3,559 research outputs found
Cosmic-Ray Induced Diffuse Emissions from the Milky Way and Local Group Galaxies
Cosmic rays fill up the entire volume of galaxies, providing an important
source of heating and ionisation of the interstellar medium, and may play a
significant role in the regulation of star formation and galactic evolution.
Diffuse emissions from radio to high-energy gamma rays (> 100 MeV) arising from
various interactions between cosmic rays and the interstellar medium,
interstellar radiation field, and magnetic field, are currently the best way to
trace the intensities and spectra of cosmic rays in the Milky Way and other
galaxies. In this contribution, I describe our recent work to model the full
spectral energy distribution of galaxies like the Milky Way from radio to
gamma-ray energies. The application to other galaxies, in particular the
Magellanic Clouds and M31 that are detected in high-energy gamma-rays by the
Fermi-LAT, is also discussed.Comment: Contribution to "The Spectral Energy Distribution of Galaxies"
Proceedings IAU Symposium No. 284, 2011, eds. R.J. Tuffs & C.C.Popescu. 4
pages with 4 figure
Search for variable gamma-ray emission from the Galactic plane in the Fermi data
High-energy gamma-ray emission from the Galactic plane above ~100 MeV is
composed of three main contributions: diffuse emission from cosmic ray
interactions in the interstellar medium, emission from extended sources, such
as supernova remnants and pulsar wind nebulae, and emission from isolated
compact source populations. The diffuse emission and emission from the extended
sources provide the dominant contribution to the flux almost everywhere in the
inner Galaxy, preventing the detection of isolated compact sources. In spite of
this difficulty, compact sources in the Galactic plane can be singled out based
on the variability properties of their gamma-ray emission. Our aim is to find
sources in the Fermi data that show long-term variability. We performed a
systematic study of the emission variability from the Galactic plane, by
constructing the variability maps. We find that emission from several
directions along the Galactic plane is significantly variable on a time scale
of months. These directions include, in addition to known variable Galactic
sources and background blazars, the Galactic ridge region at positive Galactic
longitudes and several regions containing young pulsars. We argue that
variability on the time scale of months may be common to pulsars, originating
from the inner parts of pulsar wind nebulae, similarly to what is observed in
the Crab pulsar.Comment: 4 pages, 4 figures, accepted to Astronomy & Astrophysic
Gamma-ray emission from AGNs
Blazars, radio-loud active galactic nuclei with the relativistic jet closely
aligned with the line of sight, dominate the extragalactic sky observed at
gamma-ray energies, above 100 MeV. We discuss some of the emission properties
of these sources, focusing in particular on the "blazar sequence" and the
interpretative models of the high-energy emission of BL Lac objects.Comment: 8 pages, 4 figures, to appear in the proceedings of the HEPRO II
conference, Buenos Aires, October 26-30 200
On leptonic models for blazars in the Fermi era
Some questions raised by Fermi-LAT data about blazars are summarized, along
with attempts at solutions within the context of leptonic models. These include
both spectral and statistical questions, including the origin of the GeV breaks
in low-synchrotron peaked blazars, the location of the gamma-ray emission
sites, the correlations in the spectral energy distributions with luminosity,
and the difficulty of synchrotron/SSC models to fit the spectra of some TeV
blazars.Comment: 9 pages, 1 figure, in "Beamed and Unbeamed Gamma Rays from Galaxies,"
Muonio, Finland, 11-15 April, 2011, ed. R. Wagner, L. Maraschi, A. Sillanpaa,
to appear in Journal of Physics: Conference Serie
The bright unidentified gamma-ray source 1FGL J1227.9-4852: Can it be associated with an LMXB?
We present an analysis of high energy (HE; 0.1-300 GeV) gamma-ray
observations of 1FGL J1227.9-4852 with the Fermi Gamma-ray Space Telescope,
follow-up radio observations with the Australia Telescope Compact Array, Giant
Metrewave Radio Telescope and Parkes radio telescopes of the same field and
follow-up optical observations with the ESO VLT. We also examine archival
XMM-Newton and INTEGRAL X-ray observations of the region around this source.
The gamma-ray spectrum of 1FGL J1227.9-4852 is best fit with an exponentially
cutoff power-law, reminiscent of the population of pulsars observed by Fermi. A
previously unknown, compact radio source within the 99.7% error circle of 1FGL
J1227.9-4852 is discovered and has a morphology consistent either with an AGN
core/jet structure or with two roughly symmetric lobes of a distant radio
galaxy. A single bright X-ray source XSS J12270-4859, a low-mass X-ray binary,
also lies within the 1FGL J1227.9-4852 error circle and we report the first
detection of radio emission from this source. The potential association of 1FGL
J1227.9-4852 with each of these counterparts is discussed. Based upon the
available data we find the association of the gamma-ray source to the compact
double radio source unlikely and suggest that XSS J12270-4859 is a more likely
counterpart to the new HE source. We propose that XSS J12270-4859 may be a
millisecond binary pulsar and draw comparisons with PSR J1023+0038.Comment: Accepted for publication in MNRAS; 9 pages, 8 figures, 2 table
Conservative upper limits on WIMP annihilation cross section from Fermi-LAT -rays
The spectrum of an isotropic extragalactic -ray background (EGB) has
been measured by the Fermi-LAT telescope at high latitudes. Two new models for
the EGB are derived from the subtraction of unresolved point sources and
extragalactic diffuse processes, which could explain from 30% to 70% of the
Fermi-LAT EGB. Within the hypothesis that the two residual EGBs are entirely
due to the annihilation of dark matter (DM) particles in the Galactic halo, we
obtain upper limits on their annihilation cross section \sigmav.
