342 research outputs found
Search for Early Gamma-ray Production in Supernovae Located in a Dense Circumstellar Medium with the Fermi LAT
Supernovae (SNe) exploding in a dense circumstellar medium (CSM) are
hypothesized to accelerate cosmic rays in collisionless shocks and emit GeV
gamma rays and TeV neutrinos on a time scale of several months. We perform the
first systematic search for gamma-ray emission in Fermi LAT data in the energy
range from 100 MeV to 300 GeV from the ensemble of 147 SNe Type IIn exploding
in dense CSM. We search for a gamma-ray excess at each SNe location in a one
year time window. In order to enhance a possible weak signal, we simultaneously
study the closest and optically brightest sources of our sample in a
joint-likelihood analysis in three different time windows (1 year, 6 months and
3 months). For the most promising source of the sample, SN 2010jl (PTF10aaxf),
we repeat the analysis with an extended time window lasting 4.5 years. We do
not find a significant excess in gamma rays for any individual source nor for
the combined sources and provide model-independent flux upper limits for both
cases. In addition, we derive limits on the gamma-ray luminosity and the ratio
of gamma-ray-to-optical luminosity ratio as a function of the index of the
proton injection spectrum assuming a generic gamma-ray production model.
Furthermore, we present detailed flux predictions based on multi-wavelength
observations and the corresponding flux upper limit at 95% confidence level
(CL) for the source SN 2010jl (PTF10aaxf).Comment: Accepted for publication in ApJ. Corresponding author: A. Franckowiak
([email protected]), updated author list and acknowledgement
Multiwavelength Evidence for Quasi-periodic Modulation in the Gamma-ray Blazar PG 1553+113
We report for the first time a gamma-ray and multi-wavelength nearly-periodic
oscillation in an active galactic nucleus. Using the Fermi Large Area Telescope
(LAT) we have discovered an apparent quasi-periodicity in the gamma-ray flux (E
>100 MeV) from the GeV/TeV BL Lac object PG 1553+113. The marginal significance
of the 2.18 +/-0.08 year-period gamma-ray cycle is strengthened by correlated
oscillations observed in radio and optical fluxes, through data collected in
the OVRO, Tuorla, KAIT, and CSS monitoring programs and Swift UVOT. The optical
cycle appearing in ~10 years of data has a similar period, while the 15 GHz
oscillation is less regular than seen in the other bands. Further long-term
multi-wavelength monitoring of this blazar may discriminate among the possible
explanations for this quasi-periodicity.Comment: 8 pages, 5 figures. Accepted to The Astrophysical Journal Letters.
Corresponding authors: S. Ciprini (ASDC/INFN), S. Cutini (ASDC/INFN), S.
Larsson (Stockholm Univ/KTH), A. Stamerra (INAF/SNS), D. J. Thompson (NASA
GSFC
Gamma-ray flares from the Crab Nebula
A young and energetic pulsar powers the well-known Crab Nebula. Here we
describe two separate gamma-ray (photon energy >100 MeV) flares from this
source detected by the Large Area Telescope on board the Fermi Gamma-ray Space
Telescope. The first flare occurred in February 2009 and lasted approximately
16 days. The second flare was detected in September 2010 and lasted
approximately 4 days. During these outbursts the gamma-ray flux from the nebula
increased by factors of four and six, respectively. The brevity of the flares
implies that the gamma rays were emitted via synchrotron radiation from PeV
(10^15 eV) electrons in a region smaller than 1.4 10^-2 pc. These are the
highest energy particles that can be associated with a discrete astronomical
source, and they pose challenges to particle acceleration theory.Comment: Contact authors: Rolf Buehler,[email protected]; Stefan
Funk,[email protected]; Roger Blandford,rdb3@stanford ; 16 pages,2
figure
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
Fermi Large Area Telescope Observations of the Cosmic-Ray Induced gamma-ray Emission of the Earth's Atmosphere
We report on measurements of the cosmic-ray induced gamma-ray emission of
Earth's atmosphere by the Large Area Telescope onboard the Fermi Gamma-ray
Space Telescope. The LAT has observed the Earth during its commissioning phase
and with a dedicated Earth-limb following observation in September 2008. These
measurements yielded 6.4 x 10^6 photons with energies >100MeV and ~250hours
total livetime for the highest quality data selection. This allows the study of
the spatial and spectral distributions of these photons with unprecedented
detail. The spectrum of the emission - often referred to as Earth albedo
gamma-ray emission - has a power-law shape up to 500 GeV with spectral index
Gamma = 2.79+-0.06.Comment: Accepted for publication in PR
Fermi-LAT Study of Gamma-ray Emission in the Direction of Supernova Remnant W49B
We present an analysis of the gamma-ray data obtained with the Large Area
Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope in the direction of
SNR W49B (G43.3-0.2). A bright unresolved gamma-ray source detected at a
significance of 38 sigma is found to coincide with SNR W49B. The energy
spectrum in the 0.2-200 GeV range gradually steepens toward high energies. The
luminosity is estimated to be 1.5x10^{36} (D/8 kpc)^2 erg s^-1 in this energy
range. There is no indication that the gamma-ray emission comes from a pulsar.
