322 research outputs found
Towards a Flexible User Simulation for Evaluating Spoken Dialogue Systems
Abstract. The main aim of research is to introduce a new data-driven user simulation approach for the quality and usability evaluation for spoken dialogue systems
Fermi LAT Observations of the Supernova Remnant W28 (G6.4-0.1)
We present detailed analysis of the two gamma-ray sources,1FGL J1801.3-2322c
and 1FGL J1800.5-2359c,that have been found toward the supernova remnant(SNR)
W28 with the Large Area Telescope(LAT) on board the Fermi Gamma-ray Space
Telescope.1FGL J1801.3-2322c is found to be an extended source within the
boundary of SNR W28,and to extensively overlap with the TeV gamma-ray source
HESS J1801-233,which is associated with a dense molecular cloud interacting
with the supernova remnant.The gamma-ray spectrum measured with LAT from
0.2--100 GeV can be described by a broken power-law function with a break of
~1GeV,and photon indices of 2.090.08(stat)0.28(sys) below the break
and 2.740.06(stat)0.09(sys) above the break.Given the clear
association between HESS J1801-233 and the shocked molecular cloud and a
smoothly connected spectrum in the GeV--TeV band,we consider the origin of the
gamma-ray emission in both GeV and TeV ranges to be the interaction between
particles accelerated in the SNR and the molecular cloud.The decay of neutral
pions produced in interactions between accelerated hadrons and dense molecular
gas provide a reasonable explanation for the broadband gamma-ray spectrum. 1FGL
J1800.5-2359c, located outside the southern boundary of SNR W28, cannot be
resolved.An upper limit on the size of the gamma-ray emission was estimated to
be ~16 using events above ~2GeV under the assumption of a circular shape
with uniform surface brightness. It appears to coincide with the TeV source
HESS J1800-240B,which is considered to be associated with a dense molecular
cloud that contains the ultra compact HII region W28A2(G5.89-0.39).We found no
significant gamma-ray emission in the LAT energy band at the positions of TeV
sources HESS J1800-230A and HESS J1800-230C.The LAT data for HESS J1800-230A
combined with the TeV data points indicate a spectral break between 10GeV and
100GeV.Comment: 23 pages, 6 figures. Accepted for publication in the Astrophysical
Journal. Corresponding authors: H. Katagiri, H. Tajima, T. Tanaka, and Y.
Uchiyam
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 Large Area Telescope observations of the Vela-X Pulsar Wind Nebula
We report on gamma-ray observations in the off-pulse window of the Vela
pulsar PSR B0833-45, using 11 months of survey data from the Fermi Large Area
Telescope (LAT). This pulsar is located in the 8 degree diameter Vela supernova
remnant, which contains several regions of non-thermal emission detected in the
radio, X-ray and gamma-ray bands. The gamma-ray emission detected by the LAT
lies within one of these regions, the 2*3 degrees area south of the pulsar
known as Vela-X. The LAT flux is signicantly spatially extended with a best-fit
radius of 0.88 +/- 0.12 degrees for an assumed radially symmetric uniform disk.
The 200 MeV to 20 GeV LAT spectrum of this source is well described by a
power-law with a spectral index of 2.41 +/- 0.09 +/- 0.15 and integral flux
above 100 MeV of (4.73 +/- 0.63 +/- 1.32) * 10^{-7} cm^{-2} s^{-1}. The first
errors represent the statistical error on the fit parameters, while the second
ones are the systematic uncertainties. Detailed morphological and spectral
analyses give strong constraints on the energetics and magnetic field of the
pulsar wind nebula (PWN) system and favor a scenario with two distinct electron
populations.Comment: 21 pages, 5 figures, accepted for publication in Astrophysical
Journa
Fermi observations of high-energy gamma-ray emission from GRB 090217A
The Fermi observatory is advancing our knowledge of Gamma-Ray Bursts (GRBs)
through pioneering observations at high energies, covering more than 7 decades
in energy with the two on-board detectors, the Large Area Telescope (LAT) and
the Gamma-ray Burst Monitor (GBM). Here we report on the observation of the
long GRB 090217A which triggered the GBM and has been detected by the LAT with
a significance greater than 9 sigma. We present the GBM and LAT observations
and on-ground analyses, including the time-resolved spectra and the study of
the temporal profile from 8 keV up to 1 GeV. All spectra are well reproduced by
a Band model. We compare these observations to the first two LAT-detected, long
bursts GRB 080825C and GRB 080916C. These bursts were found to have
time-dependent spectra and exhibited a delayed onset of the high-energy
emission, which are not observed in the case of GRB 090217A. We discuss some
theoretical implications for the high-energy emission of GRBs.Comment: 17 pages, 4 figures. Contact Authors: Fred, Piron; Sara, Cutini;
Andreas, von Kienli
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 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]
Fermi LAT Search for Photon Lines from 30 to 200 GeV and Dark Matter Implications
Dark matter (DM) particle annihilation or decay can produce monochromatic
-rays readily distinguishable from astrophysical sources. -ray
line limits from 30 GeV to 200 GeV obtained from 11 months of Fermi Large Area
Space Telescope data from 20-300 GeV are presented using a selection based on
requirements for a -ray line analysis, and integrated over most of the
sky. We obtain -ray line flux upper limits in the range , and give corresponding DM annihilation
cross-section and decay lifetime limits. Theoretical implications are briefly
discussed.Comment: 6 pages, 1 figure. Accepted for publication by The Physical Review
Letter
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