9,896 research outputs found
A Survey for Transient Astronomical Radio Emission at 611 MHz
We have constructed and operated the Survey for Transient Astronomical Radio
Emission (STARE) to detect transient astronomical radio emission at 611 MHz
originating from the sky over the northeastern United States. The system is
sensitive to transient events on timescales of 0.125 s to a few minutes, with a
typical zenith flux density detection threshold of approximately 27 kJy. During
18 months of around-the-clock observing with three geographically separated
instruments, we detected a total of 4,318,486 radio bursts. 99.9% of these
events were rejected as locally generated interference, determined by requiring
the simultaneous observation of an event at all three sites for it to be
identified as having an astronomical origin. The remaining 3,898 events have
been found to be associated with 99 solar radio bursts. These results
demonstrate the remarkably effective RFI rejection achieved by a coincidence
technique using precision timing (such as GPS clocks) at geographically
separated sites. The non-detection of extra-solar bursting or flaring radio
sources has improved the flux density sensitivity and timescale sensitivity
limits set by several similar experiments in the 1970s. We discuss the
consequences of these limits for the immediate solar neighborhood and the
discovery of previously unknown classes of sources. We also discuss other
possible uses for the large collection of 611 MHz monitoring data assembled by
STARE.Comment: 24 pages, 6 figures; to appear in PAS
Galois cohomology of certain field extensions and the divisible case of Milnor-Kato conjecture
We prove the "divisible case" of the Milnor-Bloch-Kato conjecture (which is
the first step of Voevodsky's proof of this conjecture for arbitrary prime l)
in a rather clear and elementary way. Assuming this conjecture, we construct a
6-term exact sequence of Galois cohomology with cyclotomic coefficients for any
finite extension of fields whose Galois group has an exact quadruple of
permutational representations over it. Examples include cyclic groups, dihedral
groups, the biquadratic group Z/2\times Z/2, and the symmetric group S_4.
Several exact sequences conjectured by Bloch-Kato, Merkurjev-Tignol, and Kahn
are proven in this way. In addition, we introduce a more sophisticated version
of the classical argument known as "Bass-Tate lemma". Some results about
annihilator ideals in Milnor rings are deduced as corollaries.Comment: LaTeX 2e, 17 pages. V5: Updated to the published version + small
mistake corrected in Section 5. Submitted also to K-theory electronic
preprint archives at http://www.math.uiuc.edu/K-theory/0589
A High-Mass Protobinary System in the Hot Core W3(H2O)
We have observed a high-mass protobinary system in the hot core W3(H2O) with
the BIMA Array. Our continuum maps at wavelengths of 1.4mm and 2.8mm both
achieve sub-arcsecond angular resolutions and show a double-peaked morphology.
The angular separation of the two sources is 1.19" corresponding to 2.43X10^3
AU at the source distance of 2.04 kpc. The flux densities of the two sources at
1.4mm and 2.8mm have a spectral index of 3, translating to an opacity law of
kappa ~ nu. The small spectral indices suggest that grain growth has begun in
the hot core. We have also observed 5 K components of the CH3CN (12-11)
transitions. A radial velocity difference of 2.81 km/s is found towards the two
continuum peaks. Interpreting these two sources as binary components in orbit
about one another, we find a minimum mass of 22 Msun for the system. Radiative
transfer models are constructed to explain both the continuum and methyl
cyanide line observations of each source. Power-law distributions of both
density and temperature are derived. Density distributions close to the
free-fall value, r^-1.5, are found for both components, suggesting continuing
accretion. The derived luminosities suggest the two sources have equivalent
zero-age main sequence (ZAMS) spectral type B0.5 - B0. The nebular masses
derived from the continuum observations are about 5 Msun for source A and 4
Msun for source C. A velocity gradient previously detected may be explained by
unresolved binary rotation with a small velocity difference.Comment: 38 pages, 9 figures, accepted by The Astrophysical Journa
On the Largest Singular Values of Random Matrices with Independent Cauchy Entries
We apply the method of determinants to study the distribution of the largest
singular values of large real rectangular random matrices with
independent Cauchy entries. We show that statistical properties of the
(rescaled by a factor of \frac{1}{m^2\*n^2})largest singular values agree in
the limit with the statistics of the inhomogeneous Poisson random point process
with the intensity and, therefore, are different
from the Tracy-Widom law. Among other corollaries of our method we show an
interesting connection between the mathematical expectations of the
determinants of complex rectangular standard Wishart ensemble
and real rectangular standard Wishart ensemble.Comment: We have shown in the revised version that the statistics of the
largest eigenavlues of a sample covariance random matrix with i.i.d. Cauchy
entries agree in the limit with the statistics of the inhomogeneous Poisson
random point process with the intensity $\frac{1}{\pi} x^{-3/2}.
