| 1. |
- Acharya, B. S., et al.
(author)
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Introducing the CTA concept
- 2013
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In: Astroparticle physics. - : Elsevier BV. - 0927-6505 .- 1873-2852. ; 43, s. 3-18
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Journal article (other academic/artistic)abstract
- The Cherenkov Telescope Array (CTA) is a new observatory for very high-energy (VHE) gamma rays. CTA has ambitions science goals, for which it is necessary to achieve full-sky coverage, to improve the sensitivity by about an order of magnitude, to span about four decades of energy, from a few tens of GeV to above 100 TeV with enhanced angular and energy resolutions over existing VHE gamma-ray observatories. An international collaboration has formed with more than 1000 members from 27 countries in Europe, Asia, Africa and North and South America. In 2010 the CTA Consortium completed a Design Study and started a three-year Preparatory Phase which leads to production readiness of CTA in 2014. In this paper we introduce the science goals and the concept of CTA, and provide an overview of the project. (C) 2013 Elsevier B.V. All rights reserved.
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| 2. |
- Actis, M., et al.
(author)
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Design concepts for the Cherenkov Telescope Array CTA : an advanced facility for ground-based high-energy gamma-ray astronomy
- 2011
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In: Experimental astronomy. - : Springer. - 0922-6435 .- 1572-9508. ; 32:3, s. 193-316
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Journal article (peer-reviewed)abstract
- Ground-based gamma-ray astronomy has had a major breakthrough with the impressive results obtained using systems of imaging atmospheric Cherenkov telescopes. Ground-based gamma-ray astronomy has a huge potential in astrophysics, particle physics and cosmology. CTA is an international initiative to build the next generation instrument, with a factor of 5-10 improvement in sensitivity in the 100 GeV-10 TeV range and the extension to energies well below 100 GeV and above 100 TeV. CTA will consist of two arrays (one in the north, one in the south) for full sky coverage and will be operated as open observatory. The design of CTA is based on currently available technology. This document reports on the status and presents the major design concepts of CTA.
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| 3. |
- Ackermann, M., et al.
(author)
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Fermi-LAT Observations of the Gamma-Ray Burst GRB 130427A
- 2014
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In: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 343:6166, s. 42-47
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Journal article (peer-reviewed)abstract
- The observations of the exceptionally bright gamma-ray burst (GRB) 130427A by the Large Area Telescope aboard the Fermi Gamma-ray Space Telescope provide constraints on the nature of these unique astrophysical sources. GRB 130427A had the largest fluence, highest-energy photon (95 GeV), longest gamma-ray duration (20 hours), and one of the largest isotropic energy releases ever observed from a GRB. Temporal and spectral analyses of GRB 130427A challenge the widely accepted model that the nonthermal high-energy emission in the afterglow phase of GRBs is synchrotron emission radiated by electrons accelerated at an external shock.
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| 4. |
- Ackermann, M., et al.
(author)
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THE FIRST FERMI-LAT GAMMA-RAY BURST CATALOG
- 2013
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In: Astrophysical Journal Supplement Series. - : American Astronomical Society. - 0067-0049 .- 1538-4365. ; 209:1
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Journal article (peer-reviewed)abstract
- In three years of observations since the beginning of nominal science operations in 2008 August, the Large Area Telescope (LAT) on board the Fermi Gamma-Ray Space Telescope has observed high-energy (greater than or similar to 20 MeV) gamma-ray emission from 35 gamma-ray bursts (GRBs). Among these, 28 GRBs have been detected above 100 MeV and 7 GRBs above similar to 20 MeV. The first Fermi-LAT catalog of GRBs is a compilation of these detections and provides a systematic study of high-energy emission from GRBs for the first time. To generate the catalog, we examined 733 GRBs detected by the Gamma-Ray Burst Monitor (GBM) on Fermi and processed each of them using the same analysis sequence. Details of the methodology followed by the LAT collaboration for the GRB analysis are provided. We summarize the temporal and spectral properties of the LAT-detected GRBs. We also discuss characteristics of LAT-detected emission such as its delayed onset and longer duration compared with emission detected by the GBM, its power-law temporal decay at late times, and the fact that it is dominated by a power-law spectral component that appears in addition to the usual Band model.
