| 1. |
- Abdo, A. A., et al.
(författare)
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FERMI DISCOVERY OF GAMMA-RAY EMISSION FROM NGC 1275
- 2009
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 699:1, s. 31-39
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Tidskriftsartikel (refereegranskat)abstract
- We report the discovery of high-energy (E > 100 MeV) gamma-ray emission from NGC 1275, a giant elliptical galaxy lying at the center of the Perseus cluster of galaxies, based on observations made with the Large Area Telescope (LAT) of the Fermi Gamma-ray Space Telescope. The positional center of the gamma-ray source is only approximate to 3' away from the NGC 1275 nucleus, well within the 95% LAT error circle of approximate to 5'. The spatial distribution of gamma-ay photons is consistent with a point source. The average flux and power-law photon index measured with the LAT from 2008 August 4 to 2008 December 5 are F-gamma = (2.10 +/- 0.23) x 10(-7) ph (>100 MeV) cm(-2) s(-1) and Gamma = 2.17 +/- 0.05, respectively. The measurements are statistically consistent with constant flux during the four-month LAT observing period. Previous EGRET observations gave an upper limit of F-gamma < 3.72 x 10(-8) ph (>100 MeV) cm(-2) s(-1) to the gamma-ray flux from NGC 1275. This indicates that the source is variable on timescales of years to decades, and therefore restricts the fraction of emission that can be produced in extended regions of the galaxy cluster. Contemporaneous and historical radio observations are also reported. The broadband spectrum of NGC 1275 is modeled with a simple one-zone synchrotron/synchrotron self-Compton model and a model with a decelerating jet flow.
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| 2. |
- Engström, Pia, 1970, et al.
(författare)
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Anaerobic ammonium oxidation by nitrite (anammox): Implications for N-2 production in coastal marine sediments
- 2005
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Ingår i: Geochimica Et Cosmochimica Acta. - : Elsevier BV. - 0016-7037. ; 69:8, s. 2057-2065
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Tidskriftsartikel (refereegranskat)abstract
- The respiratory reduction of nitrate (denitrification) is acknowledged as the most important process that converts biologically available nitrogen to gaseous dinitrogen (N-2) in marine ecosystems. Recent findings, however, indicate that anaerobic ammonium oxidation by nitrite (anammox) may be an important pathway for N-2 formation and N removal in coastal marine sediments and in anoxic water columns of the oceans. In the present study, we explored this novel mechanism during N mineralization by 15 N amendments (single and coupled additions of (NH4+)-N-15, (NO3-)-N-14 and (NO3-)-N-15) to surface sediments with a wide range of characteristics and overall reactivity. Patterns of N-29/30(2) production in the pore water during closed sediment incubations demonstrated anammox at all 7 of the investigated sites. Stoichiometric calculations revealed that 4% to 79% of total N-2 production was due to this novel route. The relative importance of anammox for N2 release was inversely correlated with remineralized solute production, benthic O-2 consumption, and surface sediment Ch1 a. The observed correlations indicate competition between reductants for pore water nitrite during early diagenesis and that additional factors (e.g. availability of Mn-oxides), superimposed on overall patterns of diagenetic activity, are important for determining absolute and relative rates of anammox in coastal marine sediments. Copyright (c) 2005 Elsevier Ltd
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| 3. |
- Hulth, Stefan, 1965, et al.
(författare)
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Nitrogen removal in marine environments: recent findings and future research challenges
- 2005
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Ingår i: Marine Chemistry. - : Elsevier BV. - 0304-4203. ; 94:1-4, s. 125-145
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Tidskriftsartikel (refereegranskat)abstract
- Respiratory reduction of nitrate (denitrification) is recognized as the most important process converting biologically available (fixed) nitrogen to N-2. In current N cycle models, a major proportion of global marine denitrification (50-70%) is assumed to take place on the sea floor, particularly in organic rich continental margin sediments. Recent observations indicate that present conceptual views of denitrification and pathways of nitrate reduction and N, formation are incomplete. Alternative N cycle pathways, particularly in sediments. include anaerobic ammonium oxidation to nitrite, nitrate and N-2 by Mn-oxides, and anaerobic ammonium oxidation coupled to nitrite reduction and subsequent N, mobilization. The discovery of new links and feedback mechanisms between the redox cycles of, e.g., C, N, S, Mn and Fe casts doubt on the present general understanding of the global N cycle. Recent models of the oceanic N budget indicate that total inputs are significantly smaller than estimated fixed N removal. The occurrence of alternative N reaction pathways further exacerbates the apparent imbalance as they introduce additional routes of N removal. In this contribution, we give a brief historical background of the conceptual understanding of N cycling in marine ecosystems, emphasizing pathways of aerobic and anaerobic N mineralization in marine sediments, and the implications of recently recognized metabolic pathways for N removal in marine environments. (c) 2004 Elsevier B.V. All rights reserved.
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