1,130 research outputs found
Air-core photonic band-gap fibers: the impact of surface modes
We study the dispersion and leakage properties for the recently reported
low-loss photonic band-gap fiber by Smith et al. [Nature 424, 657 (2003)]. We
find that surface modes have a significant impact on both the dispersion and
leakage properties of the fundamental mode. Our dispersion results are in
qualitative agreement with the dispersion profile reported recently by Ouzounov
et al. [Science 301, 1702 (2003)] though our results suggest that the observed
long-wavelength anomalous dispersion is due to an avoided crossing (with
surface modes) rather than band-bending caused by the photonic band-gap
boundary of the cladding.Comment: 7 pages including 4 figures. Accepted for Optics Expres
Probing Elemental Abundances in SNR 1987A Using XMM-Newton
We report on the latest (2007 Jan) observations of supernova remnant (SNR)
1987A from the XMM-Newton mission. Since the 2003 May observations of Haberl et
al. (2006), 11 emission lines have experienced increases in flux by factors ~ 3
to 10, with the 775 eV line of O VIII showing the greatest increase; we have
observed 6 lines of Fe XVII and Fe XVIII previously unreported by XMM-Newton. A
two-shock model representing plasmas in non-equilibrium ionization is fitted to
the EPIC-pn spectra, yielding temperatures of ~ 0.4 and ~ 3 keV, as well as
elemental abundances for N, O, Ne, Mg, Si, S and Fe. We demonstrate that the
abundance ratio of N and O can be constrained to less than ~20% accuracy.
Within the same confidence interval, the same analysis suggests that the C+N+O
abundance varies from ~ 1.1 to 1.4 X 10^-4. Normalizing our obtained abundances
by the Large Magellanic Cloud (LMC) values of Hughes, Hayashi & Koyama (1998),
we find that O, Ne, Mg and Fe are under-abundant, while Si and S are
over-abundant, consistent with the findings of Aschenbach (2007). Such a result
has implications for both the single-star and binary accretion/merger models
for the progenitor of SNR 1987A. In the context of the binary merger scenario
proposed by Morris & Podsiadlowski (2006, 2007), material forming the inner,
equatorial ring was expelled after the merger, implying that either our derived
Fe abundance is inconsistent with typical LMC values or that iron is
under-abundant at the site of the progenitor star of SNR 1987A.Comment: 14 pages, 10 diagrams (2 omitted). Accepted by Ap
A Chandra View of The Morphological And Spectral Evolution of Supernova Remnant 1987A
We present an update on the results of our monitoring observations of the
X-ray remnant of supernova (SN) 1987A with the {\it Chandra X-Ray Observatory}.
As of 2002 December, we have performed a total of seven observations of SN
1987A. The high angular resolution images from the latest data reveal
developments of new X-ray bright spots in the northwestern and the southwestern
portions of the remnant as well as changes on the eastern side. The latest
0.5-2 keV band flux ( 6 10 ergs cm
s) is four times brighter than three years earlier. The overall X-ray
emission is primarily from the blast wave shock with 2.4 keV. As
the blast wave approaches the dense circumstellar material, the contribution
from the decelerated slow shock ( 0.22 keV) to the observed X-ray
emission is becoming significant. The increase of this slow shock contribution
over the last two years is particularly noticeable in the western half of the
remnant. These results indicate that the shock front is now reaching the main
body of the inner circumstellar ring. Based on the best-fit two-shock spectral
model, we derive approximate densities of the X-ray-emitting regions (
235 cm for the fast shock and 7500 cm for the
slow shock). We obtain an upper limit on the observed X-ray luminosity of any
embedded point source ( 1.5 10 ergs s) in the
210 keV band. The X-ray remnant continues to expand linearly at a rate of
4167 km s.Comment: 22 pages (ApJ preprint style), 7 Figures, Accepted by ApJ (scheduled
on July 20, 2004), for high-quality Fig 1 and Fig 2, please contact
[email protected]
Improved large-mode area endlessly single-mode photonic crystal fibers
We numerically study the possibilities for improved large-mode area endlessly
single mode photonic crystal fibers for use in high-power delivery