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| 5. |
- Preece, R., et al.
(author)
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The First Pulse of the Extremely Bright GRB 130427A : A Test Lab for Synchrotron Shocks
- 2014
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In: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 343:6166, s. 51-54
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Journal article (peer-reviewed)abstract
- Gamma-ray burst (GRB) 130427A is one of the most energetic GRBs ever observed. The initial pulse up to 2.5 seconds is possibly the brightest well-isolated pulse observed to date. A fine time resolution spectral analysis shows power-law decays of the peak energy from the onset of the pulse, consistent with models of internal synchrotron shock pulses. However, a strongly correlated power-law behavior is observed between the luminosity and the spectral peak energy that is inconsistent with curvature effects arising in the relativistic outflow. It is difficult for any of the existing models to account for all of the observed spectral and temporal behaviors simultaneously.
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| 6. |
- Ackermann, M., et al.
(author)
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Constraining The High-Energy Emission From Gamma-Ray Bursts With Fermi
- 2012
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In: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 754:2, s. 121-
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Journal article (peer-reviewed)abstract
- We examine 288 gamma-ray bursts (GRBs) detected by the Fermi Gamma-ray Space Telescope's Gamma-ray Burst Monitor (GBM) that fell within the field of view of Fermi's Large Area Telescope (LAT) during the first 2.5 years of observations, which showed no evidence for emission above 100 MeV. We report the photon flux upper limits in the 0.1-10 GeV range during the prompt emission phase as well as for fixed 30 s and 100 s integrations starting from the trigger time for each burst. We compare these limits with the fluxes that would be expected from extrapolations of spectral fits presented in the first GBM spectral catalog and infer that roughly half of the GBM-detected bursts either require spectral breaks between the GBM and LAT energy bands or have intrinsically steeper spectra above the peak of the nu F-nu spectra (E-pk). In order to distinguish between these two scenarios, we perform joint GBM and LAT spectral fits to the 30 brightest GBM-detected bursts and find that a majority of these bursts are indeed softer above E-pk than would be inferred from fitting the GBM data alone. Approximately 20% of this spectroscopic subsample show statistically significant evidence for a cutoff in their high-energy spectra, which if assumed to be due to gamma gamma attenuation, places limits on the maximum Lorentz factor associated with the relativistic outflow producing this emission. All of these latter bursts have maximum Lorentz factor estimates that are well below the minimum Lorentz factors calculated for LAT-detected GRBs, revealing a wide distribution in the bulk Lorentz factor of GRB outflows and indicating that LAT-detected bursts may represent the high end of this distribution.
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| 7. |
- Axelsson, Magnus, et al.
(author)
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GRB110721A : AN EXTREME PEAK ENERGY AND SIGNATURES OF THE PHOTOSPHERE
- 2012
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In: Astrophysical Journal Letters. - 2041-8205. ; 757:2
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Journal article (peer-reviewed)abstract
- GRB110721A was observed by the Fermi Gamma-ray Space Telescope using its two instruments, the Large Area Telescope (LAT) and the Gamma-ray Burst Monitor (GBM). The burst consisted of one major emission episode which lasted for similar to 24.5 s (in the GBM) and had a peak flux of (5.7 +/- 0.2) x 10(-5) erg s(-1) cm(-2). The time-resolved emission spectrum is best modeled with a combination of a Band function and a blackbody spectrum. The peak energy of the Band component was initially 15 +/- 2 MeV, which is the highest value ever detected in a GRB. This measurement was made possible by combining GBM/BGO data with LAT Low Energy events to achieve continuous 10-100 MeV coverage. The peak energy later decreased as a power law in time with an index of -1.89 +/- 0.10. The temperature of the blackbody component also decreased, starting from similar to 80 keV, and the decay showed a significant break after similar to 2 s. The spectrum provides strong constraints on the standard synchrotron model, indicating that alternative mechanisms may give rise to the emission at these energies.
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| 8. |
- Jarvis, Erich D., et al.
