1,053 research outputs found
Understanding extreme quasar optical variability with CRTS: I. Major AGN flares
There is a large degree of variety in the optical variability of quasars and
it is unclear whether this is all attributable to a single (set of) physical
mechanism(s). We present the results of a systematic search for major flares in
AGN in the Catalina Real-time Transient Survey as part of a broader study into
extreme quasar variability. Such flares are defined in a quantitative manner as
being atop of the normal, stochastic variability of quasars. We have identified
51 events from over 900,000 known quasars and high probability quasar
candidates, typically lasting 900 days and with a median peak amplitude of
mag. Characterizing the flare profile with a Weibull
distribution, we find that nine of the sources are well described by a
single-point single-lens model. This supports the proposal by Lawrence et al.
(2016) that microlensing is a plausible physical mechanism for extreme
variability. However, we attribute the majority of our events to explosive
stellar-related activity in the accretion disk: superluminous supernovae, tidal
disruption events, and mergers of stellar mass black holes.Comment: 25 pages, 18 figures, accepted for publication by MNRA
A systematic search for close supermassive black hole binaries in the Catalina Real-Time Transient Survey
Hierarchical assembly models predict a population of supermassive black hole
(SMBH) binaries. These are not resolvable by direct imaging but may be
detectable via periodic variability (or nanohertz frequency gravitational
waves). Following our detection of a 5.2 year periodic signal in the quasar PG
1302-102 (Graham et al. 2015), we present a novel analysis of the optical
variability of 243,500 known spectroscopically confirmed quasars using data
from the Catalina Real-time Transient Survey (CRTS) to look for close (< 0.1
pc) SMBH systems. Looking for a strong Keplerian periodic signal with at least
1.5 cycles over a baseline of nine years, we find a sample of 111 candidate
objects. This is in conservative agreement with theoretical predictions from
models of binary SMBH populations. Simulated data sets, assuming stochastic
variability, also produce no equivalent candidates implying a low likelihood of
spurious detections. The periodicity seen is likely attributable to either jet
precession, warped accretion disks or periodic accretion associated with a
close SMBH binary system. We also consider how other SMBH binary candidates in
the literature appear in CRTS data and show that none of these are equivalent
to the identified objects. Finally, the distribution of objects found is
consistent with that expected from a gravitational wave-driven population. This
implies that circumbinary gas is present at small orbital radii and is being
perturbed by the black holes. None of the sources is expected to merge within
at least the next century. This study opens a new unique window to study a
population of close SMBH binaries that must exist according to our current
understanding of galaxy and SMBH evolution.Comment: 29 pages, 10 figures, accepted for publication in MNRAS - this
version contains extended table and figur
A possible close supermassive black-hole binary in a quasar with optical periodicity
Quasars have long been known to be variable sources at all wavelengths. Their
optical variability is stochastic, can be due to a variety of physical
mechanisms, and is well-described statistically in terms of a damped random
walk model. The recent availability of large collections of astronomical time
series of flux measurements (light curves) offers new data sets for a
systematic exploration of quasar variability. Here we report on the detection
of a strong, smooth periodic signal in the optical variability of the quasar PG
1302-102 with a mean observed period of 1,884 88 days. It was identified
in a search for periodic variability in a data set of light curves for 247,000
known, spectroscopically confirmed quasars with a temporal baseline of
years. While the interpretation of this phenomenon is still uncertain, the most
plausible mechanisms involve a binary system of two supermassive black holes
with a subparsec separation. Such systems are an expected consequence of galaxy
mergers and can provide important constraints on models of galaxy formation and
evolution.Comment: 19 pages, 6 figures. Published online by Nature on 7 January 201
Right to Serve, Right to Lead: Lives and Legacies of the USCT
This is a catalog for an exhibit that follows the evolution of African-American participation in the Civil War, from slaves, to contrabands, to soldiers of the United States Colored Troops (USCT), as well as the lives of black veterans beyond the war, and their ultimate military and social legacy. Using a variety of period items, it creates a narrative that stretches from the Antebellum Period to the current day. In doing so, the exhibit shows how black sacrifice on the battlefield redefined the war\u27s purpose throughout the divided nation, how Jim Crowe suppressed the memory of black participation after Reconstruction, and how the illustrious African-American military tradition left by the USCT endures to this day in their modern heirs
Identification of single nucleotide polymorphisms from the transcriptome of an organism with a whole genome duplication
BACKGROUND: The common ancestor of salmonid fishes, including rainbow trout (Oncorhynchus mykiss), experienced a whole genome duplication between 20 and 100 million years ago, and many of the duplicated genes have been retained in the trout genome. This retention complicates efforts to detect allelic variation in salmonid fishes. Specifically, single nucleotide polymorphism (SNP) detection is problematic because nucleotide variation can be found between the duplicate copies (paralogs) of a gene as well as between alleles. RESULTS: We present a method of differentiating between allelic and paralogous (gene copy) sequence variants, allowing identification of SNPs in organisms with multiple copies of a gene or set of genes. The basic strategy is to: 1) identify windows of unique cDNA sequences with homology to each other, 2) compare these unique cDNAs if they are not shared between individuals (i.e. the cDNA is homozygous in one individual and homozygous for another cDNA in the other individual), and 3) give a “SNP score” value between zero and one to each candidate sequence variant based on six criteria. Using this strategy we were able to detect about seven thousand potential SNPs from the transcriptomes of several clonal lines of rainbow trout. When directly compared to a pre-validated set of SNPs in polyploid wheat, we were also able to estimate the false-positive rate of this strategy as 0 to 28% depending on parameters used. CONCLUSIONS: This strategy has an advantage over traditional techniques of SNP identification because another dimension of sequencing information is utilized. This method is especially well suited for identifying SNPs in polyploids, both outbred and inbred, but would tend to be conservative for diploid organisms
Comparison of Image Registration Based Measures of Regional Lung Ventilation from Dynamic Spiral CT with Xe-CT
Purpose: Regional lung volume change as a function of lung inflation serves
as an index of parenchymal and airway status as well as an index of regional
ventilation and can be used to detect pathologic changes over time. In this
article, we propose a new regional measure of lung mechanics --- the specific
air volume change by corrected Jacobian.
