5,752 research outputs found
Twenty-one centimeter tomography with foregrounds
Twenty-one centimeter tomography is emerging as a powerful tool to explore
the end of the cosmic dark ages and the reionization epoch, but it will only be
as good as our ability to accurately model and remove astrophysical foreground
contamination. Previous treatments of this problem have focused on the angular
structure of the signal and foregrounds and what can be achieved with limited
spectral resolution (bandwidths in the 1 MHz range). In this paper we introduce
and evaluate a ``blind'' method to extract the multifrequency 21cm signal by
taking advantage of the smooth frequency structure of the Galactic and
extragalactic foregrounds. We find that 21 cm tomography is typically limited
by foregrounds on scales Mpc and limited by noise on scales Mpc, provided that the experimental bandwidth can be made substantially
smaller than 0.1 MHz. Our results show that this approach is quite promising
even for scenarios with rather extreme contamination from point sources and
diffuse Galactic emission, which bodes well for upcoming experiments such as
LOFAR, MWA, PAST, and SKA.Comment: 10 pages, 6 figures. Revised version including various cases with
high noise level. Major conclusions unchanged. Accepted for publication in
Ap
The First Reported Infrared Emission from the SN 1006 Remnant
We report results of infrared imaging and spectroscopic observations of the
SN 1006 remnant, carried out with the Spitzer Space Telescope. The 24 micron
image from MIPS clearly shows faint filamentary emission along the northwest
rim of the remnant shell, nearly coincident with the Balmer filaments that
delineate the present position of the expanding shock. The 24 micron emission
traces the Balmer filaments almost perfectly, but lies a few arcsec within,
indicating an origin in interstellar dust heated by the shock. Subsequent
decline in the IR behind the shock is presumably due largely to grain
destruction through sputtering. The emission drops far more rapidly than
current models predict, however, even for a higher proportion of small grains
than would be found closer to the Galactic plane. The rapid drop may result in
part from a grain density that has always been lower -- a relic effect from an
earlier epoch when the shock was encountering a lower density -- but higher
grain destruction rates still seem to be required. Spectra from three positions
along the NW filament from the IRS instrument all show only a featureless
continuum, consistent with thermal emission from warm dust. The dust-to-gas
mass ratio in the pre-shock interstellar medium is lower than that expected for
the Galactic ISM -- as has also been observed in the analysis of IR emission
from other SNRs but whose cause remains unclear. As with other SN Ia remnants,
SN 1006 shows no evidence for dust grain formation in the supernova ejecta.Comment: 24 pages, 6 figure
COMPASS: a 2.6m telescope for CMBR polarization studies
COMPASS (COsmic Microwave Polarization at Small Scale) is an experiment devoted to measuring the polarization of the CMBR. Its design and characteristics are presented
On the jets, kinks, and spheromaks formed by a planar magnetized coaxial gun
Measurements of the various plasma configurations produced by a planar
magnetized coaxial gun provide insight into the magnetic topology evolution
resulting from magnetic helicity injection. Important features of the
experiments are a very simple coaxial gun design so that all observed
geometrical complexity is due to the intrinsic physical dynamics rather than
the source shape and use of a fast multiple-frame digital camera which provides
direct imaging of topologically complex shapes and dynamics. Three key
experimental findings were obtained: (1) formation of an axial collimated jet
[Hsu and Bellan, Mon. Not. R. Astron. Soc. 334, 257 (2002)] that is consistent
with a magnetohydrodynamic description of astrophysical jets, (2)
identification of the kink instability when this jet satisfies the
Kruskal-Shafranov limit, and (3) the nonlinear properties of the kink
instability providing a conversion of toroidal to poloidal flux as required for
spheromak formation by a coaxial magnetized source [Hsu and Bellan, Phys. Rev.
Lett. 90, 215002 (2003)]. A new interpretation is proposed for how the n=1
central column instability provides flux amplification during spheromak
formation and sustainment, and it is shown that jet collimation can occur
within one rotation of the background poloidal field.Comment: Physics of Plasmas (accepted
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