(author)
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Whole-genome analyses resolve early branches in the tree of life of modern birds
- 2014
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In: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 346:6215, s. 1320-1331
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Journal article (peer-reviewed)abstract
- To better determine the history of modern birds, we performed a genome-scale phylogenetic analysis of 48 species representing all orders of Neoaves using phylogenomic methods created to handle genome-scale data. We recovered a highly resolved tree that confirms previously controversial sister or close relationships. We identified the first divergence in Neoaves, two groups we named Passerea and Columbea, representing independent lineages of diverse and convergently evolved land and water bird species. Among Passerea, we infer the common ancestor of core landbirds to have been an apex predator and confirm independent gains of vocal learning. Among Columbea, we identify pigeons and flamingoes as belonging to sister clades. Even with whole genomes, some of the earliest branches in Neoaves proved challenging to resolve, which was best explained by massive protein-coding sequence convergence and high levels of incomplete lineage sorting that occurred during a rapid radiation after the Cretaceous-Paleogene mass extinction event about 66 million years ago.
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| 9. |
- Zhang, Guojie, et al.
(author)
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Comparative genomics reveals insights into avian genome evolution and adaptation
- 2014
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In: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 346:6215, s. 1311-1320
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Journal article (peer-reviewed)abstract
- Birds are the most species-rich class of tetrapod vertebrates and have wide relevance across many research fields. We explored bird macroevolution using full genomes from 48 avian species representing all major extant clades. The avian genome is principally characterized by its constrained size, which predominantly arose because of lineage-specific erosion of repetitive elements, large segmental deletions, and gene loss. Avian genomes furthermore show a remarkably high degree of evolutionary stasis at the levels of nucleotide sequence, gene synteny, and chromosomal structure. Despite this pattern of conservation, we detected many non-neutral evolutionary changes in protein-coding genes and noncoding regions. These analyses reveal that pan-avian genomic diversity covaries with adaptations to different lifestyles and convergent evolution of traits.
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| 10. |
- Sun, H. Q., et al.
(author)
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Characteristics of Respiratory Distress Syndrome in Infants of Different Gestational Ages
- 2013
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In: Lung. - : Springer Science and Business Media LLC. - 0341-2040 .- 1432-1750. ; 191:4, s. 425-433
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Journal article (peer-reviewed)abstract
- The purpose of this study was to compare the risk factors, clinical characteristics, and complications of respiratory distress syndrome (RDS) in infants delivered very preterm, late preterm, and term in order to help optimize the management of RDS in neonates. A retrospective study was conducted on neonates admitted to the NICU between January 2006 and December 2010. The enrolled infants with RDS were categorized as very preterm (< 32(0/7) weeks gestation), moderately preterm (32(0/7)-33(6/7) weeks), late preterm (34(0/7)-36(6/7) weeks), and term (37(0/7)-42(0/7) weeks). The rates, potential risk factors, clinical characteristics, and complications of RDS of these four groups were comparatively analyzed. There was an increasing trend in incidence of RDS among the NICU admissions annually. Caesarean section without labor was significantly associated with RDS in term and late preterm infants (P < 0.001). Rates of requirements for ventilator and pulmonary surfactant were similar in very preterm and term infants but significantly lower in late preterm infants (P < 0.001). The oxygenation index value was not substantially lower in late preterm and term infants compared to very preterm infants, and the arterial oxygenation efficiency was improved slowly (P < 0.001). Incidence of pneumonia and occurrence of pneumothorax were significantly higher in term infants (P < 0.001). Term infants with RDS showed an association of RDS with caesarean section without labor and lung infection. These infants also showed slower improvement of oxygenation after surfactant administration and mechanical ventilation, and they experienced a high rate of pneumothorax complication, which was also noticed in late preterm neonates.
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| 11. |
- Guiriec, S., et al.
