439 research outputs found
Paleomagnetism of Late Jurassic Rocks in the Northern Canelo Hills, Southeastern Arizona
The Canelo Hills volcanics are exposed in the Canelo Hills, a northwest trending range in Santa Cruz County, southeast Arizona. The formation is composed of silicic tuffs and flows as well as volcaniclastic conglomerates and sandstones. Strikes of the rocks are generally to the northwest with moderate dips to the southwest and northeast. Apparent age results from the sequence studied paleomagnetically include two published isotopic dates of 147 ± 6 Ma (K-Ar, biotite) and 149 ± 11 Ma (whole rock, Rb-Sr) and a Rb/Sr isochron age, reported here, which indicates an age of 151 ± 2 Ma. Paleomagnetic samples were collected from 17 cooling units in the northern Canelo Hills. Samples from most of these units responded to alternating field (af) demagnetization, and secondary components were generally erased by peak af between 10 and 50 mT. Samples from five sites showed no response to af demagnetization. Thermal demagnetization of samples from these units produced no significant changes in direction of natural remanent magnetization (NRM), although within-site clustering of NRM directions was improved. Data from two sites were rejected because of failure to isolate a well-determined characteristic NRM. Of the remaining 15 sites, 10 sites were of normal polarity, while five sites showed reversed polarity. Intensities of the characteristic NRM ranged from 4 Ă 10â3 to 3 Ă 10â1 A/m. The data from these 15 cooling units yield a formation mean direction of I = 29.9°, D = 334.9° with k = 33.4 and α95 = 6.7°. The resulting paleomagnetic pole is at 62.2°N, 130.3° (dp = 4.1°, dm = 7.4°). This pole is between poles obtained from the Summerville and lower Morrison formations. The Canelo Hills pole is thus consistent both in position and age with the Late Jurassic episode of rapid apparent polar wander originally defined by paleomagnetic data from the Summerville and Morrison formations
The HST Key Project on the Extragalactic Distance Scale XVII. The Cepheid Distance to NGC 4725
The distance to NGC 4725 has been derived from Cepheid variables, as part of
the Hubble Space Telescope Key Project on the Extragalactic Distance Scale.
Thirteen F555W (V) and four F814W (I) epochs of cosmic-ray-split Wide Field and
Planetary Camera 2 observations were obtained. Twenty Cepheids were discovered,
with periods ranging from 12 to 49 days. Adopting a Large Magellanic Cloud
distance modulus and extinction of 18.50+/-0.10 mag and E(V-I)=0.13 mag,
respectively, a true reddening-corrected distance modulus (based on an analysis
employing the ALLFRAME software package) of 30.50 +/- 0.16 (random) +/- 0.17
(systematic) mag was determined for NGC 4725. The corresponding of distance of
12.6 +/- 1.0 (random) +/- 1.0 (systematic) Mpc is in excellent agreement with
that found with an independent analysis based upon the DoPHOT photometry
package. With a foreground reddening of only E(V-I)=0.02, the inferred
intrinsic reddening of this field in NGC 4725, E(V-I)=0.19, makes it one of the
most highly-reddened, encountered by the HST Key Project, to date.Comment: To be published in The Astrophysical Journal, Vol. 512 (1999). 34
pages, LaTeX, 9 jpg figure
Search for Early Gamma-ray Production in Supernovae Located in a Dense Circumstellar Medium with the Fermi LAT
Supernovae (SNe) exploding in a dense circumstellar medium (CSM) are
hypothesized to accelerate cosmic rays in collisionless shocks and emit GeV
gamma rays and TeV neutrinos on a time scale of several months. We perform the
first systematic search for gamma-ray emission in Fermi LAT data in the energy
range from 100 MeV to 300 GeV from the ensemble of 147 SNe Type IIn exploding
in dense CSM. We search for a gamma-ray excess at each SNe location in a one
year time window. In order to enhance a possible weak signal, we simultaneously
study the closest and optically brightest sources of our sample in a
joint-likelihood analysis in three different time windows (1 year, 6 months and
3 months). For the most promising source of the sample, SN 2010jl (PTF10aaxf),
we repeat the analysis with an extended time window lasting 4.5 years. We do
not find a significant excess in gamma rays for any individual source nor for
the combined sources and provide model-independent flux upper limits for both
cases. In addition, we derive limits on the gamma-ray luminosity and the ratio
of gamma-ray-to-optical luminosity ratio as a function of the index of the
proton injection spectrum assuming a generic gamma-ray production model.
