures, 3 tables, accepted for pulication in A
On the classification of Kahler-Ricci solitons on Gorenstein del Pezzo surfaces
We give a classification of all pairs (X,v) of Gorenstein del Pezzo surfaces
X and vector fields v which are K-stable in the sense of Berman-Nystrom and
therefore are expected to admit a Kahler-Ricci solition. Moreover, we provide
some new examples of Fano threefolds admitting a Kahler-Ricci soliton.Comment: 21 pages, ancillary files containing calculations in SageMath; minor
correction
The effect of ruhrstahl and hereaeus (RH) operation on the mixing of molten steel
The flow and mixing characteristics of molten steel in the RH refining process were studied. A hydraulic simulation system with the geometric similarity ratio of 1:4 was built with 210 tons of RH as the prototype. The influence of different locations of tracers and different operation processes on the mixing of molten steel in ladle was studied. The results show that the RH operation should adopt a large air blowing amount, the insertion depth of the impregnated tube should not be less than 560 mm, and the liquid level of the vacuum chamber should be kept small
Acetylene-linked conjugated polymers for sacrificial photocatalytic hydrogen evolution from water
Conjugated organic polymers have shown potential as photocatalysts for hydrogen production by water splitting. Taking advantage of a high throughput screening workflow, two series of acetylene-linked co-polymers were prepared and studied for their potential as photocatalysts for sacrificial hydrogen production from water. It was found that a triethynylbenzene-based polymer with a dibenzo[b,d]thiophene sulfone linker (TE11) had the highest performance in terms of hydrogen evolution rate under visible illumination in the presence of a sacrificial hole-scavenger. Synthetically elaborating the triethynylbenzene linker in TE11 by changing the core and by introducing nitrogen, the resulting hydrogen evolution rate was further increased by a factor of nearly two
An Unobscured type II quasar candidate: SDSS J012032.19-005501.9
We report the finding of an unobscured type II Active Galactic Nuclei (AGN)
candidate, SDSS J012032.19-005501.9 at a relatively high redshift of
0.601,which shows a number of unusual properties. It varies significantly on
timescales of years as typical type I AGNs and marginally on timescales of
weeks. The color-magnitude relation and the structure function are also
consistent with that of type I AGNs, which imply that its variability likely
originates from the black hole accretion system .However, no broad emission
line is detected in the SDSS spectrum, and the upper limit of the equivalent
width of the H broad emission line is much less than that of type I
AGNs. These properties suggest that SDSS J012032.19-005501.9 may be an
unobscured quasar without broad emission lines intrinsically, namely an
unobscured type II AGN or "true" type II AGN. Furthermore, its continuum
luminosity is at least one order of magnitude fainter than the average value of
thepast century expected from the [OIII] emission line. It indicates that SDSS
J012032.19-005501.9 may be switching off. Additional possible scenarios to
explain this intriguing source are also discussed. Future deep observations at
multi-wavelengths are needed to reveal the nature of this peculiar and
intriguing AGN.Comment: 8 pages, 7 figures, 1 table, accepted for publication in The
Astronomical Journa
POSTNATAL BRAIN DYSMORPHOLOGY INDUCED BY PRENATAL ALCOHOL EXPOSURE: A PRECLINICAL MRI STUDY
poster abstractBrain dysmorphology is one of the most critical features of Fetal Alcohol Spectrum Disorders (FASD). This study was designed to use high resolution preclinical MRI system to compare the brain structures between alcohol exposed C57BL/6 mice with control. The objective is to examine how alcohol affects a dose- and timing-dependent brain dysmorphology during development comparable to that of human FASD. Three treated groups, ALC (pre- and pregnancy alcohol with 4.2 % (v/v) alcohol liquid), PF (pre alcohol and a calorically matched liquid pregnancy diet), and CHOW (ad lib chow/water), were examined. Mouse heads were imaged using 9.4T preclinical MRI system with 3D gradient echo (GRE) sequence to acquire volumetric images with voxel size as low as 40 microns. Whole brain, olfactory bulbs, cortex, hypothalamus, and cerebellum were segmented and the volumes were calculated. Data was examined by ANOVA followed with paired comparison between treatment groups to test the effect of prenatal alcohol exposure. ALC group had shown consistently smaller mean volumes of difference brain regions than the other two groups. Volume of total brain, olfactory bulbs and cerebellum were observed to be significantly different for ALC compared to PF pups. This indicated that prenatal alcohol exposure caused retarded fetal brain development. Comparing PF with CHOW pups, only cerebellum volume was observed to be significantly different. For cortex volume, no significant difference was shown for any pairwise comparison. These results suggest that alcohol effect contribute to brain dysmorphology, and match with our previous craniofacial dysmorphology study. This could be important to assist in the understanding of clinical variants of human FASD patients in brain dysmorphology
- …