Electronic structure of the molecule based magnet Cu PM(NO3)2 (H2O)2
We present density functional calculations on the molecule based S=1/2
antiferromagnetic chain compound Cu PM(NO3)2 (H2O)2; PM = pyrimidine. The
properties of the ferro- and antiferromagnetic state are investigated at the
level of the local density approximation and with the hybrid functional B3LYP.
Spin density maps illustrate the exchange path via the pyrimidine molecule
which mediates the magnetism in the one-dimensional chain. The computed
exchange coupling is antiferromagnetic and in reasonable agreement with the
experiment. It is suggested that the antiferromagnetic coupling is due to the
possibility of stronger delocalization of the charges on the nitrogen atoms,
compared to the ferromagnetic case. In addition, computed isotropic and
anisotropic hyperfine interaction parameters are compared with recent NMR
experiments
Subtle competition between ferromagnetic and antiferromagnetic order in a Mn(II) - free radical ferrimagnetic chain
The macroscopic magnetic characterization of the Mn(II) - nitronyl nitroxide
free radical chain (Mn(hfac)2(R)-3MLNN) evidenced its transition from a
1-dimensional behavior of ferrimagnetic chains to a 3-dimensional ferromagnetic
long range order below 3 K. Neutron diffraction experiments, performed on a
single crystal around the transition temperature, led to a different conclusion
: the magnetic Bragg reflections detected below 3 K correspond to a canted
antiferromagnet where the magnetic moments are mainly oriented along the chain
axis. Surprisingly in the context of other compounds in this family of magnets,
the interchain coupling is antiferromagnetic. This state is shown to be very
fragile since a ferromagnetic interchain arrangement is recovered in a weak
magnetic field. This peculiar behavior might be explained by the competition
between dipolar interaction, shown to be responsible for the antiferromagnetic
long range order below 3 K, and exchange interaction, the balance between these
interactions being driven by the strong intrachain spin correlations. More
generally, this study underlines the need, in this kind of molecular compounds,
to go beyond macroscopic magnetization measurements.Comment: 12 pages, 10 figures, submitted to Phys. Rev.
The Strange and Spooky Battle over Bats and Black Dresses: The Commodification of Whitby Goth Weekend and the Loss of a Subculture
From counterculture to subculture to the ubiquity of every black-clad wannabe vampire hanging around the centre of Western cities, Goth has transcended a musical style to become a part of everyday leisure and popular culture. The musicâs cultural terrain has been extensively mapped in the first decade of this century. In this article, we examine the phenomenon of the Whitby Goth Weekend, a modern Goth music festival, which has contributed to (and has been altered by) the heritage-tourism marketing of Whitby as the holiday resort of Dracula (the place where Bram Stoker imagined the Vampire Count arriving one dark and stormy night). We examine marketing literature and websites that sell Whitby as a spooky town, and suggest that this strategy has driven the success of the Goth festival. We explore the development of the festival and the politics of its ownership, and its increasing visibility as a mainstream tourist destination for those who want to dress up for the weekend. By interviewing Goths from the north of England, we suggest that the mainstreaming of the festival has led to it becoming less attractive to those more established, older Goths who see the subcultureâs authenticity as being rooted in the post-punk era, and who believe that Goth subculture should be something one lives full-time
Hydrodynamical Models of Outflow Collimation in YSOs
We explore the physics of time-dependent hydrodynamic collimation of jets
from Young Stellar Objects (YSOs). Using parameters appropriate to YSOs we have
carried out high resolution hydrodynamic simulations modeling the interaction
of a central wind with an environment characterized by a moderate opening angle
toroidal density distribution. The results show that the the wind/environment
interaction produces strongly collimated supersonic jets. The jet is composed
of shocked wind gas. Using analytical models of wind blown bubble evolution we
show that the scenario studied here should be applicable to YSOs and can, in
principle, initiate collimation on the correct scales (R ~ 100 AU). The
simulations reveal a number of time-dependent non-linear features not
anticipated in previous analytical studies including: a prolate wind shock; a
chimney of cold swept-up ambient material dragged into the bubble cavity; a
plug of dense material between the jet and bow shocks. We find that the
collimation of the jet occurs through both de Laval nozzles and focusing of the
wind via the prolate wind shock. Using an analytical model for shock focusing
we demonstrate that a prolate wind shock can, by itself, produce highly
collimated supersonic jets.Comment: Accepted by ApJ, 31 pages with 12 figures (3 JPEG's) now included,
using aasms.sty, Also available in postscript via a gzipped tar file at
ftp://s1.msi.umn.edu/pub/afrank/SFIC1/SFIC.tar.g
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