479 research outputs found
Gamma-Ray Telescopes (in "400 Years of Astronomical Telescopes")
The last half-century has seen dramatic developments in gamma-ray telescopes,
from their initial conception and development through to their blossoming into
full maturity as a potent research tool in astronomy. Gamma-ray telescopes are
leading research in diverse areas such as gamma-ray bursts, blazars, Galactic
transients, and the Galactic distribution of aluminum-26.Comment: 11 pages, 6 figures/ in "400 Years of Astronomical Telescopes: A
Review of History, Science and Technology", ed. B.R. Brandl, R. Stuik, & J.K.
Katgert-Merkeli (Exp. Astron. 26, 111-122 [2009]
The MEGA Advanced Compton Telescope Project
The goal of the Medium Energy Gamma-ray Astronomy (MEGA) telescope is to
improve sensitivity at medium gamma-ray energies (0.4-50 MeV) by at least an
order of magnitude over that of COMPTEL. This will be achieved with a new
compact design that allows for a very wide field of view, permitting a
sensitive all-sky survey and the monitoring of transient and variable sources.
The key science objectives for MEGA include the investigation of cosmic
high-energy particle accelerators, studies of nucleosynthesis sites using
gamma-ray lines, and determination of the large-scale structure of galactic and
cosmic diffuse background emission. MEGA records and images gamma-ray events by
completely tracking both Compton and pair creation interactions in a tracker of
double-sided silicon strip detectors and a calorimeter of CsI crystals able to
resolve in three dimensions. We present initial laboratory calibration results
from a small prototype MEGA telescope.Comment: 7 pages LaTeX, 5 figures, to appear in New Astronomy Reviews
(Proceedings of the Ringberg Workshop "Astronomy with Radioactivities III"
The COMPTEL instrumental line background
The instrumental line background of the Compton telescope COMPTEL onboard the
Compton Gamma-Ray Observatory is due to the activation and/or decay of many
isotopes. The major components of this background can be attributed to eight
individual isotopes, namely 2D, 22Na, 24Na, 28Al, 40K, 52Mn, 57Ni, and 208Tl.
The identification of instrumental lines with specific isotopes is based on the
line energies as well as on the variation of the event rate with time,
cosmic-ray intensity, and deposited radiation dose during passages through the
South-Atlantic Anomaly. The characteristic variation of the event rate due to a
specific isotope depends on its life-time, orbital parameters such as the
altitude of the satellite above Earth, and the solar cycle. A detailed
understanding of the background contributions from instrumental lines is
crucial at MeV energies for measuring the cosmic diffuse gamma-ray background
and for observing gamma-ray line emission in the interstellar medium or from
supernovae and their remnants. Procedures to determine the event rate from each
background isotope are described, and their average activity in spacecraft
materials over the first seven years of the mission is estimated.Comment: accepted for publication in A&A, 22 pages, 21 figure
The all-sky distribution of 511 keV electron-positron annihilation emission
We present a map of 511 keV electron-positron annihilation emission, based on
data accumulated with the SPI spectrometer aboard ESA's INTEGRAL gamma-ray
observatory, that covers approximately 95% of the celestial sphere. 511 keV
line emission is significantly detected towards the galactic bulge region and,
at a very low level, from the galactic disk. The bulge emission is highly
symmetric and is centred on the galactic centre with an extension of 8 deg. The
emission is equally well described by models that represent the stellar bulge
or halo populations. The disk morphology is only weakly constrained by the
present data, being compatible with both the distribution of young and old
stellar populations. The 511 keV line flux from the bulge and disk components
is 1.05e-3 ph cm-2 s-1 and 0.7e-3 ph cm-2 s-1, respectively, corresponding to a
bulge-to-disk flux ratio in the range 1-3. Assuming a positronium fraction of
0.93 this translates into annihilation rates of 1.5e43 s-1 and 3e42 s-1,
respectively. The ratio of the bulge luminosity to that of the disk is in the
range 3-9. We find no evidence for a point-like source in addition to the
diffuse emission, down to a typical flux limit of 1e-4 ph cm-2 s-1. We also
find no evidence for the positive latitude enhancement that has been reported
from OSSE measurements; our 3 sigma upper flux limit for this feature is 1.5e-4
ph cm-2 s-1. The disk emission can be attributed to the beta+ decay of the
radioactive species 26Al and 44Ti. The bulge emission arises from a different
source which has only a weak or no disk component. We suggest that Type Ia
supernovae and/or low-mass X-ray binaries are the prime candidates for the
source of the galactic bulge positrons. Light dark matter annihilation could
also explain the observed 511 keV bulge emission characteristics.Comment: accepted for publication in A&
INTEGRAL and XMM-Newton observations towards the unidentified MeV source GRO J1411-64
The COMPTEL unidentified source GRO J1411-64 was observed by INTEGRAL, and
its central part, also by XMM-Newton. The data analysis shows no hint for new
detections at hard X-rays. The upper limits in flux herein presented constrain
the energy spectrum of whatever was producing GRO J1411-64, imposing, in the
framework of earlier COMPTEL observations, the existence of a peak in power
output located somewhere between 300-700 keV for the so-called low state. The
Circinus Galaxy is the only source detected within the 4 location error
of GRO J1411-64, but can be safely excluded as the possible counterpart: the
extrapolation of the energy spectrum is well below the one for GRO J1411-64 at
MeV energies. 22 significant sources (likelihood ) were extracted and
analyzed from XMM-Newton data. Only one of these sources, XMMU
J141255.6-635932, is spectrally compatible with GRO J1411-64 although the fact
the soft X-ray observations do not cover the full extent of the COMPTEL source
position uncertainty make an association hard to quantify and thus risky. The
unique peak of the power output at high energies (hard X-rays and gamma-rays)
resembles that found in the SED seen in blazars or microquasars. However, an
analysis using a microquasar model consisting on a magnetized conical jet
filled with relativistic electrons which radiate through synchrotron and
inverse Compton scattering with star, disk, corona and synchrotron photons
shows that it is hard to comply with all observational constrains. This and the
non-detection at hard X-rays introduce an a-posteriori question mark upon the
physical reality of this source, which is discussed in some detail
Внедрение корпоративной системы управления проектами для реализации стратегических проектов компании
В целях реализации Восточной газовой программы ООО "Газпром трансгаз Томск" разработало Стратегию развития до 2020 г. и План организационно-технических мероприятий. Для успешного выполнения перспективных задач в организации формируется новая, современная система управления - корпоративная система управления проектами. Разработана методика внедрения корпоративной системы управления проектами, состав системы, определены ключевые участники и критические факторы успеха, выполнен расчет экономической эффективности проекта
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