35 research outputs found
The Gamma-ray Albedo of the Moon
We use the GEANT4 Monte Carlo framework to calculate the gamma-ray albedo of
the Moon due to interactions of cosmic ray (CR) nuclei with moon rock. Our
calculation of the albedo spectrum agrees with the EGRET data. We show that the
spectrum of gamma rays from the Moon is very steep with an effective cutoff
around 3-4 GeV (600 MeV for the inner part of the Moon disk) and exhibits a
narrow pion-decay line at 67.5 MeV, perhaps unique in astrophysics. Apart from
other astrophysical sources, the albedo spectrum of the Moon is well
understood, including its absolute normalisation; this makes it a useful
"standard candle" for gamma-ray telescopes. The steep albedo spectrum also
provides a unique opportunity for energy calibration of gamma-ray telescopes,
such as the forthcoming Gamma Ray Large Area Space Telescope (GLAST). Since the
albedo flux depends on the incident CR spectrum which changes over the solar
cycle, it is possible to monitor the CR spectrum using the albedo gamma-ray
flux. Simultaneous measurements of CR proton and helium spectra by the Payload
for Antimatter-Matter Exploration and Light-nuclei Astrophysics (PAMELA), and
observations of the albedo gamma rays by the GLAST Large Area Telescope (LAT),
can be used to test the model predictions and will enable the LAT to monitor
the CR spectrum near the Earth beyond the lifetime of the PAMELA.Comment: 6 pages, 4 figures, emulateapj.cls; to appear in the Astrophysical
Journa
A Hard X-Ray View of Two Distant VHE Blazars: 1ES 1101-232 and 1ES 1553+113
TeV blazars are known as prominent nonthermal emitters across the entire
electromagnetic spectrum with their photon power peaking in the X-ray and TeV
bands. If distant, absorption of gamma-ray photons by the extragalactic
background light (EBL) alters the intrinsic TeV spectral shape, thereby
affecting the overall interpretation. Suzaku observations for two of the more
distant TeV blazars known to date, 1ES 1101-232 and 1ES 1553+113, were carried
out in 2006 May and July, respectively, including a quasi-simultaneous coverage
with the state-of-the-art Cerenkov telescope facilities. We report on the
resulting data sets with emphasis on the X-ray band and set in context to their
historical behavior. During our campaign, we did not detect any significant
X-ray or gamma-ray variability. 1ES 1101-232 was found in a quiescent state
with the lowest X-ray flux ever measured. The combined XIS and HXD PIN data for
1ES 1101-232 and 1ES 1553+113 clearly indicate spectral curvature up to the
highest hard X-ray data point (~30 keV), manifesting as softening with
increasing energy. We describe this spectral shape by either a broken power law
or a log-parabolic fit with equal statistical goodness of fits. The combined
1ES 1553+113 very high energy spectrum (90-500 GeV) did not show any
significant changes with respect to earlier observations. The resulting
contemporaneous broadband spectral energy distributions of both TeV blazars are
discussed in view of implications for intrinsic blazar parameter values, taking
into account the gamma-ray absorption in the EBL.Comment: 9 pages, 10 figure
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Clumping Effects on Non-Thermal Particle Spectra in Massive Star Systems
Observational evidence exists that winds of massive stars are clumped. Many massive star systems are known as non-thermal particle production sites, as indicated by their synchrotron emission in the radio band. As a consequence they are also considered as candidate sites for non-thermal high-energy photon production up to gamma-ray energies. The present work considers the effects of wind clumpiness expected on the emitting relativistic particle spectrum in colliding wind systems, built up from the pool of thermal wind particles through diffusive particle acceleration, and taking into account inverse Compton and synchrotron losses. In comparison to a homogeneous wind, a clumpy wind causes flux variations of the emitting particle spectrum when the clump enters the wind collision region. It is found that the spectral features associated with this variability moves temporally from low to high energy bands with the time shift between any two spectral bands being dependent on clump size, filling factor, and the energy-dependence of particle energy gains and losses
Fermi-LAT Detection of a Break in the Gamma-Ray Spectrum of the Supernova Remnant Cassiopeia A
We report on observations of the supernova remnant Cassiopeia A in the energy
range from 100 MeV to 100 GeV using 44 months of observations from the Large
Area Telescope on board the Fermi Gamma-ray Space Telescope. We perform a