Severe bounds on a possible Sommerfeld enhancement of the annihilation cross
section are set as well. Finally, would {\sigmav} be inversely proportional to
the WIMP velocity, very severe limits are derived for the velocity-independent
part of the annihilation cross section.Comment: Proceedings of XII Taup Conference, Munich, September 201
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Future prospects of the renewable energy sector in libya
This study investigates the options available to the energy sector in Libyan, particularly in relation to the potential of using renewable energy as one of the main sources for the country. Libyan government has set a target for renewable energy resources sharing with current energy sources to reach 30% by the year 2030 which mainly includes wind energy, Concentrating Solar Power (CSP), Photovoltaic (PV) and Solar Water Heating (SWH). The argument here is not whether this can be completed or not within the stipulated time. But the main objective is achieving a sustainable economic growth through a clean energy system and for the energy supply to maintain meeting the growing energy demand. The aim of this paper is to illustrate the current energy supply and future demands in Libya. This paper integrates data from literature review, field visits and interviews with Libyan energy experts to paint a comprehensive picture in relation to energy demand and consumption. The findings which have emerged from the analysis of this data reflect energy challenges and opportunities in Libya. Furthermore, securing alternative resources of energy and income are becoming critically important for Libya and other countries within the Middle East and North Africa region (MENA) if they wish to maintain the same standard of living for future generations and reduce pollution, fossil fuel local consumption and carbon emission
Gamma-Ray Emission from Two Blazars Behind the Galactic Plane: B2013+370 & B2023+336
B2013+370 and B2023+336 are two blazars at low-galactic latitude that were
previously proposed to be the counterparts for the EGRET unidentified sources,
3EG J2016+3657 and 3EG J2027+3429. Gamma-ray emission associated with the EGRET
sources has been detected by the Fermi Gamma-ray Space Telescope, and the two
sources, 1FGL J2015.7+3708 and 1FGL J2027.6+3335, have been classified as
unidentified in the 1-year catalog. This analysis of the Fermi-LAT data
collected during 31 months reveals that the 1FGL sources are spatially
compatible with the blazars, and are significantly variable, supporting the
hypothesis of extragalactic origin for the gamma-ray emission. The gamma-ray
light curves are compared with 15 GHz radio light curves from the 40-m
telescope at the Owens Valley Radio Observatory (OVRO). Simultaneous
variability is seen in both bands for the two blazar candidates. The study is
completed with the X-ray analysis of 1FGL J2015.7+3708 using Swift observations
that were triggered in August 2010 by a Fermi-detected flare. The resulting
spectral energy distribution shows a two-component structure typical of
blazars. We also identify a second source in the field of view of 1FGL
J2027.6+3335 with similar characteristics to the known LAT pulsars. This study
gives solid evidence favoring blazar counterparts for these two unidentified
EGRET and Fermi sources, supporting the hypothesis that a number of
unidentified gamma-ray sources at low galactic latitudes are indeed of
extragalactic origin.Comment: 10 pages, 7 figures, 6 tables, accepted for publication in The
Astrophysical Journa
Fermi Large Area Telescope Observations of the Cygnus Loop Supernova Remnant
We present an analysis of the gamma-ray measurements by the Large Area
Telescope(LAT) onboard the \textit{Fermi Gamma-ray Space Telescope} in the
region of the supernova remnant(SNR) Cygnus Loop(G74.08.5). We detect
significant gamma-ray emission associated with the SNR in the energy band
0.2--100 GeV. The gamma-ray spectrum shows a break in the range 2--3 GeV. The
gamma-ray luminosity is erg s between 1--100
GeV, much lower than those of other GeV-emitting SNRs. The morphology is best
represented by a ring shape, with inner/outer radii 0.7
0.1 and 1.6 0.1. Given the association among
X-ray rims, \halpha filaments and gamma-ray emission, we argue that gamma rays
originate in interactions between particles accelerated in the SNR and
interstellar gas or radiation fields adjacent to the shock regions. The decay
of neutral pions produced in nucleon-nucleon interactions between accelerated
hadrons and interstellar gas provides a reasonable explanation for the
gamma-ray spectrum.Comment: accepted by ApJ, 34 pages, 6 figure
Extragalactic Very-High-Energy gamma-ray background
We study the origin of the extragalactic diffuse gamma-ray background using
the data from the Fermi telescope. To estimate the background level, we count
photons at high Galactic latitudes |b|>60 degrees. Subtracting photons
associated to known sources and the residual cosmic ray and Galactic diffuse
backgrounds, we estimate the Extragalactic Gamma-ray Background (EGB) flux. We
find that the spectrum of EGB in the very-high-energy (VHE) band above 30 GeV
follows the stacked spectrum of BL Lacs. LAT data reveal the positive (1+z)^k,
1<k<4 cosmological evolution of the BL Lac source population consistent with
that of their parent population, FR I radio galaxies. We show that EGB at E>30
GeV could be completely explained by emission from unresolved BL Lacs if k~3.Comment: 8 pages, 6 figures, accepted to Astrophysics Journa
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