18 research outputs found
Understanding hadronic gamma-ray emission from supernova remnants
We aim to test the plausibility of a theoretical framework in which the
gamma-ray emission detected from supernova remnants may be of hadronic origin,
i.e., due to the decay of neutral pions produced in nuclear collisions
involving relativistic nuclei. In particular, we investigate the effects
induced by magnetic field amplification on the expected particle spectra,
outlining a phenomenological scenario consistent with both the underlying
Physics and the larger and larger amount of observational data provided by the
present generation of gamma experiments, which seem to indicate rather steep
spectra for the accelerated particles. In addition, in order to study to study
how pre-supernova winds might affect the expected emission in this class of
sources, the time-dependent gamma-ray luminosity of a remnant with a massive
progenitor is worked out. Solid points and limitations of the proposed scenario
are finally discussed in a critical way.Comment: 30 pages, 5 figures; Several comments, references and a figure added.
Some typos correcte
Three Years of Fermi LAT Flare Advocate Activity
The Fermi Flare Advocate (also known as Gamma-ray Sky Watcher, FA-GSW)
service provides for a daily quicklook analysis and review of the high-energy
gamma-ray sky seen by the Fermi Gamma-ray Space Telescope. The duty offers
alerts for potentially new gamma-ray sources, interesting transients and
relevant flares. A public weekly digest containing the main highlights about
the GeV gamma-ray sky is published in the web-based Fermi Sky Blog. During the
first 3 years of all-sky survey, more than 150 Astronomical Telegrams, several
alerts to the TeV Cherenkov telescopes, and targets of opportunity to Swift and
other observatories have been realized. This increased the rate of simultaneous
multi-frequency observing campaigns and the level of international cooperation.
Many gamma-ray flares from blazars (like extraordinary outbursts of 3C 454.3,
intense flares of PKS 1510-089, 4C 21.35, PKS 1830-211, AO 0235+164, PKS
1502+106, 3C 279, 3C 273, PKS 1622-253), short/long flux duty cycles,
unidentified transients near the Galactic plane (like J0910-5041, J0109+6134,
the Galactic center region), flares associated to Galactic sources (like the
Crab nebula, the nova V407 Cyg, the microquasar Cyg X-3), emission of the quiet
and active sun, were observed by Fermi and communicated by FA-GSWs.Comment: 2011 Fermi Symposium proceedings - eConf C11050
Dark Stars and Boosted Dark Matter Annihilation Rates
Dark Stars (DS) may constitute the first phase of stellar evolution, powered
by dark matter (DM) annihilation. We will investigate here the properties of DS
assuming the DM particle has the required properties to explain the excess
positron and elec- tron signals in the cosmic rays detected by the PAMELA and
FERMI satellites. Any possible DM interpretation of these signals requires
exotic DM candidates, with an- nihilation cross sections a few orders of
magnitude higher than the canonical value required for correct thermal relic
abundance for Weakly Interacting Dark Matter can- didates; additionally in most
models the annihilation must be preferentially to lep- tons. Secondly, we study
the dependence of DS properties on the concentration pa- rameter of the initial
DM density profile of the halos where the first stars are formed. We restrict
our study to the DM in the star due to simple (vs. extended) adiabatic
contraction and minimal (vs. extended) capture; this simple study is sufficient
to illustrate dependence on the cross section and concentration parameter. Our
basic results are that the final stellar properties, once the star enters the
main sequence, are always roughly the same, regardless of the value of boosted
annihilation or concentration parameter in the range between c=2 and c=5:
stellar mass ~ 1000M\odot, luminosity ~ 10^7 L\odot, lifetime ~ 10^6 yrs (for
the minimal DM models considered here; additional DM would lead to more massive
dark stars). However, the lifetime, final mass, and final luminosity of the DS
show some dependence on boost factor and concentration parameter as discussed
in the paper.Comment: 37 pages, 11 figure
Fermi Gamma-ray Space Telescope: High-Energy Results from the First Year
The Fermi Gamma-ray Space Telescope (Fermi) was launched on June 11, 2008 and
began its first year sky survey on August 11, 2008. The Large Area Telescope
(LAT), a wide field-of-view pair-conversion telescope covering the energy range
from 20 MeV to more than 300 GeV, is the primary instrument on Fermi. While
this review focuses on results obtained with the LAT, the Gamma-ray Burst
Monitor (GBM) complements the LAT in its observations of transient sources and
is sensitive to X-rays and gamma-rays with energies between 8 keV and 40 MeV.
During the first year in orbit, the Fermi LAT has observed a large number of
sources that include active galaxies, pulsars, compact binaries, globular
clusters, supernova remnants, as well as the Sun, the Moon and the Earth. The
GBM and LAT together have uncovered surprising characteristics in the
high-energy emission of gamma-ray bursts (GRBs) that have been used to set
significant new limits on violations of Lorentz invariance. The Fermi LAT has
also made important new measurements of the Galactic diffuse radiation and has
made precise measurements of the spectrum of cosmic-ray electrons and positrons
from 20 GeV to 1 TeV.Comment: 39 pages, 16 figure
Using Likelihood for Combined Data Set Analysis
2014 Fermi Symposium proceedings - eConf C14102.1International audienceThe joint likelihood is a simple extension of the standard likelihood formalism that enables the estimation of common parameters across disjoint datasets. Joining the likelihood, rather than the data itself, means nuisance parameters can be dealt with independently. Application of this technique, particularly to Fermi-LAT dwarf spheroidal analyses, has already been met with great success. We present a description of the method's general implementation along with a toy Monte-Carlo study of its properties and limitations