detailed spectral analysis of this source and report on a low-energy break in
the spectrum at GeV. By comparing the results with
models for the gamma-ray emission, we find that hadronic emission is preferred
for the GeV energy range.Comment: 18 pages, 5 figures, 2 tables, to be published in Ap
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Bright AGN Source List from the First Three Months of the Fermi Large Area Telescope All-Sky Survey
Gravitomagnetism and spinor quantum mechanics
We give a systematic treatment of a spin 1/2 particle in a combined
electromagnetic field and a weak gravitational field that is produced by a
slowly moving matter source. This paper continues previous work on a spin zero
particle, but it is largely self-contained and may serve as an introduction to
spinors in a Riemann space. The analysis is based on the Dirac equation
expressed in generally covariant form and coupled minimally to the
electromagnetic field. The restriction to a slowly moving matter source, such
as the earth, allows us to describe the gravitational field by a
gravitoelectric (Newtonian) potential and a gravitomagnetic (frame-dragging)
vector potential, the existence of which has recently been experimentally
verified. Our main interest is the coupling of the orbital and spin angular
momenta of the particle to the gravitomagnetic field. Specifically we calculate
the gravitational gyromagnetic ratio as gsubg=1 ; this is to be compared with
the electromagnetic gyromagnetic ratio of gsube=2 for a Dirac electron.Comment: 12 pages, 1 figur
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Normal Modes of Black Hole Accretion Disks
This paper studies the hydrodynamical problem of normal modes of small adiabatic oscillations of relativistic barotropic thin accretion disks around black holes (and compact weakly magnetic neutron stars). Employing WKB techniques, we obtain the eigen frequencies and eigenfunctions of the modes for different values of the mass and angular momentum of the central black hole. We discuss the properties of the various types of modes and examine the role of viscosity, as it appears to render some of the modes unstable to rapid growth
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The Gamma-Ray Albedo of the Moon
We use the GEANT4 Monte Carlo framework to calculate the {gamma}-ray albedo of the Moon due to interactions of cosmic ray (CR) nuclei with moon rock. Our calculation of the albedo spectrum agrees with the EGRET data. We show that the spectrum of {gamma}-rays from the Moon is very steep with an effective cutoff around 3-4 GeV (600 MeV for the inner part of the Moon disk) and exhibits a narrow pion-decay line at 67.5 MeV, perhaps unique in astrophysics. Apart from other astrophysical sources, the albedo spectrum of the Moon is well understood, including its absolute normalization; this makes it a useful 'standard candle' for {gamma}-ray telescopes. The steep albedo spectrum also provides a unique opportunity for energy calibration of {gamma}-ray telescopes, such as the forthcoming Gamma Ray Large Area Space Telescope (GLAST). Since the albedo flux depends on the incident CR spectrum which changes over the solar cycle, it is possible to monitor the CR spectrum using the albedo {gamma}-ray flux. Simultaneous measurements of CR proton and helium spectra by the Payload for Antimatter-Matter Exploration and Light-nuclei Astrophysics (PAMELA), and observations of the albedo {gamma}-rays by the GLAST Large Area Telescope (LAT), can be used to test the model predictions and will enable the LAT to monitor the CR spectrum near the Earth beyond the lifetime of the PAMELA
Dissipation of Magnetohydrodynamic Waves on Energetic Particles: Impact on Interstellar Turbulence and Cosmic Ray Transport
The physical processes involved in diffusion of Galactic cosmic rays in the
interstellar medium are addressed. We study the possibility that the nonlinear
MHD cascade sets the power-law spectrum of turbulence which scatters charged
energetic particles. We find that the dissipation of waves due to the resonant
interaction with cosmic ray particles may terminate the Kraichnan-type cascade
below wavelengths 10^13 cm. The effect of this wave dissipation has been
incorporated in the GALPROP numerical propagation code in order to asses the
impact on measurable astrophysical data. The energy-dependence of the
cosmic-ray diffusion coefficient found in the resulting self-consistent model
may explain the peaks in the secondary to primary nuclei ratios observed at
about 1 GeV/nucleon.Comment: 15 pages, 20 figures, 1 table, emulateapj.cls; To be published in ApJ
10 May 2006, v.64