77 research outputs found
Evaluating the Maximum Likelihood Method for Detecting Short-Term Variability of AGILE gamma-ray Sources
The AGILE space mission (whose instrument is sensitive in the energy ranges
18-60 keV, and 30 MeV - 50 GeV) has been operating since 2007. Assessing the
statistical significance of time variability of gamma-ray sources above 100 MeV
is a primary task of the AGILE data analysis. In particular, it is important to
check the instrument sensitivity in terms of Poisson modeling of the data
background, and to determine the post-trial confidence of detections. The goals
of this work are: (i) evaluating the distributions of the likelihood ratio test
for "empty" fields, and for regions of the Galactic plane; (ii) calculating the
probability of false detection over multiple time intervals. In this paper we
describe in detail the techniques used to search for short-term variability in
the AGILE gamma-ray source database. We describe the binned maximum likelihood
method used for the analysis of AGILE data, and the numerical simulations that
support the characterization of the statistical analysis. We apply our method
to both Galactic and extra-galactic transients, and provide a few examples.
After having checked the reliability of the statistical description tested with
the real AGILE data, we obtain the distribution of p-values for blind and
specific source searches. We apply our results to the determination of the
post-trial statistical significance of detections of transient gamma-ray
sources in terms of pre-trial values. The results of our analysis allow a
precise determination of the post-trial significance of {\gamma}-ray sources
detected by AGILE.Comment: 12 pages, 13 figures, 8 tables, accepted to A&
Neutral pion emission from accelerated protons in the supernova remnant W44
We present the AGILE gamma-ray observations in the energy range 50 MeV - 10
GeV of the supernova remnant (SNR) W44, one of the most interesting systems for
studying cosmic-ray production. W44 is an intermediate-age SNR (20, 000 years)
and its ejecta expand in a dense medium as shown by a prominent radio shell,
nearby molecular clouds, and bright [SII] emitting regions. We extend our
gamma-ray analysis to energies substantially lower than previous measurements
which could not conclusively establish the nature of the radiation. We find
that gamma-ray emission matches remarkably well both the position and shape of
the inner SNR shocked plasma. Furthermore, the gamma-ray spectrum shows a
prominent peak near 1 GeV with a clear decrement at energies below a few
hundreds of MeV as expected from neutral pion decay. Here we demonstrate that:
(1) hadron-dominated models are consistent with all W44 multiwavelength
constraints derived from radio, optical, X-ray, and gamma-ray observations; (2)
ad hoc lepton-dominated models fail to explain simultaneously the
well-constrained gamma-ray and radio spectra, and require a circumstellar
density much larger than the value derived from observations; (3) the hadron
energy spectrum is well described by a power-law (with index s = 3.0 \pm 0.1)
and a low-energy cut-off at Ec = 6 \pm 1 GeV. Direct evidence for pion emission
is then established in an SNR for the first time.Comment: accepted for publication on ApJ
Calibration of AGILE-GRID with In-Flight Data and Monte Carlo Simulations
Context: AGILE is a gamma-ray astrophysics mission which has been in orbit
since 23 April 2007 and continues to operate reliably. The gamma-ray detector,
AGILE-GRID, has observed Galactic and extragalactic sources, many of which were
collected in the first AGILE Catalog. Aims: We present the calibration of the
AGILE-GRID using in-flight data and Monte Carlo simulations, producing
Instrument Response Functions (IRFs) for the effective area A_eff), Energy
Dispersion Probability (EDP), and Point Spread Function (PSF), each as a
function of incident direction in instrument coordinates and energy. Methods:
We performed Monte Carlo simulations at different gamma-ray energies and
incident angles, including background rejection filters and Kalman filter-based
gamma-ray reconstruction. Long integrations of in-flight observations of the
Vela, Crab and Geminga sources in broad and narrow energy bands were used to
validate and improve the accuracy of the instrument response functions.
Results: The weighted average PSFs as a function of spectra correspond well to
the data for all sources and energy bands. Conclusions: Changes in the
interpolation of the PSF from Monte Carlo data and in the procedure for
construction of the energy-weighted effective areas have improved the
correspondence between predicted and observed fluxes and spectra of celestial
calibration sources, reducing false positives and obviating the need for
post-hoc energy-dependent scaling factors. The new IRFs have been publicly
available from the Agile Science Data Centre since November 25, 2011, while the
changes in the analysis software will be distributed in an upcoming release
Direct Evidence for Hadronic Cosmic-Ray Acceleration in the Supernova Renmant IC 443
The Supernova Remnant (SNR) IC 443 is an intermediate-age remnant well known
for its radio, optical, X-ray and gamma-ray energy emissions. In this Letter we
study the gamma-ray emission above 100 MeV from IC 443 as obtained by the AGILE
satellite. A distinct pattern of diffuse emission in the energy range 100 MeV-3
GeV is detected across the SNR with its prominent maximum (source "A")
localized in the Northeastern shell with a flux F = (47 \pm 10) 10^{-8} photons
cm^{-2} s^{-1} above 100 MeV. This location is the site of the strongest shock
interaction between the SNR blast wave and the dense circumstellar medium.
