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11.
  • Adriani, O., et al. (författare)
  • An anomalous positron abundance in cosmic rays with energies 1.5-100 GeV
  • 2009
  • Ingår i: ; 458:7238, s. 607-609
  • Tidskriftsartikel (refereegranskat)abstract
    • Antiparticles account for a small fraction of cosmic rays and are known to be produced in interactions between cosmic-ray nuclei and atoms in the interstellar medium(1), which is referred to as a 'secondary source'. Positrons might also originate in objects such as pulsars(2) and microquasars(3) or through dark matter annihilation(4), which would be 'primary sources'. Previous statistically limited measurements(5-7) of the ratio of positron and electron fluxes have been interpreted as evidence for a primary source for the positrons, as has an increase in the total electron+positron flux at energies between 300 and 600 GeV (ref. 8). Here we report a measurement of the positron fraction in the energy range 1.5-100 GeV. We find that the positron fraction increases sharply overmuch of that range, in a way that appears to be completely inconsistent with secondary sources. We therefore conclude that a primary source, be it an astrophysical object or dark matter annihilation, is necessary.
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12.
  • Aharonian, Felix, et al. (författare)
  • Solar abundance ratios of the iron-peak elements in the Perseus cluster
  • 2017
  • Ingår i: Nature. - 0028-0836 .- 1476-4687. ; 551:7681, s. 478-
  • Tidskriftsartikel (refereegranskat)abstract
    • The metal abundance of the hot plasma that permeates galaxy clusters represents the accumulation of heavy elements produced by billions of supernovae(1). Therefore, X-ray spectroscopy of the intracluster medium provides an opportunity to investigate the nature of supernova explosions integrated over cosmic time. In particular, the abundance of the iron-peak elements (chromium, manganese, iron and nickel) is key to understanding how the progenitors of typical type Ia supernovae evolve and explode(2-6). Recent X-ray studies of the intracluster medium found that the abundance ratios of these elements differ substantially from those seen in the Sun(7-11), suggesting differences between the nature of type Ia supernovae in the clusters and in the Milky Way. However, because the K-shell transition lines of chromium and manganese are weak and those of iron and nickel are very close in photon energy, highresolution spectroscopy is required for an accurate determination of the abundances of these elements. Here we report observations of the Perseus cluster, with statistically significant detections of the resonance emission from chromium, manganese and nickel. Our measurements, combined with the latest atomic models, reveal that these elements have near-solar abundance ratios with respect to iron, in contrast to previous claims. Comparison between our results and modern nucleosynthesis calculations(12-14) disfavours the hypothesis that type Ia supernova progenitors are exclusively white dwarfs with masses well below the Chandrasekhar limit (about 1.4 times the mass of the Sun). The observed abundance pattern of the iron-peak elements can be explained by taking into account a combination of near-and sub-Chandrasekhar-mass type Ia supernova systems, adding to the mounting evidence that both progenitor types make a substantial contribution to cosmic chemical enrichment(5,15,16).
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13.
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14.
  • Ahlberg, Per E. (författare)
  • Birth of the jawed vertebrates
  • 2009
  • Ingår i: Nature. - 0028-0836 .- 1476-4687. ; 457, s. 1094-1095
  • Tidskriftsartikel (övrigt vetenskapligt)abstract
    • The discovery of embryos in certain fossil fishes not only shows that internal fertilization and live birth evolved early in vertebrate history, but also raises questions about the origin of jawed vertebrates.
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15.
  • Ahlberg, Per E. (författare)
  • Coelacanth fins and evolution
  • 1992
  • Ingår i: Nature. - 0028-0836 .- 1476-4687. ; 358, s. 459-
  • Tidskriftsartikel (refereegranskat)
  •  
16.
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17.
  • Ahlberg, Per E. (författare)
  • Fossil fishes from Gogo
  • 1989
  • Ingår i: Nature. - 0028-0836 .- 1476-4687. ; 337, s. 511-512
  • Tidskriftsartikel (övrigt vetenskapligt)
  •  
18.
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19.
  • Ahlberg, Per Erik (författare)
  • Glimpsing the hidden majority
  • 1990
  • Ingår i: Nature. - 0028-0836 .- 1476-4687. ; 344, s. 23-
  • Tidskriftsartikel (övrigt vetenskapligt)
  •  
20.
  • Ahlberg, Per E. (författare)
  • How to keep a head in order
  • 1997
  • Ingår i: Nature. - 0028-0836 .- 1476-4687. ; 385, s. 489-490
  • Tidskriftsartikel (övrigt vetenskapligt)
  •  
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