(author)
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Evidence for a photospheric component in the prompt emission of the short grb 120323a and its effects on the grb hardness-luminosity relation
- 2013
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In: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 770:1, s. 32-
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Journal article (peer-reviewed)abstract
- The short GRB 120323A had the highest flux ever detected with the Gamma-Ray Burst Monitor on board the Fermi Gamma-Ray Space Telescope. Here we study its remarkable spectral properties and their evolution using two spectral models: (1) a single emission component scenario, where the spectrum is modeled by the empirical Band function (a broken power law), and (2) a two-component scenario, where thermal (a Planck-like function) emission is observed simultaneously with a non-thermal component (a Band function). We find that the latter model fits the integrated burst spectrum significantly better than the former, and that their respective spectral parameters are dramatically different: when fit with a Band function only, the E peak of the event is unusually soft for a short gamma-ray burst (GRB; 70 keV compared to an average of 300 keV), while adding a thermal component leads to more typical short GRB values (E peak 300 keV). Our time-resolved spectral analysis produces similar results. We argue here that the two-component model is the preferred interpretation for GRB 120323A based on (1) the values and evolution of the Band function parameters of the two component scenario, which are more typical for a short GRB, and (2) the appearance in the data of a significant hardness-intensity correlation, commonly found in GRBs, when we employee two-component model fits; the correlation is non-existent in the Band-only fits. GRB 110721A, a long burst with an intense photospheric emission, exhibits the exact same behavior. We conclude that GRB 120323A has a strong photospheric emission contribution, observed for the first time in a short GRB. Magnetic dissipation models are difficult to reconcile with these results, which instead favor photospheric thermal emission and fast cooling synchrotron radiation from internal shocks. Finally, we derive a possibly universal hardness-luminosity relation in the source frame using a larger set of GRBs (), which could be used as a possible redshift estimator for cosmology.
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| 12. |
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| 13. |
- Song, F., et al.
(author)
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Extracting the near surface stoichiometry of BiFe0.5Mn0.5O3 thin films; a finite element maximum entropy approach
- 2012
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In: Surface Science. - : Elsevier BV. - 0039-6028. ; 606:23-24, s. 1771-1776
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Journal article (peer-reviewed)abstract
- The surface and near-surface chemical composition of BiFe0.5Mn0.5O3 has been studied using a combination of low photon energy synchrotron photoemission spectroscopy, and a newly developed maximum entropy finite element model from which it is possible to extract the depth dependent chemical composition. In the uppermost few unit cells, an overabundance of Bi, and a deficiency of Fe and Mn are observed. In deeper layers, the measurements are consistent with bulk-like stoichiometry. Additionally, a definitive identification of all the observed species together with their abundance and depth dependence is given, and the mixed Fe and Mn valencies are estimated. In addition to the expected bulk valencies Mn3+ and Fe3+, some Fe2+ and a small amount of Mn4+ are also observed. The maximum entropy finite element model demonstrated here is also discussed in more general terms and its potential application to the broader field of perovskite thin films is made apparent. (C) 2012 Elsevier B.V. All rights reserved.
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| 14. |
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| 15. |
- Zhao, Y., et al.
(author)
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Nanocomposite electrode materials for low temperature solid oxide fuel cells using the ceria-carbonate composite electrolytes
- 2012
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In: International journal of hydrogen energy. - : Elsevier BV. - 0360-3199 .- 1879-3487. ; 37:24, s. 19351-19356
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Journal article (peer-reviewed)abstract
- Low temperature solid oxide fuel cell (LTSOFC, 300-600 °C) is one of the hot areas in recent fuel cell developments. In order to develop high performance LTSOFCs, compatible electrodes are highly demanded. We used NANOCOFC (nanocomposites for advanced fuel cell technology) approach to develop nanocomposite electrodes based on metal oxides Ni-Cu-Zn-oxide and samarium doped ceria (SDC). It was found that the materials consist of individual metal oxide and SDC phase, indicating the material as a composite with a homogenous distribution for all constituent components. Highly homogenous distribution of the particles enhanced the catalyst function for electrode applications in LTSOFC devices. We constructed the devices using the SDC-carbonate nanocomposite (NSDC) as the electrolyte and above as prepared composite as electrodes in a symmetrical configuration. We found that the prepared composite electrodes had good catalytic function for both H2 and O2, to prove its anode and cathode functions. Based on the material properties, the LTSOFC devices have reached a power output more than 730 mW cm-2 at 550 °C.
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