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Träfflista för sökning "WFRF:(Zook A) "

Sökning: WFRF:(Zook A)

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1.
  • Grün, E., et al. (författare)
  • Dust measurements in the Jovian magnetosphere
  • 1997
  • Ingår i: Geophysical Research Letters. - 1944-8007. ; 24:17, s. 2171-2174
  • Tidskriftsartikel (refereegranskat)abstract
    • Dust measurements have been obtained with the dust detector onboard the Galileo spacecraft inside a distance of about 60RJ from Jupiter (Jupiter radius, RJ = 71,492 km) during two periods of about 8 days around Galileo's closest approaches to Ganymede on 27 June and on 6 Sept 1996. The impact rate of submicrometer-sized particles fluctuated by a factor of several hundred with a period of about 10 hours, implying that their trajectories are strongly affected by the interaction with the Jovian magnetic field. Concentrations of small dust impacts were detected at the times of Ganymede closest approaches that could be secondary ejecta particles generated upon impact of other particles onto Ganymede's surface. Micrometer-sized dust particles, which could be on bound orbits about Jupiter, are concentrated in the inner Jovian system inside about 20RJ from Jupiter.
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2.
  • Grün, E., et al. (författare)
  • Galileo observes electromagnetically coupled dust in the Jovian magnetosphere
  • 1998
  • Ingår i: Journal of Geophysical Research. - 2156-2202. ; 103:E9, s. 20011-20022
  • Tidskriftsartikel (refereegranskat)abstract
    • Measurements of dust coupled to the Jovian magnetosphere have been obtained with the dust detector on board the Galileo spacecraft. We report on data obtained during the first four orbits about Jupiter that had flybys of the Galilean satellites: Ganymede (orbits 1 and 2), Callisto (orbit 3), and Europa (orbit 4). The most prominent features observed are highly time variable dust streams recorded throughout the Jovian system. The impact rate varied by up to 2 orders of magnitude with a 5 and 10 hour periodicity, which shows a correlation with Galileo's position relative to the Jovian magnetic field. Around 20 RJ (Jupiter radius, RJ=71, 492 km) in bound a dip in the impact rate has been found consistently. At the same times, reversals by 180° in impact direction occurred. This behavior can be qualitatively explained by strong coupling of nanometer-sized dust to the Jovian magnetic field. At times of satellite flybys, enhanced rates of dust impacts have been observed, which suggests that all Galilean satellites are sources of ejecta particles. Inside about 20 RJ impacts of micrometer-sized particles have been recorded that could be particles on bound orbits about Jupiter.
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3.
  • Buson, S., et al. (författare)
  • Unusual flaring activity in the blazar PKS 1424-418 during 2008-2011
  • 2014
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 569, s. A40-
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. Blazars are a subset of active galactic nuclei (AGN) with jets that are oriented along our line of sight. Variability and spectral energy distribution (SED) studies are crucial tools for understanding the physical processes responsible for observed AGN emission. Aims. We report peculiar behavior in the bright gamma-ray blazar PKS 1424 418 and use its strong variability to reveal information about the particle acceleration and interactions in the jet. Methods. Correlation analysis of the extensive optical coverage by the ATOM telescope and nearly continuous gamma-ray coverage by the Fermi Large Area Telescope is combined with broadband, time-dependent modeling of the SED incorporating supplemental information from radio and X-ray observations of this blazar. Results. We analyse in detail four bright phases at optical-GeV energies. These flares of PKS 1424-418 show high correlation between these energy ranges, with the exception of one large optical flare that coincides with relatively low gamma-ray activity. Although the optical/gamma-ray behavior of PKS 1424-418 shows variety, the multiwavelength modeling indicates that these differences can largely be explained by changes in the flux and energy spectrum of the electrons in the jet that are radiating. We find that for all flares the SED is adequately represented by a leptonic model that includes inverse Compton emission from external radiation fields with similar parameters. Conclusions. Detailed studies of individual blazars like PKS 1424 418 during periods of enhanced activity in different wavebands are helping us identify underlying patterns in the physical parameters in this class of AGN.
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4.
  • Gruen, E., et al. (författare)
  • Ulysses Dust Detection System V3.1
  • 2010
  • Ingår i: NASA Planetary Data System. ; 140
  • Tidskriftsartikel (refereegranskat)abstract
    • This data set contains the data from the Ulysses dust detector system (UDDS) from start of mission through the end of mission, 1990-2007. (As the dust detector was turned off after Nov. 30, 2007, this is the last date for which UDDS data is recorded.) Included are the dust impact data, noise data, laboratory calibration data, and location and orientation of the spacecraft and instrument.
