591 research outputs found
Measurement of the Cosmic Ray Energy Spectrum and Composition from 10^{17} to 10^{18.3} eV Using a Hybrid Fluorescence Technique
We study the spectrum and average mass composition of cosmic rays with
primary energies between 10^{17} eV and 10^{18} eV using a hybrid detector
consisting of the High Resolution Fly's Eye (HiRes) prototype and the MIA muon
array. Measurements have been made of the change in the depth of shower maximum
as a function of energy. A complete Monte Carlo simulation of the detector
response and comparisons with shower simulations leads to the conclusion that
the cosmic ray intensity is changing f rom a heavier to a lighter composition
in this energy range. The spectrum is consistent with earlier Fly's Eye
measurements and supports the previously found steepening near 4 \times 10^{17}
eV .Comment: 39 pages, 15 figures, in revtex4 epsf style, submited to AP
Constraints on Gamma-ray Emission from the Galactic Plane at 300 TeV
We describe a new search for diffuse ultrahigh energy gamma-ray emission
associated with molecular clouds in the galactic disk. The Chicago Air Shower
Array (CASA), operating in coincidence with the Michigan muon array (MIA), has
recorded over 2.2 x 10^{9} air showers from April 4, 1990 to October 7, 1995.
We search for gamma rays based upon the muon content of air showers arriving
from the direction of the galactic plane. We find no significant evidence for
diffuse gamma-ray emission, and we set an upper limit on the ratio of gamma
rays to normal hadronic cosmic rays at less than 2.4 x 10^{-5} at 310 TeV (90%
confidence limit) from the galactic plane region: (50 degrees < l < 200
degrees); -5 degrees < b < 5 degrees). This limit places a strong constraint on
models for emission from molecular clouds in the galaxy. We rule out
significant spectral hardening in the outer galaxy, and conclude that emission
from the plane at these energies is likely to be dominated by the decay of
neutral pions resulting from cosmic rays interactions with passive target gas
molecules.Comment: Astrophysical Journal, submitted, 11 pages, AASTeX Latex, 3
Postscript figure
Galaxy Zoo: evidence for diverse star formation histories through the green valley
Does galaxy evolution proceed through the green valley via multiple pathways or as a single population? Motivated by recent results highlighting radically different evolutionary pathways between early- and late-type galaxies, we present results from a simple Bayesian approach to this problem wherein we model the star formation history (SFH) of a galaxy with two parameters, [t, Ï] and compare the predicted and observed optical and near-ultraviolet colours. We use a novel method to investigate the morphological differences between the most probable SFHs for both disc-like and smooth-like populations of galaxies, by using a sample of 126â316 galaxies (0.01 < z < 0.25) with probabilistic estimates of morphology from Galaxy Zoo. We find a clear difference between the quenching time-scales preferred by smooth- and disc-like galaxies, with three possible routes through the green valley dominated by smooth- (rapid time-scales, attributed to major mergers), intermediate- (intermediate time-scales, attributed to minor mergers and galaxy interactions) and disc-like (slow time-scales, attributed to secular evolution) galaxies. We hypothesize that morphological changes occur in systems which have undergone quenching with an exponential time-scale Ï < 1.5âGyr, in order for the evolution of galaxies in the green valley to match the ratio of smooth to disc galaxies observed in the red sequence. These rapid time-scales are instrumental in the formation of the red sequence at earlier times; however, we find that galaxies currently passing through the green valley typically do so at intermediate time-scales.
Possibility of an ultra-precise optical clock using the transition in Yb atoms held in an optical lattice
We report calculations designed to assess the ultimate precision of an atomic
clock based on the 578 nm transition in Yb atoms
confined in an optical lattice trap. We find that this transition has a natural
linewidth less than 10 mHz in the odd Yb isotopes, caused by hyperfine
coupling. The shift in this transition due to the trapping light acting through
the lowest order AC polarizability is found to become zero at the magic trap
wavelength of about 752 nm. The effects of Rayleigh scattering, higher-order
polarizabilities, vector polarizability, and hyperfine induced electronic
magnetic moments can all be held below a mHz (about a part in 10^{18}), except
in the case of the hyperpolarizability larger shifts due to nearly resonant
terms cannot be ruled out without an accurate measurement of the magic
wavelength.Comment: 4 pages, 1 figur
A Multi-Component Measurement of the Cosmic Ray Composition Between 10^{17} eV and 10^{18} eV
The average mass composition of cosmic rays with primary energies between
eV and eV has been studied using a hybrid detector consisting
of the High Resolution Fly's Eye (HiRes) prototype and the MIA muon array.
