Author
X. J. Bi
Bio: X. J. Bi is an academic researcher from Chinese Academy of Sciences. The author has contributed to research in topics: Air shower & Cosmic ray. The author has an hindex of 11, co-authored 28 publications receiving 496 citations.
Topics: Air shower, Cosmic ray, Crab Nebula, Pulsar, Cherenkov detector
Papers
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TL;DR: In this article, the authors reported the detection of more than 530 photons at energies above 100 teraelectronvolts and up to 1.4 PeV from 12 sources in the Galaxy.
Abstract: The extension of the cosmic-ray spectrum beyond 1 petaelectronvolt (PeV; 1015 electronvolts) indicates the existence of the so-called PeVatrons—cosmic-ray factories that accelerate particles to PeV energies. We need to locate and identify such objects to find the origin of Galactic cosmic rays1. The principal signature of both electron and proton PeVatrons is ultrahigh-energy (exceeding 100 TeV) γ radiation. Evidence of the presence of a proton PeVatron has been found in the Galactic Centre, according to the detection of a hard-spectrum radiation extending to 0.04 PeV (ref. 2). Although γ-rays with energies slightly higher than 0.1 PeV have been reported from a few objects in the Galactic plane3–6, unbiased identification and in-depth exploration of PeVatrons requires detection of γ-rays with energies well above 0.1 PeV. Here we report the detection of more than 530 photons at energies above 100 teraelectronvolts and up to 1.4 PeV from 12 ultrahigh-energy γ-ray sources with a statistical significance greater than seven standard deviations. Despite having several potential counterparts in their proximity, including pulsar wind nebulae, supernova remnants and star-forming regions, the PeVatrons responsible for the ultrahigh-energy γ-rays have not yet been firmly localized and identified (except for the Crab Nebula), leaving open the origin of these extreme accelerators. Observations of γ-rays with energies up to 1.4 PeV find that 12 sources in the Galaxy are PeVatrons, one of which is the Crab Nebula.
184 citations
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TL;DR: In this paper, a measurement of the mixed proton and helium energy spectrum, obtained with the combined data of the ARGO-YBJ experiment and a wide field of view Cherenkov telescope, a prototype of the future LHAASO experiment, is reported.
Abstract: The measurement of the cosmic ray energy spectrum, in particular for individual species of nuclei, is an important tool to investigate cosmic ray production and propagation mechanisms. The determination of the ``knees'' in the spectra of different species remains one of the main challenges in cosmic ray physics. In fact, experimental results are still conflicting. In this paper we report a measurement of the mixed proton and helium energy spectrum, obtained with the combined data of the ARGO-YBJ experiment and a wide field of view Cherenkov telescope, a prototype of the future LHAASO experiment. By means of a multiparameter technique, we have selected a high-purity proton plus helium sample. The reconstructed energy resolution is found to be about 25% throughout the investigated energy range from 100 TeV to 3 PeV, with a systematic uncertainty in the absolute energy scale of 9.7%. The found energy spectrum can be fitted with a broken power-law function, with a break at the energy ${\mathrm{E}}_{k}=700\ifmmode\pm\else\textpm\fi{}230(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}70(\mathrm{sys})\text{ }\text{ }\mathrm{TeV}$, where the spectral index changes from $\ensuremath{-}2.56\ifmmode\pm\else\textpm\fi{}0.05$ to $\ensuremath{-}3.24\ifmmode\pm\else\textpm\fi{}0.36$. The statistical significance of the observed spectral break is 4.2 standard deviations.
106 citations
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University of Copenhagen1, Kunming Institute of Zoology2, Landcare Research3, University of Otago4, Chinese Academy of Sciences5, University of Oxford6, Bowling Green State University7, Otago Polytechnic8, Copenhagen Zoo9, University of Washington10, University of La Rochelle11, Canterbury of New Zealand12, University of Western Australia13, University of Tasmania14, University of Missouri–St. Louis15, Natural Environment Research Council16, University of Giessen17, Percy FitzPatrick Institute of African Ornithology18, Hastings Entertainment19, National Institute of Water and Atmospheric Research20, Norwegian University of Science and Technology21, Museum of New Zealand Te Papa Tongarewa22, Ludwig Maximilian University of Munich23, Massey University24, Wellington Management Company25, La Trobe University26, National Scientific and Technical Research Council27
TL;DR: A novel dataset of 19 high-coverage genomes that, together with 2 previously published genomes, encompass all extant penguin species and demonstrates that the genus Aptenodytes is basal and sister to all other extant Penguin genera, providing intriguing new insights into the adaptation of penguins to Antarctica.
