558 research outputs found
Vacancy assisted arsenic diffusion and time dependent clustering effects in silicon
We present results of kinetic lattice Monte Carlo (KLMC) simulations of
substitutional arsenic diffusion in silicon mediated by lattice vacancies.
Large systems are considered, with 1000 dopant atoms and long range \textit{ab
initio} interactions, to the 18th nearest lattice neighbor, and the diffusivity
of each defect species over time is calculated. The concentration of vacancies
is greater than equilibrium concentrations in order to simulate conditions
shortly after ion implantation. A previously unreported time dependence in the
applicability of the pair diffusion model, even at low temperatures, is
demonstrated. Additionally, long range interactions are shown to be of critical
importance in KLMC simulations; when shorter interaction ranges are considered
only clusters composed entirely of vacancies form. An increase in arsenic
diffusivity for arsenic concentrations up to is
observed, along with a decrease in arsenic diffusivity for higher arsenic
concentrations, due to the formation of arsenic dominated clusters. Finally,
the effect of vacancy concentration on diffusivity and clustering is studied,
and increasing vacancy concentration is found to lead to a greater number of
clusters, more defects per cluster, and a greater vacancy fraction within the
clusters.Comment: 22 pages, 16 figure
The spectroscopic evolution of the symbiotic-like recurrent nova V407 Cygni during its 2010 outburst. I. The shock and its evolution
On 2010 Mar 10, V407 Cyg was discovered in outburst, eventually reaching V< 8
and detected by Fermi. Using medium and high resolution ground-based optical
spectra, visual and Swift UV photometry, and Swift X-ray spectrophotometry, we
describe the behavior of the high-velocity profile evolution for this nova
during its first three months. The peak of the X-ray emission occurred at about
day 40 with a broad maximum and decline after day 50. The main changes in the
optical spectrum began at around that time. The He II 4686A line first appeared
between days 7 and 14 and initially displayed a broad, symmetric profile that
is characteristic of all species before day 60. Low-excitation lines remained
comparatively narrow, with v(rad,max) of order 200-400 km/s. They were
systematically more symmetric than lines such as [Ca V], [Fe VII], [Fe X], and
He II, all of which showed a sequence of profile changes going from symmetric
to a blue wing similar to that of the low ionization species but with a red
wing extended to as high as 600 km/s . The Na I D doublet developed a broad
component with similar velocity width to the other low-ionization species. The
O VI Raman features were not detected. We interpret these variations as
aspherical expansion of the ejecta within the Mira wind. The blue side is from
the shock penetrating into the wind while the red wing is from the low-density
periphery. The maximum radial velocities obey power laws, v(rad,max) t^{-n}
with n ~ 1/3 for red wing and ~0.8 for the blue. (truncated)Comment: Accepted for publication, A&A (submitted: 9 Oct 2010; accepted: 1 Dec
2010) in press; based on data obtained with Swift, Nordic Optical Telescope,
Ondrejov Observatory. Corrected typo, Fermi?LAT detection was at energies
above 100 MeV (with thanks to C. C. Cheung
The SMC SNR 1E0102.2-7219 as a Calibration Standard for X-ray Astronomy in the 0.3-2.5 keV Bandpass
The flight calibration of the spectral response of CCD instruments below 1.5
keV is difficult in general because of the lack of strong lines in the on-board
calibration sources typically available. We have been using 1E 0102.2-7219, the
brightest supernova remnant in the Small Magellanic Cloud, to evaluate the
response models of the ACIS CCDs on the Chandra X-ray Observatory (CXO), the
EPIC CCDs on the XMM-Newton Observatory, the XIS CCDs on the Suzaku
Observatory, and the XRT CCD on the Swift Observatory. E0102 has strong lines
of O, Ne, and Mg below 1.5 keV and little or no Fe emission to complicate the
spectrum. The spectrum of E0102 has been well characterized using
high-resolution grating instruments, namely the XMM-Newton RGS and the CXO
HETG, through which a consistent spectral model has been developed that can
