375 research outputs found
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Collisional Ionization Equilibrium for Optically Thin Plasmas
Reliably interpreting spectra from electron-ionized cosmic plasmas requires accurate ionization balance calculations for the plasma in question. However, much of the atomic data needed for these calculations have not been generated using modern theoretical methods and their reliability are often highly suspect. We have utilized state-of-the-art calculations of dielectronic recombination (DR) rate coefficients for the hydrogenic through Na-like ions of all elements from He to Zn. We have also utilized state-of-the-art radiative recombination (RR) rate coefficient calculations for the bare through Na-like ions of all elements from H to Zn. Using our data and the recommended electron impact ionization data of Mazzotta et al. (1998), we have calculated improved collisional ionization equilibrium calculations. We compare our calculated fractional ionic abundances using these data with those presented by Mazzotta et al. (1998) for all elements from H to Ni, and with the fractional abundances derived from the modern DR and RR calculations of Gu (2003a,b, 2004) for Mg, Si, S, Ar, Ca, Fe, and Ni
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Dielectronic recombination data for dynamic finite-density plasmas II. The oxygen isoelectronic sequence
Mistakes in an earlier publication (Zatsarinny et al. 2003) have been discovered and are corrected below (in boldface). The first is a missing exponent in an equation, the second is some wrong fitting coefficients for several ions, the third and fourth are an incorrect caption and an incorrect label in a figure, and the fifth is an inaccuracy in high-temperature radiative recombination rate coefficients
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New Collisional Ionization Equilibrium Calculations for Optically Thin Plasmas
Reliably interpreting spectra from electron-ionized laboratory and cosmic plasmas requires accurate ionization balance calculations for the plasma in question. However, much of the atomic data needed for these calculations have not been generated using modern theoretical methods and their reliability are often highly suspect. We have carried out state-of-the-art calculations of dielectronic recombination (DR) rate coefficients for the hydrogenic through Mg-like ions of all elements from He to Zn as well as for Al- like to Ar-like ions of Fe. We have also carried out state-of-the-art radiative recombination (RR) rate coefficient calculations for the bare through Na-like ions of all elements from H to Zn. Using our data and the most recently recommended electron impact ionization data, we present improved collisional ionization equilibrium (CIE) calculations. Here, as an example, we present our calculated fractional ionic abundances for iron using these data and compare them with those from the previously recommended CIE calculations
Collisional Ionization Equilibrium for Optically Thin Plasmas. I. Updated Recombination Rate Coefficients for Bare though Sodium-like Ions
Reliably interpreting spectra from electron-ionized cosmic plasmas requires
accurate ionization balance calculations for the plasma in question. However,
much of the atomic data needed for these calculations have not been generated
using modern theoretical methods and are often highly suspect. This translates
directly into the reliability of the collisional ionization equilibrium (CIE)
calculations. We make use of state-of-the-art calculations of dielectronic
recombination (DR) rate coefficients for the hydrogenic through Na-like ions of
all elements from He up to and including Zn. We also make use of
state-of-the-art radiative recombination (RR) rate coefficient calculations for
the bare through Na-like ions of all elements from H through to Zn. Here we
present improved CIE calculations for temperatures from to K
using our data and the recommended electron impact ionization data of
\citet{Mazz98a} for elements up to and including Ni and Mazzotta (private
communication) for Cu and Zn. DR and RR data for ionization stages that have
not been updated are also taken from these two additional sources. We compare
our calculated fractional ionic abundances using these data with those
presented by Mazzotta et al. for all elements from H to Ni. The differences in
peak fractional abundance are up to 60%. We also compare with the fractional
ionic abundances for Mg, Si, S, Ar, Ca, Fe, and Ni derived from the modern DR
calculations of \citet{Gu03a,Gu04a} for the H-like through Na-like ions, and
the RR calculations of \citet{Gu03b} for the bare through F-like ions. These
results are in better agreement with our work, with differences in peak
fractional abundance of less than 10%.Comment: 83 pages, 38 figures, 41 tables Accepted to ApJ
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Electron-Impact Ionization of Be-like C III, N IV, and O V
We present recent measurements of absolute electron-impact ionization cross sections for Be-like C III, N IV, and O V forming Li-like C IV, N V, and O VI. The measurements were taken using the crossed-beams apparatus at Oak Ridge National Laboratory. A gas cell beam attenuation method was used to independently measure the metastable fractions present in the ion beams. The measured ionization cross sections were compared with calculations using the R-matrix with pseudostates and distorted-wave theoretical methods. Best agreement is found with the R-matrix with pseudostates cross sections results that account for the metastable fractions inferred from the gas attenuation measurements. We present a set of recommended rate coefficients for electron-impact single ionization from the ground state and metastable term of each ion
