| 1. |
- Judge, Philip, et al.
(författare)
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On the formation of extreme-ultraviolet helium lines in the Sun: Analysis of SOHO data
- 2004
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 606:2, s. 1258-1275
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Tidskriftsartikel (refereegranskat)abstract
- The resonance lines of helium in the Sun are several times stronger than expected, relative to lines of other ions. To explore the origins of this "helium enhancement,'' we have studied data from the SUMER, CDS, MDI, and EIT instruments on the Solar and Heliospheric Observatory (SOHO). Time series data obtained in a quiet region and a coronal hole indicate that the spatio-temporal properties of the He I 584 Angstrom, and He II 304 and 1084 Angstrom lines are qualitatively unlike other chromospheric and transition region lines. Helium line intensities vary slowly compared to chromospheric oscillations and to transient transition region brightenings seen in other lines, yet they vary rapidly (both spatially and temporally) compared to the coronal radiation. This suggests that photoionization/recombination plays a minor excitation role in these lines. The Doppler shift of the 584 Angstrom line shows a remarkably clear 4-8 mHz oscillation, with no accompanying line intensity oscillation. Transient brightenings are used to show that nonradial photon scattering might explain the previously found diffuse appearance of the network but cannot account for the reduced network/internetwork intensity contrast in the 584 Angstrom line. We propose a new enhancement mechanism, based on arguments in a companion paper, in which predominantly neutral species such as helium diffuse across magnetic field lines into regions of hot coronal plasma, but charged ions do not. The enhanced internetwork helium emission may arise from the diffusion of helium atoms across the chromospheric canopy; the enhanced network emission may have contributions from diffusion from spicules. This mechanism may naturally explain why the 584 Angstrom line shows Doppler shift, but not intensity, oscillations. Finally, if a mechanism such as "velocity redistribution'' dominates helium emission, spectroheliograms of helium lines will be bright in regions of large temperature gradients parallel to the magnetic field. Differences between helium and other spectroheliograms may then reveal the relative contributions of "classical'' and "cool loop'' models to the observed emission.
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| 2. |
- Ahrens, J., et al.
(författare)
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Search for point sources of high-energy neutrinos with AMANDA
- 2003
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Ingår i: Astrophysical Journal Letters. - : Institute of Physics (IOP). - 2041-8205 .- 0004-637X .- 1538-4357. ; 583:2 I, s. 1040-1057
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Tidskriftsartikel (refereegranskat)abstract
- This paper describes the search for astronomical sources of high-energy neutrinos using the AMANDA-B10 detector, an array of 302 photomultiplier tubes used for the detection of Cerenkov light from upward-traveling neutrino-induced muons, buried deep in ice at the South Pole. The absolute pointing accuracy and angular resolution were studied by using coincident events between the AMANDA detector and two independent telescopes on the surface, the GASP air Cerenkov telescope and the SPASE extensive air shower array. Using data collected from 1997 April to October (130.1 days of live time), a general survey of the northern hemisphere revealed no statistically significant excess of events from any direction. The sensitivity for a flux of muon neutrinos is based on the effective detection area for through-going muons. Averaged over the northern sky, the effective detection area exceeds 10,000 m2 for E μ ≈ 10 TeV. Neutrinos generated in the atmosphere by cosmic-ray interactions were used to verify the predicted performance of the detector. For a source with a differential energy spectrum proportional to Eν -2 and declination larger than +40°, we obtain E2(dNν/dE) ≤ 10-6 GeV cm-2 s-1 for an energy threshold of 10 GeV.
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| 3. |
- Andersson, Karl E., et al.
(författare)
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Complex structure of galaxy cluster A1689 : Evidence for a merger from X-ray data?
- 2004
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 607:1, s. 190-201
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Tidskriftsartikel (refereegranskat)abstract
- A1689 is a galaxy cluster at z = 0.183 for which previous measurements of its mass by using various techniques gave discrepant results. We present a new detailed measurement of the mass with the data based on X-ray observations with the European Photon Imaging Camera aboard the XMM-Newton Observatory, determined by using an unparameterized deprojection technique. Fitting the total mass profile to a Navarro-Frenk-White model yields halo concentration c = 7.2(-2.4)(+1.6) and r(200) = 1.13 +/- 0.21 h(-1) Mpc, corresponding to a mass that is less than half of that found from gravitational lensing. Adding to the evidence of substructure from optical observations, X-ray analysis shows a highly asymmetric temperature profile and a nonuniform redshift distribution, implying large-scale relative motion of the gas. A lower than expected gas mass fraction f(gas) = 0.072 +/- 0: 008 (for a flat LambdaCDM cosmology) suggests a complex spatial and/or dynamical structure. We also find no sign of any additional absorbing component previously reported on the basis of the Chandra data, confirming the XMM-Newton low-energy response by using data from ROSAT.
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| 4. |
- Bessell, M.S., et al.
(författare)
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On the oxygen abundance of HE0107-5240
- 2004
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Ingår i: Astrophysical Journal: Letters. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 612:1, s. 61-63
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Tidskriftsartikel (refereegranskat)
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| 5. |
- Biemont, E, et al.
(författare)
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Doubly ionized thorium: Laser lifetime measurements and transition probability determination of interest in cosmochronology
- 2002
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 567:2, s. 1276-1283
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Tidskriftsartikel (refereegranskat)abstract
- The first lifetime measurements have been performed for six levels of doubly ionized thorium, an important cosmochronometer in astrophysics for estimating the age of the Galaxy. The levels, belonging to the 5f(2), 5f7p, 7s7p, and 6d7p configurations of Th III, have been measured by the time-resolved laser-induced fluorescence method and compared with relativistic Hartree-Fock calculations including configuration interaction and core-polarization effects. Taking advantage of the excellent agreement between theory and experiment, a first set of transition probabilities of astrophysical interest has been deduced for this ion from a combination of the experimental lifetimes and of the theoretical branching fractions.