Source "A" is not coincident with the TeV source located 0.4 degree away and
associated with a dense molecular cloud complex in the SNR central region. From
our observations, and from the lack of detectable diffuse TeV emission from its
Northeastern rim, we demonstrate that electrons cannot be the main emitters of
gamma-rays in the range 0.1-10 GeV at the site of the strongest SNR shock. The
intensity, spectral characteristics, and location of the most prominent
gamma-ray emission together with the absence of co-spatial detectable TeV
emission are consistent only with a hadronic model of cosmic-ray acceleration
in the SNR. A high-density molecular cloud (cloud "E") provides a remarkable
"target" for nucleonic interactions of accelerated hadrons: our results show
enhanced gamma-ray production near the molecular cloud/shocked shell
interaction site. IC 443 provides the first unambiguous evidence of cosmic-ray
acceleration by SNRs.Comment: 5 pages, 2 figures; accepted by ApJLetters on Jan 21, 201
Multiwavelength observations of 3C 454.3 II. The AGILE 2007 December campaign
We report on the second AGILE multiwavelength campaign of the blazar 3C 454.3
during the first half of December 2007. This campaign involved AGILE, Spitzer,
Swift,Suzaku,the WEBT consortium,the REM and MITSuME telescopes,offering a
broad band coverage that allowed for a simultaneous sampling of the synchrotron
and inverse Compton (IC) emissions.The 2-week AGILE monitoring was accompanied
by radio to optical monitoring by WEBT and REM and by sparse observations in
mid-Infrared and soft/hard X-ray energy bands performed by means of Target of
Opportunity observations by Spitzer, Swift and Suzaku, respectively.The source
was detected with an average flux of~250x10^{-8}ph cm^-2s^-1 above 100
MeV,typical of its flaring states.The simultaneous optical and gamma-ray
monitoring allowed us to study the time-lag associated with the variability in
the two energy bands, resulting in a possible ~1-day delay of the gamma-ray
emission with respect to the optical one. From the simultaneous optical and
gamma-ray fast flare detected on December 12, we can constrain the delay
between the gamma-ray and optical emissions within 12 hours. Moreover, we
obtain three Spectral Energy Distributions (SEDs) with simultaneous data for
2007 December 5, 13, 15, characterized by the widest multifrequency coverage.
We found that a model with an external Compton on seed photons by a standard
disk and reprocessed by the Broad Line Regions does not describe in a
satisfactory way the SEDs of 2007 December 5, 13 and 15. An additional
contribution, possibly from the hot corona with T=10^6 K surrounding the jet,
is required to account simultaneously for the softness of the synchrotron and
the hardness of the inverse Compton emissions during those epochs.Comment: 13 pages, 8 figures, 2 tables, Accepted for publication in Ap
Agile Detection of Delayed Gamma-Ray Emission from the Short Gamma-Ray Burst GRB 090510
Short gamma-ray bursts (GRBs), typically lasting less than 2 s, are a special
class of GRBs of great interest. We report the detection by the AGILE satellite
of the short GRB 090510 which shows two clearly distinct emission phases: a
prompt phase lasting ~ 200 msec and a second phase lasting tens of seconds. The
prompt phase is relatively intense in the 0.3-10 MeV range with a spectrum
characterized by a large peak/cutoff energy near 3 MeV, in this phase, no
significant high-energy gamma-ray emission is detected. At the end of the
prompt phase, intense gamma-ray emission above 30 MeV is detected showing a
power-law time decay of the flux of the type t^-1.3 and a broad-band spectrum
remarkably different from that of the prompt phase. It extends from sub-MeV to
hundreds of MeV energies with a photon index alpha ~ 1.5. GRB 090510 provides
the first case of a short GRB with delayed gamma-ray emission. We present the
timing and spectral data of GRB 090510 and briefly discuss its remarkable
properties within the current models of gamma-ray emission of short GRBs.Comment: Accepted by the Astrophysical Journal Letters on September 11, 200
Detection of Gamma-Ray Emission from the Vela Pulsar Wind Nebula with AGILE
Pulsars are known to power winds of relativistic particles that can produce
bright nebulae by interacting with the surrounding medium. These pulsar wind
nebulae (PWNe) are observed in the radio, optical, x-rays and, in some cases,
also at TeV energies, but the lack of information in the gamma-ray band
prevents from drawing a comprehensive multiwavelength picture of their
phenomenology and emission mechanisms. Using data from the AGILE satellite, we
detected the Vela pulsar wind nebula in the energy range from 100 MeV to 3 GeV.
This result constrains the particle population responsible for the GeV
emission, probing multivavelength PWN models, and establishes a class of
gamma-ray emitters that could account for a fraction of the unidentified
Galactic gamma-ray sources.Comment: Accepted by Science; first published online on December 31, 2009 in
Science Express. Science article and Supporting Online Material are available
at http://www.sciencemag.or
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