Furthermore, we present detailed flux predictions based on multi-wavelength
observations and the corresponding flux upper limit at 95% confidence level
(CL) for the source SN 2010jl (PTF10aaxf).Comment: Accepted for publication in ApJ. Corresponding author: A. Franckowiak
([email protected]), updated author list and acknowledgement
Constraints on the Cosmic-Ray Density Gradient beyond the Solar Circle from Fermi gamma-ray Observations of the Third Galactic Quadrant
We report an analysis of the interstellar -ray emission in the third
Galactic quadrant measured by the {Fermi} Large Area Telescope. The window
encompassing the Galactic plane from longitude 210\arcdeg to 250\arcdeg has
kinematically well-defined segments of the Local and the Perseus arms, suitable
to study the cosmic-ray densities across the outer Galaxy. We measure no large
gradient with Galactocentric distance of the -ray emissivities per
interstellar H atom over the regions sampled in this study. The gradient
depends, however, on the optical depth correction applied to derive the \HI\
column densities. No significant variations are found in the interstellar
spectra in the outer Galaxy, indicating similar shapes of the cosmic-ray
spectrum up to the Perseus arm for particles with GeV to tens of GeV energies.
The emissivity as a function of Galactocentric radius does not show a large
enhancement in the spiral arms with respect to the interarm region. The
measured emissivity gradient is flatter than expectations based on a cosmic-ray
propagation model using the radial distribution of supernova remnants and
uniform diffusion properties. In this context, observations require a larger
halo size and/or a flatter CR source distribution than usually assumed. The
molecular mass calibrating ratio, , is
found to be
in the Local-arm clouds and is not significantly sensitive to the choice of
\HI\ spin temperature. No significant variations are found for clouds in the
interarm region.Comment: Corresponding authors: I. A. Grenier ([email protected]); T.
Mizuno ([email protected]); L. Tibaldo
([email protected]) accepted for publication in Ap
Fermi Large Area Telescope Measurements of the Diffuse Gamma-Ray Emission at Intermediate Galactic Latitudes
The diffuse Galactic gamma-ray emission is produced by cosmic rays (CRs)
interacting with the interstellar gas and radiation field. Measurements by the
Energetic Gamma-Ray Experiment Telescope (EGRET) instrument on the Compton
Gamma-Ray Observatory indicated excess gamma-ray emission > 1 GeV relative to
diffuse Galactic gamma-ray emission models consistent with directly measured CR
spectra (the so-called ``EGRET GeV excess''). The excess emission was observed
in all directions on the sky, and a variety of explanations have been proposed,
including beyond-the-Standard-Model scenarios like annihilating or decaying
dark matter. The Large Area Telescope (LAT) instrument on the Fermi Gamma-ray
Space Telescope has measured the diffuse gamma-ray emission with improved
sensitivity and resolution compared to EGRET. We report on LAT measurements of
the diffuse gamma-ray emission for energies 100 MeV to 10 GeV and Galactic
latitudes 10 deg. <= |b| <= 20 deg. The LAT spectrum for this region of the sky
is well reproduced by a diffuse Galactic gamma-ray emission model that is
consistent with local CR spectra and inconsistent with the EGRET GeV excess.Comment: 2 figures, 1 table, accepted by Physical Review Letters, available
online Dec. 18th, 200
Fermi LAT observations of the Geminga pulsar
We report on the \textit{Fermi}-LAT observations of the Geminga pulsar, the
second brightest non-variable GeV source in the -ray sky and the first
example of a radio-quiet -ray pulsar. The observations cover one year,
from the launch of the satellite through 2009 June 15. A data sample of
over 60,000 photons enabled us to build a timing solution based solely on
rays. Timing analysis shows two prominent peaks, separated by = 0.497 0.004 in phase, which narrow with increasing energy. Pulsed
rays are observed beyond 18 GeV, precluding emission below 2.7 stellar
radii because of magnetic absorption. The phase-averaged spectrum was fitted
with a power law with exponential cut-off of spectral index = (1.30
0.01 0.04), cut-off energy = (2.46 0.04 0.17)
GeV and an integral photon flux above 0.1 GeV of (4.14 0.02 0.32)
10 cm s. The first uncertainties are statistical
and the second are systematic. The phase-resolved spectroscopy shows a clear
evolution of the spectral parameters, with the spectral index reaching a
minimum value just before the leading peak and the cut-off energy having maxima
around the peaks. Phase-resolved spectroscopy reveals that pulsar emission is
present at all rotational phases. The spectral shape, broad pulse profile, and
maximum photon energy favor the outer magnetospheric emission scenarios.Comment: 32 pages, 12 figures, 3 tables. Accepted for publication in The
Astrophysical Journal. Corresponding authors: Denis Dumora
([email protected]), Fabio Gargano ([email protected]),
Massimiliano Razzano ([email protected]
Gamma-ray flares from the Crab Nebula
A young and energetic pulsar powers the well-known Crab Nebula. Here we
describe two separate gamma-ray (photon energy >100 MeV) flares from this
source detected by the Large Area Telescope on board the Fermi Gamma-ray Space
Telescope. The first flare occurred in February 2009 and lasted approximately
16 days. The second flare was detected in September 2010 and lasted
approximately 4 days. During these outbursts the gamma-ray flux from the nebula
increased by factors of four and six, respectively. The brevity of the flares
implies that the gamma rays were emitted via synchrotron radiation from PeV
(10^15 eV) electrons in a region smaller than 1.4 10^-2 pc. These are the
highest energy particles that can be associated with a discrete astronomical
source, and they pose challenges to particle acceleration theory.Comment: Contact authors: Rolf Buehler,[email protected]; Stefan
Funk,[email protected]; Roger Blandford,rdb3@stanford ; 16 pages,2
figure
Fermi LAT observations of cosmic-ray electrons from 7 GeV to 1 TeV
We present the results of our analysis of cosmic-ray electrons using about 8
million electron candidates detected in the first 12 months on-orbit by the
Fermi Large Area Telescope. This work extends our previously-published
cosmic-ray electron spectrum down to 7 GeV, giving a spectral range of
approximately 2.5 decades up to 1 TeV. We describe in detail the analysis and
its validation using beam-test and on-orbit data. In addition, we describe the
spectrum measured via a subset of events selected for the best energy
resolution as a cross-check on the measurement using the full event sample. Our
electron spectrum can be described with a power law with no prominent spectral features within systematic uncertainties.