Assuming that the SNR shell is the site of gamma-ray production, the observed
spectrum can be explained either by the decay of neutral pi mesons produced
through the proton-proton collisions or by electron bremsstrahlung. The
calculated energy density of relativistic particles responsible for the LAT
flux is estimated to be remarkably large, U_{e,p}>10^4 eV cm^-3, for either
gamma-ray production mechanism.Comment: 9 pages, 10 figure
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 LAT observations of the Geminga pulsar
We report on the \textit{Fermi}-LAT observations of the Geminga pulsar, the
second brightest non-variable GeV source in the -ray sky and the first
example of a radio-quiet -ray pulsar. The observations cover one year,
from the launch of the satellite through 2009 June 15. A data sample of
over 60,000 photons enabled us to build a timing solution based solely on
rays. Timing analysis shows two prominent peaks, separated by = 0.497 0.004 in phase, which narrow with increasing energy. Pulsed
rays are observed beyond 18 GeV, precluding emission below 2.7 stellar
radii because of magnetic absorption. The phase-averaged spectrum was fitted
with a power law with exponential cut-off of spectral index = (1.30
0.01 0.04), cut-off energy = (2.46 0.04 0.17)
GeV and an integral photon flux above 0.1 GeV of (4.14 0.02 0.32)
10 cm s. The first uncertainties are statistical
and the second are systematic. The phase-resolved spectroscopy shows a clear
evolution of the spectral parameters, with the spectral index reaching a
minimum value just before the leading peak and the cut-off energy having maxima
around the peaks. Phase-resolved spectroscopy reveals that pulsar emission is
present at all rotational phases. The spectral shape, broad pulse profile, and
maximum photon energy favor the outer magnetospheric emission scenarios.Comment: 32 pages, 12 figures, 3 tables. Accepted for publication in The
Astrophysical Journal. Corresponding authors: Denis Dumora
([email protected]), Fabio Gargano ([email protected]),
Massimiliano Razzano ([email protected]
Fermi Large Area Telescope View of the Core of the Radio Galaxy Centaurus A
We present gamma-ray observations with the LAT on board the Fermi Gamma-Ray
Telescope of the nearby radio galaxy Centaurus~A. The previous EGRET detection
is confirmed, and the localization is improved using data from the first 10
months of Fermi science operation. In previous work, we presented the detection
of the lobes by the LAT; in this work, we concentrate on the gamma-ray core of
Cen~A. Flux levels as seen by the LAT are not significantly different from that
found by EGRET, nor is the extremely soft LAT spectrum
(\G=2.67\pm0.10_{stat}\pm0.08_{sys} where the photon flux is \Phi\propto
E^{-\G}). The LAT core spectrum, extrapolated to higher energies, is
marginally consistent with the non-simultaneous HESS spectrum of the source.
The LAT observations are complemented by simultaneous observations from Suzaku,
the Swift Burst Alert Telescope and X-ray Telescope, and radio observations
with the Tracking Active Galactic Nuclei with Austral Milliarcsecond
Interferometry (TANAMI) program, along with a variety of non-simultaneous
archival data from a variety of instruments and wavelengths to produce a
spectral energy distribution (SED). We fit this broadband data set with a
single-zone synchrotron/synchrotron self-Compton model, which describes the
radio through GeV emission well, but fails to account for the non-simultaneous
higher energy TeV emission observed by HESS from 2004-2008. The fit requires a
low Doppler factor, in contrast to BL Lacs which generally require larger
values to fit their broadband SEDs. This indicates the \g-ray emission
originates from a slower region than that from BL Lacs, consistent with
previous modeling results from Cen~A. This slower region could be a slower
moving layer around a fast spine, or a slower region farther out from the black
hole in a decelerating flow.Comment: Accepted by ApJ. 32 pages, 5 figures, 2 tables. J. Finke and Y.
Fukazawa corresponding author
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