then be used to fit the lower-resolution CCD spectra. We have also used the
measured intensities of the lines to investigate the consistency of the
effective area models for the various instruments around the bright O (~570 eV
and 654 eV) and Ne (~910 eV and 1022 eV) lines. We find that the measured
fluxes of the O VII triplet, the O VIII Ly-alpha line, the Ne IX triplet, and
the Ne X Ly-alpha line generally agree to within +/-10 % for all instruments,
with 28 of our 32 fitted normalizations within +/-10% of the RGS-determined
value. The maximum discrepancies, computed as the percentage difference between
the lowest and highest normalization for any instrument pair, are 23% for the O
VII triplet, 24% for the O VIII Ly-alpha line, 13% for the Ne IX triplet, and
19% for the Ne X Ly-alpha line. If only the CXO and XMM are compared, the
maximum discrepancies are 22% for the O VII triplet, 16% for the O VIII
Ly-alpha line, 4% for the Ne IX triplet, and 12% for the Ne X Ly-alpha line.Comment: 16 pages, 11 figures, to be published in Proceedings of the SPIE
7011: Space Telescopes and Instrumentation II: Ultraviolet to Gamma Ray 200
Calibration of X-ray absorption in our Galaxy
Prediction of the soft X-ray absorption along lines of sight through our
Galaxy is crucial for understanding the spectra of extragalactic sources, but
requires a good estimate of the foreground column density of photoelectric
absorbing species. Assuming uniform elemental abundances this reduces to having
a good estimate of the total hydrogen column density, N(Htot)=N(HI)+2N(H2). The
atomic component, N(HI), is reliably provided using the mapped 21 cm radio
emission but estimating the molecular hydrogen column density, N(H2), expected
for any particular direction, is difficult. The X-ray afterglows of GRBs are
ideal sources to probe X-ray absorption in our Galaxy because they are
extragalactic, numerous, bright, have simple spectra and occur randomly across
the entire sky. We describe an empirical method, utilizing 493 afterglows
detected by the Swift XRT, to determine N(Htot) through the Milky Way which
provides an improved estimate of the X-ray absorption in our Galaxy and thereby
leads to more reliable measurements of the intrinsic X-ray absorption and,
potentially, other spectral parameters, for extragalactic X-ray sources. We
derive a simple function, dependent on the product of the atomic hydrogen
column density, N(HI), and dust extinction, E(B-V), which describes the
variation of the molecular hydrogen column density, N(H2), of our Galaxy, over
the sky. Using the resulting N(Htot) we show that the dust-to-hydrogen ratio is
correlated with the carbon monoxide emission and use this ratio to estimate the
fraction of material which forms interstellar dust grains. Our resulting recipe
represents a significant revision in Galactic absorption compared to previous
standard methods, particularly at low Galactic latitudes.Comment: 12 pages, 12 figures, MNRAS in pres
Swift observations of the 2006 outburst of the recurrent nova RS Ophiuchi: I. Early X-ray emission from the shocked ejecta and red giant wind
RS Ophiuchi began its latest outburst on 2006 February 12. Previous outbursts
have indicated that high velocity ejecta interact with a pre-existing red giant
wind, setting up shock systems analogous to those seen in Supernova Remnants.
However, in the previous outburst in 1985, X-ray observations did not commence
until 55 days after the initial explosion. Here we report on Swift observations
covering the first month of the 2006 outburst with the Burst Alert (BAT) and
X-ray Telescope (XRT) instruments. RS Oph was clearly detected in the BAT 14-25
keV band from t=0 to days. XRT observationsfrom 0.3-10 keV, started at
3.17 days after outburst. The rapidly evolving XRT spectra clearly show the
presence of both line and continuum emission which can be fitted by thermal
emission from hot gas whose characteristic temperature, overlying absorbing
column, , and resulting unabsorbed total flux decline monotonically
after the first few days. Derived shock velocities are in good agreement with
those found from observations at other wavelengths. Similarly, is in
accord with that expected from the red giant wind ahead of the forward shock.