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| 6. |
- Blinnikov, Sergei, et al.
(författare)
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Radiation Hydrodynamics of SN 1987A. I. Global Analysis of the Light Curve for the First 4 Months
- 2000
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 532, s. 1132-1149
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Tidskriftsartikel (refereegranskat)abstract
- The optical/UV light curves of SN 1987A are analyzed with the multienergy group radiation hydrodynamics code STELLA. The calculated monochromatic and bolometric light curves are compared with observations shortly after shock breakout, during the early plateau, through the broad second maximum, and during the earliest phase of the radioactive tail. We have concentrated on a progenitor model calculated by Nomoto & Hashimoto and Saio, Nomoto, & Kato, which assumes that 14 Msolar of the stellar mass is ejected. Using this model, we have updated constraints on the explosion energy and the extent of mixing in the ejecta. In particular, we determine the most likely range of E/M (explosion energy over ejecta mass) and R0 (radius of the progenitor). In general, our best models have energies in the range E=(1.1+/-0.3)x1051 ergs, and the agreement is better than in earlier, flux-limited diffusion calculations for the same explosion energy. Our modeled B and V fluxes compare well with observations, while the flux in U undershoots after ~10 days by a factor of a few, presumably owing to NLTE and line transfer effects. We also compare our results with IUE observations, and a very good quantitative agreement is found for the first days, and for one IUE band (2500-3000 Å) as long as for 3 months. We point out that the V flux estimated by McNaught & Zoltowski should probably be revised to a lower value.
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| 7. |
- Borgonovo, L., et al.
(författare)
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On the hardness-intensity correlation in gamma-ray burst pulses
- 2001
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 548:2, s. 770-786
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Tidskriftsartikel (refereegranskat)abstract
- We study the hardness-intensity correlation (HIC) in gamma-ray bursts (GRBs). In particular, we analyze the decay phase of pulse structures in their light curves. The study comprises a sample of 82 long pulses selected from 66 long bursts observed by the Burst and Transient Source Experiment (BATSE) on the Compton Gamma-Ray Observatory. We find that at least 57% of these pulses have HICs that can be well described by a power law. A number of the other cases can still be explained with the power-law model if various limitations of the observations are taken into account. The distribution of the power-law indices gamma, obtained by modeling the HIC of pulses from different bursts, is broad, with a mean of 1.9 and a standard deviation of 0.7. We also compare indices among pulses from the same bursts and find that their distribution is significantly narrower. The probability p of a random coincidence is shown to be very small (< 2 x 10(-5)). In most cases, the indices are equal to within the uncertainties. These results demand a physical model to be able to reproduce multiple pulses with similar characteristics for an individual burst, but with a large diversity for pulses from an ensemble of bursts. This is particularly relevant when comparing the external versus the internal models. In our analysis, we also use a new method for studying the hardness-intensity correlation, in which the intensity is represented by the peak value of the EFE spectrum, where E is the energy and F-E is the energy flux spectrum. We compare it to the traditional method in which the intensity over a finite energy range is used instead, which may be an incorrect measure of the bolometric intensity. This new method gives stronger correlations and is useful in the study of various aspects of the HIC. In particular, it produces a better agreement between indices of different pulses within the same burst. Also, we find that some pulses exhibit a track jump in their HICs, in which the correlation jumps between two power laws with the same index. We discuss the possibility that the track jump is caused by strongly overlapping pulses. Based on our findings, the constancy of the index is proposed to be used as a tool for pulse identification in overlapping pulses and examples of its application are given.
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| 8. |
- Brage, Tomas, et al.
(författare)
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Spectral lines for polarization measurements of the coronal magnetic field. III. Atomic data for SiIX
- 2000
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 540:2, s. 1114-1118
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Tidskriftsartikel (refereegranskat)abstract
- We report on large-scale ab initio multiconfiguration Dirac-Fock (MCDF) calculations for spectral lines of Si IX, with emphasis on the forbidden transitions 2p(2) P-3(J-->J'). The J = 0 --> J' = 1 transition at 3.9346 mu m holds promise as a diagnostic of coronal magnetic fields if and when future coronagraphic instruments can measure the polarized light at this wavelength.
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| 9. |
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| 10. |
- Geppert, W. D., et al.
(författare)
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Dissociative recombination of N2H+ : Evidence for fracture of the N-N bond
- 2004
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 609:1, s. 459-464
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Tidskriftsartikel (refereegranskat)abstract
- Branching ratios and absolute cross sections have been measured for the dissociative recombination of N2H+ using the CRYRING ion storage ring. It has been found that the channel N2H+ + e(-) --> N-2 + H accounts for only 36% of the total reaction and that the branching into the other exoergic pathway, N2H+ + e(-) --> NH + N, consequently amounts to 64%. The cross section of the reaction could be fitted by the expression sigma = (2.4 +/- 0.4) x 10(-16) E-1.04 +/- 0.02 cm(2), which leads to a thermal reaction rate of k(T) = (1.0 +/- 0.2) x 10(-7)(T/300)(-0.51 +/- 0.02) cm(3) s(-1), in favorable agreement with previous flowing afterglow Langmuir probe measurements at room temperature, although our temperature dependence is very different. The implications of these measurements for the chemistry of interstellar clouds are discussed. A standard model calculation for a dark cloud predicts a slight increase of N2H+ in the dark clouds but a five- to sevenfold increase of the NH concentration as steady state is reached.
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