4,571 research outputs found
Coupling virtual watersheds with ecosystem services assessment: A 21st century platform to support river research and management
The demand for freshwater is projected to increase worldwide over the coming decades, resulting in severe water stress and threats to riverine biodiversity, ecosystem functioning and services. A major societal challenge is to determine where environmental changes will have the greatest impacts on riverine ecosystem services and where resilience can be incorporated into adaptive resource planning. Both water managers and scientists need new integrative tools to guide them towards the best solutions that meet the demands of a growing human population but also ensure riverine biodiversity and ecosystem integrity. Resource planners and scientists could better address a growing set of riverine management and risk mitigation issues by (1) using a âVirtual Watershedsâ approach based on improved digital river networks and better connections to terrestrial systems; (2) integrating Virtual Watersheds with ecosystem services technology (ARtificial Intelligence for Ecosystem Services: ARIES), and (3) incorporating the role of riverine biotic interactions in shaping ecological responses. This integrative platform can support both interdisciplinary scientific analyses of pressing societal issues and effective dissemination of findings across river research and management communities. It should also provide new integrative tools to identify the best solutions and trade-offs to ensure the conservation of riverine biodiversity and ecosystem services
Fermi observations of TeV-selected AGN
We report on observations of TeV-selected AGN made during the first 5.5
months of observations with the Large Area Telescope (LAT) on-board the Fermi
Gamma-ray Space Telescope (Fermi). In total, 96 AGN were selected for study,
each being either (i) a source detected at TeV energies (28 sources) or (ii) an
object that has been studied with TeV instruments and for which an upper-limit
has been reported (68 objects). The Fermi observations show clear detections of
38 of these TeV-selected objects, of which 21 are joint GeV-TeV sources and 29
were not in the third EGRET catalog. For each of the 38 Fermi-detected sources,
spectra and light curves are presented. Most can be described with a power law
of spectral index harder than 2.0, with a spectral break generally required to
accommodate the TeV measurements. Based on an extrapolation of the Fermi
spectrum, we identify sources, not previously detected at TeV energies, which
are promising targets for TeV instruments. Evidence for systematic evolution of
the -ray spectrum with redshift is presented and discussed in the
context of interaction with the EBL.Comment: 51 pages, 6 figures, accepted for The Astronomical Journa
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]
Detection of 16 Gamma-Ray Pulsars Through Blind Frequency Searches Using the Fermi LAT
Pulsars are rapidly-rotating, highly-magnetized neutron stars emitting
radiation across the electromagnetic spectrum. Although there are more than
1800 known radio pulsars, until recently, only seven were observed to pulse in
gamma rays and these were all discovered at other wavelengths. The Fermi Large
Area Telescope makes it possible to pinpoint neutron stars through their
gamma-ray pulsations. We report the detection of 16 gamma-ray pulsars in blind
frequency searches using the LAT. Most of these pulsars are coincident with
previously unidentified gamma-ray sources, and many are associated with
supernova remnants. Direct detection of gamma-ray pulsars enables studies of
emission mechanisms, population statistics and the energetics of pulsar wind
nebulae and supernova remnants.Comment: Corresponding authors: Michael Dormody, Paul S. Ray, Pablo M. Saz
Parkinson, Marcus Ziegle
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
Bright AGN Source List from the First Three Months of the Fermi Large Area Telescope All-Sky Survey
The first three months of sky-survey operation with the Fermi Gamma Ray Space
Telescope (Fermi) Large Area Telescope (LAT) reveals 132 bright sources at
|b|>10 deg with test statistic greater than 100 (corresponding to about 10
sigma). Two methods, based on the CGRaBS, CRATES and BZCat catalogs, indicate
high-confidence associations of 106 of these sources with known AGNs. This
sample is referred to as the LAT Bright AGN Sample (LBAS). It contains two
radio galaxies, namely Centaurus A and NGC 1275, and 104 blazars consisting of
57 flat spectrum radio quasars (FSRQs), 42 BL Lac objects, and 5 blazars with
uncertain classification. Four new blazars were discovered on the basis of the
LAT detections. Remarkably, the LBAS includes 10 high-energy peaked BL Lacs
(HBLs), sources which were so far hard to detect in the GeV range. Another 10
lower-confidence associations are found. Only thirty three of the sources, plus
two at |b|>10 deg, were previously detected with EGRET, probably due to the
variable nature of these sources. The analysis of the gamma-ray properties of
the LBAS sources reveals that the average GeV spectra of BL Lac objects are
significantly harder than the spectra of FSRQs. No significant correlation
between radio and peak gamma-ray fluxes is observed. Blazar log N - log S and
luminosity functions are constructed to investigate the evolution of the
different blazar classes, with positive evolution indicated for FSRQs but none
for BLLacs. The contribution of LAT-blazars to the total extragalactic
gamma-ray intensity is estimated.Comment: Submitted to ApJ. Not yet refereed. 61 pages, 26 figure
The Spectral Energy Distribution of Fermi bright blazars
(Abridged) We have conducted a detailed investigation of the broad-band