Methods: 4DCT and Xe-CT data sets from four adult sheep are used in this
study. Nonlinear, 3D image registration is applied to register an image
acquired near end inspiration to an image acquired near end expiration.
Approximately 200 annotated anatomical points are used as landmarks to evaluate
registration accuracy. Three different registration-based measures of regional
lung mechanics are derived and compared: the specific air volume change
calculated from the Jacobian (SAJ); the specific air volume change calculated
by the corrected Jacobian (SACJ); and the specific air volume change by
intensity change (SAI).
Results: After registration, the mean registration error is on the order of 1
mm. For cubical ROIs in cubes with size 20 mm 20 mm 20 mm,
the SAJ and SACJ measures show significantly higher correlation (linear
regression, average and ) with the Xe-CT based measure of
specific ventilation (sV) than the SAI measure. For ROIs in slabs along the
ventral-dorsal vertical direction with size of 150 mm 8 mm 40
mm, the SAJ, SACJ, and SAI all show high correlation (linear regression,
average , and ) with the Xe-CT based sV without
significant differences when comparing between the three methods.
Conclusion: Given a deformation field by an image registration algorithm,
significant differences between the SAJ, SACJ, and SAI measures were found at a
regional level compared to the Xe-CT sV in four sheep that were studied
Acoustic metamaterial absorbers based on multilayered sonic crystals
Through the use of a layered arrangement, it is shown that lossy sonic crystals can be arranged to create a structure with extreme acoustic properties, namely, an acoustic metamaterial. This artificial structure shows different effective fluids and absorptive properties in different orientations. Theoretical, numerical, and experimental results examining thermoviscous losses in sonic crystals are presented, enabling the fabrication and characterization of an acoustic metamaterial absorber with complex-valued anisotropic inertia. To accurately describe and fabricate such an acoustic metamaterial in a realizable experimental configuration, confining structures are needed which modify the effective properties, due to the thermal and viscous boundary layer effects within the sonic crystal lattice. Theoretical formulations are presented which describe the effects of these confined sonic crystals, both individually and as part of an acoustic metamaterial structure. Experimental demonstrations are also reported using an acoustic impedance tube. The formulations developed can be written with no unknown or empirical coefficients, due to the structured lattice of the sonic crystals and organized layering scheme; and it is shown that higher filling fraction arrangements can be used to provide a large enhancement in the loss factor. (C) 2015 AIP Publishing LLC.This work was supported by the U.S. Office of Naval Research (Award No. N000141210216) and by the Spanish Ministerio de Economia y Competitividad (MINECO) under Contract No. TEC2010-19751.Guild, M.; García Chocano, VM.; Kan, W.; Sánchez-Dehesa Moreno-Cid, J. (2015). Acoustic metamaterial absorbers based on multilayered sonic crystals. Journal of Applied Physics. 117(11):114902-1-114902-14. https://doi.org/10.1063/1.4915346S114902-1114902-1411711Dowling, J. P. (1992). Sonic band structure in fluids with periodic density variations. 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The Journal of the Acoustical Society of America, 132(4), 2896-2908. doi:10.1121/1.4744933Naify, C. J., Chang, C.-M., McKnight, G., & Nutt, S. (2010). Transmission loss and dynamic response of membrane-type locally resonant acoustic metamaterials. Journal of Applied Physics, 108(11), 114905. doi:10.1063/1.3514082Yang, Z., Dai, H. M., Chan, N. H., Ma, G. C., & Sheng, P. (2010). Acoustic metamaterial panels for sound attenuation in the 50–1000 Hz regime. Applied Physics Letters, 96(4), 041906. doi:10.1063/1.3299007Naify, C. J., Chang, C.-M., McKnight, G., Scheulen, F., & Nutt, S. (2011). Membrane-type metamaterials: Transmission loss of multi-celled arrays. Journal of Applied Physics, 109(10), 104902. doi:10.1063/1.3583656Hussein, M. I., & Frazier, M. J. (2013). Metadamping: An emergent phenomenon in dissipative metamaterials. Journal of Sound and Vibration, 332(20), 4767-4774. doi:10.1016/j.jsv.2013.04.041Zhang, Y., Wen, J., Zhao, H., Yu, D., Cai, L., & Wen, X. (2013). 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A Characterization of the Shallow-Water Coral Reefs and Associated Habitats of Puerto Rico
We mapped bottom types and shelf zones of 1600 km2 or about one fourth of Puerto Rico’s insular shelf from the shoreline to the shelf edge. Overall map accuracy for these bottom types is estimated as 93.6% correct. Maps were produced through visual interpretation of benthic features using orthorectified aerial photographs within a Geographic Information System with customizable software. The maps are one component of an integrated mapping and monitoring program underway by NOAA and its partners in the US Coral Reef Task Force to assess all US reef ecosystems. Maps are currently being used to enhance coastal research and management activities in Puerto Rico such as fisheries assessments and designation of important fish habitats
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