4,820 research outputs found
A Monte-Carlo Implementation of the SAGE Algorithm for Joint Soft Multiuser and Channel Parameter Estimation
An efficient, joint transmission delay and channel parameter estimation
algorithm is proposed for uplink asynchronous direct-sequence code-division
multiple access (DS-CDMA) systems based on the space-alternating generalized
expectation maximization (SAGE) framework. The marginal likelihood of the
unknown parameters, averaged over the data sequence, as well as the expectation
and maximization steps of the SAGE algorithm are derived analytically. To
implement the proposed algorithm, a Markov Chain Monte Carlo (MCMC) technique,
called Gibbs sampling, is employed to compute the {\em a posteriori}
probabilities of data symbols in a computationally efficient way. Computer
simulations show that the proposed algorithm has excellent estimation
performance. This so-called MCMC-SAGE receiver is guaranteed to converge in
likelihood.Comment: 5 pages, 3 figures, 10th IEEE International Workshop on Signal
Processing Advances in Wireless Communications (SPAWC) 200
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
Discovery of very high energy gamma rays from PKS 1424+240 and multiwavelength constraints on its redshift
We report the first detection of very-high-energy (VHE) gamma-ray emission
above 140 GeV from PKS 1424+240, a BL Lac object with an unknown redshift. The
photon spectrum above 140 GeV measured by VERITAS is well described by a power
law with a photon index of 3.8 +- 0.5_stat +- 0.3_syst and a flux normalization
at 200 GeV of (5.1 +- 0.9_stat +- 0.5_syst) x 10^{-11} TeV^-1 cm^-2 s^-1, where
stat and syst denote the statistical and systematical uncertainty,
respectively. The VHE flux is steady over the observation period between MJD
54881 and 55003 (2009 February 19 to June 21). Flux variability is also not
observed in contemporaneous high energy observations with the Fermi Large Area
Telescope (LAT). Contemporaneous X-ray and optical data were also obtained from
the Swift XRT and MDM observatory, respectively. The broadband spectral energy
distribution (SED) is well described by a one-zone synchrotron self-Compton
(SSC) model favoring a redshift of less than 0.1. Using the photon index
measured with Fermi in combination with recent extragalactic background light
(EBL) absorption models it can be concluded from the VERITAS data that the
redshift of PKS 1424+240 is less than 0.66.Comment: accepted for publication, Ap
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
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
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 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
Fermi LAT Observations of LS I +61 303: First detection of an orbital modulation in GeV Gamma Rays
This Letter presents the first results from the observations of LSI +61 303
using Large Area Telescope data from the Fermi Gamma-Ray Space Telescope
between 2008 August and 2009 March. Our results indicate variability that is
consistent with the binary period, with the emission being modulated at 26.6
+/- 0.5 days. This constitutes the first detection of orbital periodicity in
high-energy gamma rays (20 MeV-100 GeV, HE). The light curve is characterized
by a broad peak after periastron, as well as a smaller peak just before
apastron. The spectrum is best represented by a power law with an exponential
cutoff, yielding an overall flux above 100 MeV of 0.82 +/- 0.03(stat) +/-
0.07(syst) 10^{-6} ph cm^{-2} s^{-1}, with a cutoff at 6.3 +/- 1.1(stat) +/-
0.4(syst) GeV and photon index Gamma = 2.21 +/- 0.04(stat) +/- 0.06(syst).
There is no significant spectral change with orbital phase. The phase of
maximum emission, close to periastron, hints at inverse Compton scattering as
the main radiation mechanism. However, previous very high-energy gamma ray
(>100 GeV, VHE) observations by MAGIC and VERITAS show peak emission close to
apastron. This and the energy cutoff seen with Fermi suggest the link between
HE and VHE gamma rays is nontrivial.Comment: 7 pages, 5 figures, accepted for publication in ApJ Letters 21 July
200
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]
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