We confirm the basic models of the 1985 outburst and conclude that standard
Phase I remnant evolution terminated by days and the remnant then
rapidly evolved to display behaviour characteristic of Phase III. Around t=26
days however, a new, luminous and highly variable soft X-ray source began to
appear whose origin will be explored in a subsequent paper.Comment: 20 pages, 4 figures (2 updated), accepted by Ap
ORFEUS II Far-UV Spectroscopy of AM Herculis
Six high-resolution (\lambda/\Delta\lambda ~ 3000) far-UV (\lambda\lambda =
910-1210 \AA) spectra of the magnetic cataclysmic variable AM Herculis were
acquired in 1996 November during the flight of the ORFEUS-SPAS II mission. AM
Her was in a high optical state at the time of the observations, and the
spectra reveal emission lines of O VI \lambda\lambda 1032, 1038, C III \lambda
977, \lambda 1176, and He II \lambda 1085 superposed on a nearly flat
continuum. Continuum flux variations can be described as per Gansicke et al. by
a ~ 20 kK white dwarf with a ~ 37 kK hot spot covering a fraction f~0.15 of the
surface of the white dwarf, but we caution that the expected Lyman absorption
lines are not detected. The O VI emission lines have narrow and broad component
structure similar to that of the optical emission lines, with radial velocities
consistent with an origin in the irradiated face of the secondary and the
accretion funnel, respectively. The density of the narrow- and broad-line
regions is n_{nlr} ~ 3\times 10^{10} cm^{-3} and n_{blr} ~ 1\times 10^{12}
cm^{-3}, respectively, yet the narrow-line region is optically thick in the O
VI line and the broad-line region is optically thin; apparently, the velocity
shear in the broad-line region allows the O VI photons to escape, rendering the
gas effectively optically thin. Unexplained are the orbital phase variations of
the emission-line fluxes.Comment: 15 pages, 6 Postscript figures; LaTeX format, uses aaspp4.sty;
table2.tex included separately because it must be printed sideways - see
instructions in the file; accepted on April 17, 1998 for publication in The
Astrophysical Journa
An Efficient Molecular Dynamics Scheme for the Calculation of Dopant Profiles due to Ion Implantation
We present a highly efficient molecular dynamics scheme for calculating the
concentration depth profile of dopants in ion irradiated materials. The scheme
incorporates several methods for reducing the computational overhead, plus a
rare event algorithm that allows statistically reliable results to be obtained
over a range of several orders of magnitude in the dopant concentration.
We give examples of using this scheme for calculating concentration profiles
of dopants in crystalline silicon. Here we can predict the experimental profile
over five orders of magnitude for both channeling and non-channeling implants
at energies up to 100s of keV.
The scheme has advantages over binary collision approximation (BCA)
simulations, in that it does not rely on a large set of empirically fitted
parameters. Although our scheme has a greater computational overhead than the
BCA, it is far superior in the low ion energy regime, where the BCA scheme
becomes invalid.Comment: 17 pages, 21 figures, 2 tables. See: http://bifrost.lanl.gov/~reed
The accretion flow in the discless intermediate polar V2400 Ophiuchi
RXTE observations confirm that the X-ray lightcurve of V2400 Oph is pulsed at
the beat cycle, as expected in a discless intermediate polar. There are no
X-ray modulations at the orbital or spin cycles, but optical line profiles vary
with all three cycles. We construct a model for line-profile variations in a
discless accretor, based on the idea that the accretion stream flips from one
magnetic pole to the other, and show that this accounts for the observed
behaviour over the spin and beat cycles. The minimal variability over the
orbital cycle implies that 1) V2400 Oph is at an inclination of only ~10 deg,
and 2) much of the accretion flow is not in a coherent stream, but is circling
the white dwarf, possibly as a ring of denser, diamagnetic blobs. We discuss
the light this sheds on disc formation in intermediate polars.Comment: 10 pages, 12 figures, To appear in MNRAS, includes low-res figures to
reduce siz
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