Abstract: Penguins (Sphenisciformes) are a remarkable order of flightless wing-propelled diving seabirds distributed widely across the southern hemisphere. They share a volant common ancestor with Procellariiformes close to the Cretaceous-Paleogene boundary (66 million years ago) and subsequently lost the ability to fly but enhanced their diving capabilities. With ∼20 species among 6 genera, penguins range from the tropical Galapagos Islands to the oceanic temperate forests of New Zealand, the rocky coastlines of the sub-Antarctic islands, and the sea ice around Antarctica. To inhabit such diverse and extreme environments, penguins evolved many physiological and morphological adaptations. However, they are also highly sensitive to climate change. Therefore, penguins provide an exciting target system for understanding the evolutionary processes of speciation, adaptation, and demography. Genomic data are an emerging resource for addressing questions about such processes.
89 citations
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TL;DR: In this paper, the authors reported a 40-Gb chromosome-level assembly of the African lungfish (Protopterus annectens) genome, which is the largest genome assembly ever reported and has a contig and chromosome N50 of 1.60 mb and 2.81 Gb, respectively.
77 citations
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TL;DR: In this paper, a detailed analysis of the ARGO-YBJ experiment is reported, showing that during the brightest gamma-gamma-rays flaring episodes from October 17 to November 22, 2011, an excess of the event rate over 6σigma$ is detected by ARGOYBj in the direction of Mrk 501, corresponding to an increase of the gamma-Gamma-ray flux above 1 TeV by a factor of 6.6$\pm$2.2 from its steady emission.
Abstract: As one of the brightest active blazars in both X-ray and very high energy $\gamma$-ray bands, Mrk 501 is very useful for physics associated with jets from AGNs. The ARGO-YBJ experiment is monitoring it for $\gamma$-rays above 0.3 TeV since November 2007. Starting from October 2011 the largest flare since 2005 is observed, which lasts to about April 2012. In this paper, a detailed analysis is reported. During the brightest $\gamma$-rays flaring episodes from October 17 to November 22, 2011, an excess of the event rate over 6 $\sigma$ is detected by ARGO-YBJ in the direction of Mrk 501, corresponding to an increase of the $\gamma$-ray flux above 1 TeV by a factor of 6.6$\pm$2.2 from its steady emission. In particular, the $\gamma$-ray flux above 8 TeV is detected with a significance better than 4 $\sigma$. Based on time-dependent synchrotron self-Compton (SSC) processes, the broad-band energy spectrum is interpreted as the emission from an electron energy distribution parameterized with a single power-law function with an exponential cutoff at its high energy end. The average spectral energy distribution for the steady emission is well described by this simple one-zone SSC model. However, the detection of $\gamma$-rays above 8 TeV during the flare challenges this model due to the hardness of the spectra. Correlations between X-rays and $\gamma$-rays are also investigated.
62 citations
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TL;DR: The comparison of related genomes has emerged as a powerful lens for genome interpretation as mentioned in this paper, which reveals a small number of new coding exons, candidate stop codon readthrough events and over 10,000 regions of overlapping synonymous constraint within protein-coding exons.
Abstract: The comparison of related genomes has emerged as a powerful lens for genome interpretation. Here we report the sequencing and comparative analysis of 29 eutherian genomes. We confirm that at least 5.5% of the human genome has undergone purifying selection, and locate constrained elements covering ∼4.2% of the genome. We use evolutionary signatures and comparisons with experimental data sets to suggest candidate functions for ∼60% of constrained bases. These elements reveal a small number of new coding exons, candidate stop codon readthrough events and over 10,000 regions of overlapping synonymous constraint within protein-coding exons. We find 220 candidate RNA structural families, and nearly a million elements overlapping potential promoter, enhancer and insulator regions. We report specific amino acid residues that have undergone positive selection, 280,000 non-coding elements exapted from mobile elements and more than 1,000 primate- and human-accelerated elements. Overlap with disease-associated variants indicates that our findings will be relevant for studies of human biology, health and disease.