Fermi Large Area Telescope Measurements of the Diffuse Gamma-Ray Emission at Intermediate Galactic Latitudes
The diffuse Galactic gamma-ray emission is produced by cosmic rays (CRs)
interacting with the interstellar gas and radiation field. Measurements by the
Energetic Gamma-Ray Experiment Telescope (EGRET) instrument on the Compton
Gamma-Ray Observatory indicated excess gamma-ray emission > 1 GeV relative to
diffuse Galactic gamma-ray emission models consistent with directly measured CR
spectra (the so-called ``EGRET GeV excess''). The excess emission was observed
in all directions on the sky, and a variety of explanations have been proposed,
including beyond-the-Standard-Model scenarios like annihilating or decaying
dark matter. The Large Area Telescope (LAT) instrument on the Fermi Gamma-ray
Space Telescope has measured the diffuse gamma-ray emission with improved
sensitivity and resolution compared to EGRET. We report on LAT measurements of
the diffuse gamma-ray emission for energies 100 MeV to 10 GeV and Galactic
latitudes 10 deg. <= |b| <= 20 deg. The LAT spectrum for this region of the sky
is well reproduced by a diffuse Galactic gamma-ray emission model that is
consistent with local CR spectra and inconsistent with the EGRET GeV excess.Comment: 2 figures, 1 table, accepted by Physical Review Letters, available
online Dec. 18th, 200
Fermi Large Area Telescope Observations of the Cosmic-Ray Induced gamma-ray Emission of the Earth's Atmosphere
We report on measurements of the cosmic-ray induced gamma-ray emission of
Earth's atmosphere by the Large Area Telescope onboard the Fermi Gamma-ray
Space Telescope. The LAT has observed the Earth during its commissioning phase
and with a dedicated Earth-limb following observation in September 2008. These
measurements yielded 6.4 x 10^6 photons with energies >100MeV and ~250hours
total livetime for the highest quality data selection. This allows the study of
the spatial and spectral distributions of these photons with unprecedented
detail. The spectrum of the emission - often referred to as Earth albedo
gamma-ray emission - has a power-law shape up to 500 GeV with spectral index
Gamma = 2.79+-0.06.Comment: Accepted for publication in PR
Fermi/LAT discovery of gamma-ray emission from a relativistic jet in the narrow-line quasar PMN J0948+0022
We report the discovery by the Large Area Telescope (LAT) onboard the Fermi
Gamma-ray Space Telescope of high-energy gamma-ray emission from the peculiar
quasar PMN J0948+0022 (z=0.5846). The optical spectrum of this object exhibits
rather narrow Hbeta (FWHM(Hbeta) ~ 1500 km s^-1), weak forbidden lines and is
therefore classified as a narrow-line type I quasar. This class of objects is
thought to have relatively small black hole mass and to accrete at high
Eddington ratio. The radio loudness and variability of the compact radio core
indicates the presence of a relativistic jet. Quasi simultaneous
radio-optical-X-ray and gamma-ray observations are presented. Both radio and
gamma-ray emission (observed over 5-months) are strongly variable. The
simultaneous optical and X-ray data from Swift show a blue continuum attributed
to the accretion disk and a hard X-ray spectrum attributed to the jet. The
resulting broad band spectral energy distribution (SED) and, in particular, the
gamma-ray spectrum measured by Fermi are similar to those of more powerful
FSRQ. A comparison of the radio and gamma-ray characteristics of PMN J0948+0022
with the other blazars detected by LAT shows that this source has a relatively
low radio and gamma-ray power, with respect to other FSRQ. The physical
parameters obtained from modelling the SED also fall at the low power end of
the FSRQ parameter region discussed in Celotti & Ghisellini (2008). We suggest
that the similarity of the SED of PMN J0948+0022 to that of more massive and
more powerful quasars can be understood in a scenario in which the SED
properties depend on the Eddington ratio rather than on the absolute power.Comment: 10 pages, 5 figures, accepted for publication on ApJ Main Journal.
Corresponding author: L. Foschin
Fermi LAT Observation of Diffuse Gamma-Rays Produced Through Interactions between Local Interstellar Matter and High Energy Cosmic Rays
Observations by the Large Area Telescope (LAT) on the \textit{Fermi} mission
of diffuse -rays in a mid-latitude region in the third quadrant
(Galactic longitude from 200\arcdeg to 260\arcdeg and latitude
from 22\arcdeg to 60\arcdeg) are reported. The region contains no known
large molecular cloud and most of the atomic hydrogen is within 1 kpc of the
solar system. The contributions of -ray point sources and inverse
Compton scattering are estimated and subtracted. The residual -ray
intensity exhibits a linear correlation with the atomic gas column density in
energy from 100 MeV to 10 GeV. The measured integrated -ray emissivity
is (1.63 \pm 0.05) \times 10^{-26} {\rm photons s^{-1} sr^{-1}
H\mathchar`-atom^{-1}} and (0.66 \pm 0.02) \times 10^{-26} {\rm photons
s^{-1} sr^{-1} H\mathchar`-atom^{-1}} above 100 MeV and above 300 MeV,
respectively, with additional systematic error of . The differential
emissivity in 100 MeV--10 GeV agrees with calculations based on cosmic ray
spectra consistent with those directly measured, at the 10% level. The results
obtained indicate that cosmic ray nuclei spectra within 1 kpc from the solar
system in regions studied are close to the local interstellar spectra inferred
from direct measurements at the Earth within .Comment: accepted for publication in the Astrophysical Journal. Revised
according to the author proof.(correction of typos and minor revisions
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