4,597 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 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
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 Large Area Telescope Observations of the Crab Pulsar and Nebula
We report on gamma-ray observations of the Crab Pulsar and Nebula using 8
months of survey data with the Fermi Large Area Telescope (LAT). The high
quality light curve obtained using the ephemeris provided by the Nancay and
Jodrell Bank radio telescopes shows two main peaks stable in phase with energy.
The first gamma-ray peak leads the radio main pulse by (281 \pm 12 \pm 21) mus,
giving new constraints on the production site of non-thermal emission in pulsar
magnetospheres. The improved sensitivity and the unprecedented statistics
afforded by the LAT enable precise measurement of the Crab Pulsar spectral
parameters: cut-off energy at E_c = (5.8 \pm 0.5 \pm 1.2) GeV, spectral index
of Gamma = (1.97 \pm 0.02 \pm 0.06) and integral photon flux above 100 MeV of
(2.09 \pm 0.03 \pm 0.18) x 10^{-6} cm^{-2} s^{-1}. The first errors represent
the statistical error on the fit parameters, while the second ones are the
systematic uncertainties. Pulsed gamma-ray photons are observed up to ~ 20 GeV
which precludes emission near the stellar surface, below altitudes of around 4
to 5 stellar radii in phase intervals encompassing the two main peaks. The
spectrum of the nebula in the energy range 100 MeV - 300 GeV is well described
by the sum of two power-laws of indices Gamma_{sync} = (3.99 \pm 0.12 \pm 0.08)
and Gamma_{IC} = (1.64 \pm 0.05 \pm 0.07), corresponding to the falling edge of
the synchrotron and the rising edge of the inverse Compton components,
respectively. This latter, which links up naturally with the spectral data
points of Cherenkov experiments, is well reproduced via inverse Compton
scattering from standard Magnetohydrodynamics (MHD) nebula models, and does not
require any additional radiation mechanism.Comment: 17 pages, 9 figures, Accepted for publications in Astrophysical
Journa
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
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
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
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
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]
- …