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5.
  • Grün, E., et al. (författare)
  • Dust Measurements During Galileo's Approach to Jupiter and Io Encounter
  • 1996
  • Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 1095-9203 .- 0036-8075. ; 274:5286, s. 399-401
  • Tidskriftsartikel (refereegranskat)abstract
    • About a hundred dust impacts per day were detected during the first week in December 1995 by Galileo during its approach to Jupiter. These impacts were caused by submicrometer-sized particles that were just above the detection limit. After the closest approach to Io on 7 December, impacts of these small particles ceased. This effect is expected for dust grains emitted from Io that exit the field of view of the instrument after the flyby. The impact rate of bigger micrometer-sized dust grains continued to increase toward Jupiter. These dust particles are in orbit about Jupiter or are interplanetary grains that are gravitationally concentrated near Jupiter.
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6.
  • Krueger, H., et al. (författare)
  • Five years of Ulysses dust data: 2000-2004
  • 2006
  • Ingår i: Planetary and Space Science. - : Elsevier BV. - 1873-5088 .- 0032-0633. ; 54:9-10, s. 932-956
  • Tidskriftsartikel (refereegranskat)abstract
    • The Ulysses spacecraft has been orbiting the Sun on a highly inclined ellipse (i = 79 degrees, perihelion distance 1.3 AU, aphelion distance 5.4 AU) since it encountered Jupiter in 1992. Between January 2000 and December 2004, the spacecraft completed almost an entire revolution about the Sun, passing through perihelion in May 2001 and aphelion in July 2004. In this five-year period the dust detector on board recorded 4415 dust impacts. We publish and analyse the complete data set of both raw and reduced data for particles with masses 10(-16) g <= M <= 10(-7) g. Together with. 1695 dust impacts recorded between launch of Ulysses and the end of 1999 published earlier (Grain, E., Baguhl, M., Divine, N., Fechtig, H., Hamilton, D.P, Harmer, M.S., Kissel, J., Lindblad, B.A., Linkert, D., Linkert, G., Mann, L, McDonnell, J.A.M., Morfill, G.E., Polanskey, C., Riemann, R., Schwehm, G.H., Siddique, N., Staubach, P., Zook, H.A., 1995a. Two years of Ulysses dust data. Planetary Space Sci. 43, 971-999, Paper III; Kruger, H., Grun, E., Landgraf, M., Baguhl, M., Dermott, S.F., Fechtig, H., Gustafson, B.A., Hamilton, D.P., Harmer, M.S., Horanyi, M., Kissel, J., Lindblad, B., Linkert, D., Linkert, G., Mann, L, McDonnell, J.A.M., Morfill, G.E., Polanskey, C., Schwehm, G.H., Srama, R., Zook, H.A., 1995. Three years of Ulysses dust data: 1993 to 1995. Planetary and Space Sci. 47, 363-383, Paper V; Kruger, H., Grun, E., Landgraf, M., Dermott, S.F., Fechtig, H., Gustafson, B.A., Hamilton, D.P., Harmer, M.S., Horanyi, M., Kissel, J., Lindblad, B., Linkert, D., Linkert, G., Mann, I., McDonnell, J.A.M., Morfill, G.E., Polanskey, C., Schwehm, G.H., Srama, R., Zook, H.A., 2001b. Four years of Ulysses dust data: 1996 to 1999. Planetary Space Sci. 49, 1303-1324, Paper VII), a data set of 6110 dust impacts detected with the Ulysses sensor between October 1990 and December 2004 is now available. The impact rate measured between 2000 and 2002 was relatively constant with about 0.3 impacts per day showing a maximum at 1.5 per day around ecliptic plane crossing in early-2001. The impact direction of the majority of impacts between 2000 and 2002 is compatible with particles of interstellar origin, the rest are most likely interplanetary particles. In 2003 and 2004 dust stream particles originating from the jovian system dominated the overall impact rate. Twenty-two individual dust streams were measured between November 2002 and December 2004. The observed impact rates are compared with models for interplanetary and interstellar dust. The dust measurements from the entire mission since Ulysses launch give good agreement with the interplanetary flux model of Staubach, P., Grun, E., Jehn, R., 1997. The meteoroid environment near Earth, Adv. Space Res. 19, 301-308. (c) 2006 Elsevier Ltd. All rights reserved.