Within the limits of our uncertainties, we can accommodate a slight spectral
hardening at around 100 GeV and a slight softening above 500 GeV.Comment: 20 pages, 23 figures, 2 tables, published in Physical Review D 82,
092004 (2010) - contact authors: C. Sgro', A. Moisee
Early Fermi Gamma-ray Space Telescope Observations of the Quasar 3C 454.3
This is the first report of Fermi Gamma-ray Space Telescope observations of
the quasar 3C 454.3, which has been undergoing pronounced long-term outbursts
since 2000. The data from the Large Area Telescope (LAT), covering 2008 July 7
- October 6, indicate strong, highly variable gamma-ray emission with an
average flux of ~3 x 10^{-6} photons cm^{-2} s^{-1}, for energies above 100
MeV. The gamma-ray flux is variable, with strong, distinct,
symmetrically-shaped flares for which the flux increases by a factor of several
on a time scale of about three days. This variability indicates a compact
emission region, and the requirement that the source is optically thin to
pair-production implies relativistic beaming with Doppler factor delta > 8,
consistent with the values inferred from VLBI observations of superluminal
expansion (delta ~ 25). The observed gamma-ray spectrum is not consistent with
a simple power-law, but instead steepens strongly above ~2 GeV, and is well
described by a broken power-law with photon indices of ~2.3 and ~3.5 below and
above the break, respectively. This is the first direct observation of a break
in the spectrum of a high luminosity blazar above 100 MeV, and it is likely
direct evidence for an intrinsic break in the energy distribution of the
radiating particles. Alternatively, the spectral softening above 2 GeV could be
due to gamma-ray absorption via photon-photon pair production on the soft X-ray
photon field of the host AGN, but such an interpretation would require the
dissipation region to be located very close (less than 100 gravitational radii)
to the black hole, which would be inconsistent with the X-ray spectrum of the
source.Comment: Accepted by the Astrophysical Journal; corresponding authors: Greg
Madejski ([email protected]) and Benoit Lott ([email protected]
Fermi Observations of the Very Hard Gamma-ray Blazar PG 1553+113
We report the observations of PG 1553+113 during the first ~200 days of Fermi
Gamma-ray Space Telescope science operations, from 4 August 2008 to 22 February
2009 (MJD 54682.7-54884.2). This is the first detailed study of PG 1553+113 in
the GeV gamma-ray regime and it allows us to fill a gap of three decades in
energy in its spectral energy distribution. We find PG 1553+113 to be a steady
source with a hard spectrum that is best fit by a simple power-law in the Fermi
energy band. We combine the Fermi data with archival radio, optical, X-ray and
very high energy (VHE) gamma-ray data to model its broadband spectral energy
distribution and find that a simple, one-zone synchrotron self-Compton model
provides a reasonable fit. PG 1553+113 has the softest VHE spectrum of all
sources detected in that regime and, out of those with significant detections
across the Fermi energy bandpass so far, the hardest spectrum in that energy
regime. Thus, it has the largest spectral break of any gamma-ray source studied
to date, which could be due to the absorption of the intrinsic gamma-ray
spectrum by the extragalactic background light (EBL). Assuming this to be the
case, we selected a model with a low level of EBL and used it to absorb the
power-law spectrum from PG 1553+113 measured with Fermi (200 MeV - 157 GeV) to
find the redshift which gave the best fit to the measured VHE data (90 GeV -
1.1 TeV) for this parameterisation of the EBL. We show that this redshift can
be considered an upper limit on the distance to PG 1553+113.Comment: Accepted for publication in the Astrophysical Journal (28 pages, 5
figures
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