926 citations
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Princeton University1, University of Toronto2, City University of New York3, University of British Columbia4, University of Oxford5, Cardiff University6, Pontifical Catholic University of Chile7, Sapienza University of Rome8, Carnegie Mellon University9, University of Pittsburgh10, Goddard Space Flight Center11, Johns Hopkins University12, Argonne National Laboratory13, Lawrence Berkeley National Laboratory14, University of Pennsylvania15, National Institute of Standards and Technology16, Max Planck Society17, University of Nottingham18, University of KwaZulu-Natal19, University of Miami20, National Institute of Astrophysics, Optics and Electronics21, Rutgers University22, West Chester University of Pennsylvania23, Columbia University24, Academia Sinica Institute of Astronomy and Astrophysics25, University of Tokyo26, University of California, Santa Barbara27, Arizona State University28, Cornell University29, Haverford College30, Stony Brook University31, University of Massachusetts Amherst32
TL;DR: In this article, a model of primary cosmological and secondary foreground parameters is fit to the map power spectra and lensing deflection power spectrum, including contributions from both the thermal Sunyaev-Zeldovich (tSZ) effect and the kinematic SZ effect, Poisson and correlated anisotropy from unresolved infrared sources, radio sources and the correlation between the tSZ effect and infrared sources.
Abstract: We present constraints on cosmological and astrophysical parameters from high-resolution microwave background maps at 148 GHz and 218 GHz made by the Atacama Cosmology Telescope (ACT) in three seasons of observations from 2008 to 2010. A model of primary cosmological and secondary foreground parameters is fit to the map power spectra and lensing deflection power spectrum, including contributions from both the thermal Sunyaev-Zeldovich (tSZ) effect and the kinematic Sunyaev-Zeldovich (kSZ) effect, Poisson and correlated anisotropy from unresolved infrared sources, radio sources, and the correlation between the tSZ effect and infrared sources. The power l2Cl/2π of the thermal SZ power spectrum at 148 GHz is measured to be 3.4±1.4 μK2 at l = 3000, while the corresponding amplitude of the kinematic SZ power spectrum has a 95% confidence level upper limit of 8.6 μK2. Combining ACT power spectra with the WMAP 7-year temperature and polarization power spectra, we find excellent consistency with the LCDM model. We constrain the number of effective relativistic degrees of freedom in the early universe to be Neff = 2.79±0.56, in agreement with the canonical value of Neff = 3.046 for three massless neutrinos. We constrain the sum of the neutrino masses to be Σmν < 0.39 eV at 95% confidence when combining ACT and WMAP 7-year data with BAO and Hubble constant measurements. We constrain the amount of primordial helium to be Yp = 0.225±0.034, and measure no variation in the fine structure constant α since recombination, with α/α0 = 1.004±0.005. We also find no evidence for any running of the scalar spectral index, dns/dln k = −0.004±0.012.
451 citations
01 Apr 2002
TL;DR: In this article, direct and indirect measurements of cosmic rays are reviewed, with emphasis given to the understanding of the knee in the energy spectrum, and the data are compared to contemporary models for the knee.
Abstract: Results from direct and indirect measurements of cosmic rays are reviewed. Emphasis is given to the understanding of the knee in the energy spectrum. The data are compared to contemporary models for the knee. Implications on the present understanding of the origin of galactic cosmic rays are discussed.
319 citations
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Michigan State University1, National Autonomous University of Mexico2, Universidad Michoacana de San Nicolás de Hidalgo3, Michigan Technological University4, University of Utah5, University of Maryland, College Park6, Polytechnic University of Puerto Rico7, University of Wisconsin-Madison8, National Institute of Astrophysics, Optics and Electronics9, Benemérita Universidad Autónoma de Puebla10, University of Guadalajara11, Pennsylvania State University12, Los Alamos National Laboratory13, George Mason University14, Colorado State University15, University of California, Irvine16, University of New Mexico17, Georgia Institute of Technology18, Instituto Politécnico Nacional19, Universidad Autónoma del Estado de Hidalgo20, University of New Hampshire21, University of California, Santa Cruz22, University of Alabama23, CINVESTAV24
TL;DR: The High Altitude Water Cherenkov (HAWC) observatory as mentioned in this paper is an array of large water-cherenkov detectors sensitive to gamma rays and hadronic cosmic rays in the energy band between 100 GeV and 100 TeV.
228 citations