applications. By carefully choosing the optimal hole diameter we find that a
triangular core formed by three missing neighboring air holes considerably
improves the mode area and loss properties compared to the case with a core
formed by one missing air hole. In a realized fiber we demonstrate an
enhancement of the mode area by ~30 % without a corresponding increase in the
attenuation.Comment: 3 pages including 3 eps-figures. Accepted for Optics Letter
Recommended from our members
Elimination of spurious modes in full-vectorial finite element method based acoustic modal solution
Finite element method is a powerful technique for solving a wide range of engineering problems. However, the existence of the spurious solutions in full-vectorial finite element method has been a major problem for both acoustic and optic modal analyses. For emerging photonic devices exploiting light-sound interactions in high index contrast waveguides, this problem is a major limitation. A penalty function is introduced to remove these unwanted spurious modes in acoustic waveguides, which also identifies the acoustic modes more easily. Numerically simulated results also show considerably improved vector mode profiles. The proposed penalty method has been applied for the characterization of low index contrast single mode fiber and also for high index contrast silicon nanowire to demonstrate its effectiveness
Signal for supernova and neutrinos in water \v{C}erenkov detectors
We suggest that photons with energies between 5 and 10 MeV, generated by the
() and () reactions on O, constitute a
signal which allows a unique identification of supernova and
neutrinos in water \v{C}erenkov detectors. We calculate the yield of
such events and estimate that a few hundred of them would be detected
in Superkamiokande for a supernova at 10 kpc distance.Comment: 8 pages, RevTex 3.0, figures and text available at
http://www.krl.caltech.edu/preprints/MAP.htm
Do Hadronic Charge Exchange Reactions Measure Electroweak L = 1 Strength?
An eikonal model has been used to assess the relationship between calculated
strengths for first forbidden beta decay and calculated cross sections for
(p,n) charge exchange reactions. It is found that these are proportional for
strong transitions, suggesting that hadronic charge exchange reactions may be
useful in determining the spin-dipole matrix elements for astrophysically
interesting leptonic transitions.Comment: 14 pages, 5 figures, Submitted to Physical Review
Novel diffusion mechanism on the GaAs(001) surface: the role of adatom-dimer interaction
Employing first principles total energy calculations we have studied the
behavior of Ga and Al adatoms on the GaAs(001)-beta2 surface. The adsorption
site and two relevant diffusion channels are identified. The channels are
characterized by different adatom-surface dimer interaction. Both affect in a
novel way the adatom migration: in one channel the diffusing adatom jumps
across the surface dimers and leaves the dimer bonds intact, in the other one
the surface dimer bonds are broken. The two channels are taken into account to
derive effective adatom diffusion barriers. From the diffusion barriers we
conclude a strong diffusion anisotropy for both Al and Ga adatoms with the
direction of fastest diffusion parallel to the surface dimers. In agreement
with experimental observations we find higher diffusion barriers for Al than
for Ga.Comment: 4 pages, 2 figures, Phys. Rev. Lett. 79 (1997). Other related
publications can be found at http://www.rz-berlin.mpg.de/th/paper.htm
Supernova Neutrinos and the LSND Evidence for Neutrino Oscillations
The observation of the energy spectrum from a supernova burst
can provide constraints on neutrino oscillations. We derive formulas for
adiabatic oscillations of supernova antineutrinos for a variety of 3- and
4-neutrino mixing schemes and mass hierarchies which are consistent with the
LSND evidence for oscillations.
Finally, we explore the constraints on these models and LSND given by the
supernova SN1987A 's observed by the Kamiokande-2 and IMB-3
detectors.Comment: 8 pages, 3 figures. Changes with respect to original version:
appendix added; minor changes in text, figures, reference
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