spectral properties of the \gamma-ray selected blazars of the Fermi LAT Bright
AGN Sample (LBAS). By combining our accurately estimated Fermi gamma-ray
spectra with Swift, radio, infra-red, optical and other hard X-ray/gamma-ray
data, collected within three months of the LBAS data taking period, we were
able to assemble high-quality and quasi-simultaneous Spectral Energy
Distributions (SED) for 48 LBAS blazars.The SED of these gamma-ray sources is
similar to that of blazars discovered at other wavelengths, clearly showing, in
the usual Log - Log F representation, the typical broad-band
spectral signatures normally attributed to a combination of low-energy
synchrotron radiation followed by inverse Compton emission of one or more
components. We have used these SEDs to characterize the peak intensity of both
the low and the high-energy components. The results have been used to derive
empirical relationships that estimate the position of the two peaks from the
broad-band colors (i.e. the radio to optical and optical to X-ray spectral
slopes) and from the gamma-ray spectral index. Our data show that the
synchrotron peak frequency is positioned between 10 and
10 Hz in broad-lined FSRQs and between and Hz in
featureless BL Lacertae objects.We find that the gamma-ray spectral slope is
strongly correlated with the synchrotron peak energy and with the X-ray
spectral index, as expected at first order in synchrotron - inverse Compton
scenarios. However, simple homogeneous, one-zone, Synchrotron Self Compton
(SSC) models cannot explain most of our SEDs, especially in the case of FSRQs
and low energy peaked (LBL) BL Lacs. (...)Comment: 85 pages, 38 figures, submitted to Ap
PSR J1907+0602: A Radio-Faint Gamma-Ray Pulsar Powering a Bright TeV Pulsar Wind Nebula
We present multiwavelength studies of the 106.6 ms gamma-ray pulsar PSR
J1907+06 near the TeV source MGRO J1908+06. Timing observations with Fermi
result in a precise position determination for the pulsar of R.A. =
19h07m547(2), decl. = +06:02:16(2) placing the pulsar firmly within the TeV
source extent, suggesting the TeV source is the pulsar wind nebula of PSR
J1907+0602. Pulsed gamma-ray emission is clearly visible at energies from 100
MeV to above 10 GeV. The phase-averaged power-law index in the energy range E >
0.1 GeV is = 1.76 \pm 0.05 with an exponential cutoff energy E_{c} = 3.6 \pm
0.5 GeV. We present the energy-dependent gamma-ray pulsed light curve as well
as limits on off-pulse emission associated with the TeV source. We also report
the detection of very faint (flux density of ~3.4 microJy) radio pulsations
with the Arecibo telescope at 1.5 GHz having a dispersion measure DM = 82.1 \pm
1.1 cm^{-3}pc. This indicates a distance of 3.2 \pm 0.6 kpc and a
pseudo-luminosity of L_{1400} ~ 0.035 mJy kpc^2. A Chandra ACIS observation
revealed an absorbed, possibly extended, compact <(4 arcsec) X-ray source with
significant non-thermal emission at R.A. = 19h07m54.76, decl. = +06:02:14.6
with a flux of 2.3^{+0.6}_{-1.4} X 10^{-14} erg cm^{-2} s^{-1}. From archival
ASCA observations, we place upper limits on any arcminute scale 2--10 keV X-ray
emission of ~ 1 X 10^{-13} erg cm^{-2} s^{-1}. The implied distance to the
pulsar is compatible with that of the supernova remnant G40.5-0.5, located on
the far side of the TeV nebula from PSR J1907+0602, and the S74 molecular cloud
on the nearer side which we discuss as potential birth sites
Fermi LAT Observation of Diffuse Gamma-Rays Produced Through Interactions between Local Interstellar Matter and High Energy Cosmic Rays
Observations by the Large Area Telescope (LAT) on the \textit{Fermi} mission
of diffuse -rays in a mid-latitude region in the third quadrant
(Galactic longitude from 200\arcdeg to 260\arcdeg and latitude
from 22\arcdeg to 60\arcdeg) are reported. The region contains no known
large molecular cloud and most of the atomic hydrogen is within 1 kpc of the
solar system. The contributions of -ray point sources and inverse
Compton scattering are estimated and subtracted. The residual -ray
intensity exhibits a linear correlation with the atomic gas column density in
energy from 100 MeV to 10 GeV. The measured integrated -ray emissivity
is (1.63 \pm 0.05) \times 10^{-26} {\rm photons s^{-1} sr^{-1}
H\mathchar`-atom^{-1}} and (0.66 \pm 0.02) \times 10^{-26} {\rm photons
s^{-1} sr^{-1} H\mathchar`-atom^{-1}} above 100 MeV and above 300 MeV,
respectively, with additional systematic error of . The differential
emissivity in 100 MeV--10 GeV agrees with calculations based on cosmic ray
spectra consistent with those directly measured, at the 10% level. The results
obtained indicate that cosmic ray nuclei spectra within 1 kpc from the solar
system in regions studied are close to the local interstellar spectra inferred
from direct measurements at the Earth within .Comment: accepted for publication in the Astrophysical Journal. Revised
according to the author proof.(correction of typos and minor revisions
Fermi Large Area Telescope Bright Gamma-ray Source List
Following its launch in June 2008, the Fermi Gamma-ray Space Telescope
(Fermi) began a sky survey in August. The Large Area Telescope (LAT) on Fermi
in 3 months produced a deeper and better-resolved map of the gamma-ray sky than
any previous space mission. We present here initial results for energies above
100 MeV for the 205 most significant (statistical significance greater than
~10-sigma) gamma-ray sources in these data. These are the best-characterized
and best-localized point-like (i.e., spatially unresolved) gamma-ray sources in
the early-mission data.Comment: Accepted by ApJS. Many helpful comments by referee incorporated 57
pages, 12 figure
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