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7.
  • Krueger, H., et al. (författare)
  • Galileo dust data from the jovian system: 1997-1999
  • 2006
  • Ingår i: Planetary and Space Science. - : Elsevier BV. - 1873-5088 .- 0032-0633. ; 54:9-10, s. 879-910
  • Tidskriftsartikel (refereegranskat)abstract
    • The dust detector system on board the Galileo spacecraft recorded dust impacts in circumjovian space during the craft's orbital mission about Jupiter. This is the eighth in a series of papers dedicated to presenting Galileo and Ulysses dust data. We present data from the Galileo dust instrument for the period January 1997-December 1999 when the spacecraft completed 21 revolutions about Jupiter. In this time interval data were obtained as high resolution realtime science data or recorded data during 449 days (representing 41% of the entire period), or via memory readouts during the remaining times. Because the data transmission rate of the spacecraft was very low, the complete data set (i.e. all parameters measured by the instrument during impact of a dust particle) of only 3% (7625) of all particles detected could be transmitted to Earth; the other particles were only counted. Together with the data of 2883 particles detected during, Galileo's interplanetary cruise and 53 53 particles detected in the jovian system in 1996, complete data of 15 861 particles detected by the Galileo dust instrument from 1989 to 1999 are now available. The majority of the detected particles were tiny grains (about 10 nm in radius), most of them originating from Jupiter's innermost Galilean moon Io. They were detected throughout the jovian system and the highest impact rates exceeded 100 min(-1) (C21 orbit; 01 July 1999). With the new data set the times of onset, cessation and a 180 degrees shift in the impact direction of the grains measured during 19 Galileo orbits about Jupiter are well reproduced by simulated 9 nm particles charged up to a potential of +3 V, confirming earlier results obtained for only two Galileo orbits (Horanyi, M., Grun, E., Heck, A., 1997. Modeling the Galileo dust measurements at Jupiter. Geophys. Res. Lett. 24, 2175-2178). Galileo has detected a large number of bigger particles mostly in the region between the Galilean moons. The average radius of 370 of these grains measured in the 1996-1999 period is about 2 mu m (assuming spherical grains with density 1 g cm(-3)) and the size distribution rises steeply towards smaller grains. The biggest detected particles have a radius of about 10 mu m. (c) 2006 Elsevier Ltd. All rights reserved.
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8.
  • Krüger, H., et al. (författare)
  • One year of Galileo dust data from the Jovian system: 1996
  • 2001
  • Ingår i: Planetary and Space Science. - 1873-5088. ; 49:13, s. 1285-1301
  • Tidskriftsartikel (refereegranskat)abstract
    • The dust detector system onboard Galileo has recoding dust impacts incircumjovian space since the spacecraft was injected into a bound orbitabout Jupiter in December 1995. This is the sixth in a series of papersdedicated to presenting Galileo and Ulysses dust data. We present datafrom the Galileo dust instrument for the period January to December 1996when the spacecraft completed four orbits about Jupiter (G1, G2, C3 andE4). Data were obtained as high-resolution realtime science data orrecorded data during a time period of 100 days, or via memory read-outsduring the remaining times. Because the data transmission rate of thespacecraft is very low, the complete data set (i.e. all parametersmeasured by the instrument during impact of a dust particle) for only 2%(5353) of all particles detected could be transmitted to Earth; theother particles were only counted. Together with the data for 2883particles detected during Galileo's interplanetary cruise and publishedearlier, complete data of 8236 particles detected by the Galileo dustinstrument from 1989 to 1996 are now available. The majority ofparticles detected are tiny grains (about 10nm in radius) originatingfrom Jupiter's innermost Galilean moon Io. These grains have beendetected throughout the Jovian system and the highest impact ratesexceeded 100min-1. A small number of grains has been detectedin the close vicinity of the Galilean moons Europa, Ganymede andCallisto which belong to impact-generated dust clouds formed by (mostlysubmicrometer sized) ejecta from the surfaces of the moons (Krügeret al., /1999e. Nature 399, 558). Impacts of submicrometer to micrometersized grains have been detected throughout the Jovian system andespecially in the region between the Galilean moons.
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9.