Measurements have been made of the change in the depth of shower maximum,
, and in the change in the muon density at a fixed core location,
, as a function of energy. The composition has also been
evaluated in terms of the combination of and . The
results show that the composition is changing from a heavy to lighter mix as
the energy increases.Comment: 14 pages including 3 figures in revtex epsf style, submited to PR
A new limit on the permanent electric dipole moment of ^{199}Hg
We present the first results of a new search for a permanent electric dipole
moment of the ^{199}Hg atom using a UV laser. Our measurements give d(Hg)= -
(1.06 +/- 0.49 +/- 0.40) 10^{-28} e cm. We interpret the result as an upper
limit |d(Hg)| < 2.1 10^{-28} e cm (95% C.L.), which sets new constraints on
theta_{QCD}, chromo-EDMs of the quarks, and CP violation in Supersymmetric
models.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
The Milky Way Project: A statistical study of massive star formation associated with infrared bubbles
The Milky Way Project citizen science initiative recently increased the
number of known infrared bubbles in the inner Galactic plane by an order of
magnitude compared to previous studies. We present a detailed statistical
analysis of this dataset with the Red MSX Source catalog of massive young
stellar sources to investigate the association of these bubbles with massive
star formation. We particularly address the question of massive triggered star
formation near infrared bubbles. We find a strong positional correlation of
massive young stellar objects (MYSOs) and H II regions with Milky Way Project
bubbles at separations of < 2 bubble radii. As bubble sizes increase, a
statistically significant overdensity of massive young sources emerges in the
region of the bubble rims, possibly indicating the occurrence of triggered star
formation. Based on numbers of bubble-associated RMS sources we find that
67+/-3% of MYSOs and (ultra)compact H II regions appear associated with a
bubble. We estimate that approximately 22+/-2% of massive young stars may have
formed as a result of feedback from expanding H II regions. Using MYSO-bubble
correlations, we serendipitously recovered the location of the recently
discovered massive cluster Mercer 81, suggesting the potential of such analyses
for discovery of heavily extincted distant clusters.Comment: 16 pages, 17 figures. Accepted for publication in ApJ, comments
welcome. Milky Way Project public data release available at
http://www.milkywayproject.org/dat
The neutron electric dipole form factor in the perturbative chiral quark model
We calculate the electric dipole form factor of the neutron in a perturbative
chiral quark model, parameterizing CP-violation of generic origin by means of
effective electric dipole moments of the constituent quarks and their
CP-violating couplings to the chiral fields. We discuss the relation of these
effective parameters to more fundamental ones such as the intrinsic electric
and chromoelectric dipole moments of quarks and the Weinberg parameter. From
the existing experimental upper limits on the neutron EDM we derive constraints
on these CP-violating parameters.Comment: 20 pages, 3 figure
Galaxy Zoo Supernovae
This paper presents the first results from a new citizen science project:
Galaxy Zoo Supernovae. This proof of concept project uses members of the public
to identify supernova candidates from the latest generation of wide-field
imaging transient surveys. We describe the Galaxy Zoo Supernovae operations and
scoring model, and demonstrate the effectiveness of this novel method using
imaging data and transients from the Palomar Transient Factory (PTF). We
examine the results collected over the period April-July 2010, during which
nearly 14,000 supernova candidates from PTF were classified by more than 2,500
individuals within a few hours of data collection. We compare the transients
selected by the citizen scientists to those identified by experienced PTF
scanners, and find the agreement to be remarkable - Galaxy Zoo Supernovae
performs comparably to the PTF scanners, and identified as transients 93% of
the ~130 spectroscopically confirmed SNe that PTF located during the trial
period (with no false positive identifications). Further analysis shows that
only a small fraction of the lowest signal-to-noise SN detections (r > 19.5)
are given low scores: Galaxy Zoo Supernovae correctly identifies all SNe with >
8{\sigma} detections in the PTF imaging data. The Galaxy Zoo Supernovae project
has direct applicability to future transient searches such as the Large
Synoptic Survey Telescope, by both rapidly identifying candidate transient
events, and via the training and improvement of existing machine classifier
algorithms.Comment: 13 pages, 10 figures, accepted MNRA
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