  • Abdo, A A, et al. (författare)
  • Fermi large area telescope observations of Markarian 421 : The missing piece of its spectral energy distribution
  • 2011
  • Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 736:2, s. 131-
  • Tidskriftsartikel (refereegranskat)abstract
    • We report on the gamma-ray activity of the high-synchrotron-peaked BL Lacertae object Markarian 421 (Mrk 421) during the first 1.5 years of Fermi operation, from 2008 August 5 to 2010 March 12. We find that the Large Area Telescope (LAT) gamma-ray spectrum above 0.3 GeV can be well described by a power-law function with photon index Gamma = 1.78 +/- 0.02 and average photon flux F(>0.3 GeV) = (7.23 +/- 0.16) x 10(-8) ph cm(-2) s(-1). Over this time period, the Fermi-LAT spectrum above 0.3 GeV was evaluated on seven-day-long time intervals, showing significant variations in the photon flux (up to a factor similar to 3 from the minimum to the maximum flux) but mild spectral variations. The variability amplitude at X-ray frequencies measured by RXTE/ASM and Swift/BAT is substantially larger than that in gamma-rays measured by Fermi-LAT, and these two energy ranges are not significantly correlated. We also present the first results from the 4.5 month long multifrequency campaign on Mrk 421, which included the VLBA, Swift, RXTE, MAGIC, the F-GAMMA, GASP-WEBT, and other collaborations and instruments that provided excellent temporal and energy coverage of the source throughout the entire campaign (2009 January 19 to 2009 June 1). During this campaign, Mrk 421 showed a low activity at all wavebands. The extensive multi-instrument (radio to TeV) data set provides an unprecedented, complete look at the quiescent spectral energy distribution (SED) for this source. The broadband SED was reproduced with a leptonic (one-zone synchrotron self-Compton) and a hadronic model (synchrotron proton blazar). Both frameworks are able to describe the average SED reasonably well, implying comparable jet powers but very different characteristics for the blazar emission site.
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10.
  • Abdo, A. A., et al. (författare)
  • Insights into the high-energy γ-ray emission of Markarian 501 from extensive multifrequency observations in the Fermi era
  • 2011
  • Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 727:2, s. 129-
  • Tidskriftsartikel (refereegranskat)abstract
    • We report on the gamma-ray activity of the blazar Mrk 501 during the first 480 days of Fermi operation. We find that the average Large Area Telescope (LAT) gamma-ray spectrum of Mrk 501 can be well described by a single power-law function with a photon index of 1.78 +/- 0.03. While we observe relatively mild flux variations with the Fermi-LAT (within less than a factor of two), we detect remarkable spectral variability where the hardest observed spectral index within the LAT energy range is 1.52 +/- 0.14, and the softest one is 2.51 +/- 0.20. These unexpected spectral changes do not correlate with the measured flux variations above 0.3 GeV. In this paper, we also present the first results from the 4.5 month long multifrequency campaign (2009 March 15-August 1) on Mrk 501, which included the Very Long Baseline Array (VLBA), Swift, RXTE, MAGIC, and VERITAS, the F-GAMMA, GASP-WEBT, and other collaborations and instruments which provided excellent temporal and energy coverage of the source throughout the entire campaign. The extensive radio to TeV data set from this campaign provides us with the most detailed spectral energy distribution yet collected for this source during its relatively low activity. The average spectral energy distribution of Mrk 501 is well described by the standard one-zone synchrotron self-Compton (SSC) model. In the framework of this model, we find that the dominant emission region is characterized by a size less than or similar to 0.1 pc (comparable within a factor of few to the size of the partially resolved VLBA core at 15-43 GHz), and that the total jet power (similar or equal to 10(44) erg s(-1)) constitutes only a small fraction (similar to 10(-3)) of the Eddington luminosity. The energy distribution of the freshly accelerated radiating electrons required to fit the time-averaged data has a broken power-law form in the energy range 0.3 GeV-10 TeV, with spectral indices 2.2 and 2.7 below and above the break energy of 20 GeV. We argue that such a form is consistent with a scenario in which the bulk of the energy dissipation within the dominant emission zone of Mrk 501 is due to relativistic, proton-mediated shocks. We find that the ultrarelativistic electrons and mildly relativistic protons within the blazar zone, if comparable in number, are in approximate energy equipartition, with their energy dominating the jet magnetic field energy by about two orders of magnitude.
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