929 research outputs found
Spectral constraints on unidentified EGRET gamma-ray sources from COMPTEL MeV observations
We investigated the MeV properties of 173 unidentified or only tentatively
identified EGRET sources listed in the third EGRET catalogue, by analyzing the
simultaneously collected COMPTEL MeV data for each individual source. The
sources can generally be divided into 4 groups. In this paper we focus on one
of these, a group of 22 EGRET sources for which we can provide additional
constraining information: their spectral extrapolations from the energy range
above 100 MeV towards lower energies overshoot the fluxes or upper limits
derived simultaneously at MeV energies. This means that for these sources a
spectral turnover/break between 1 MeV and 100 MeV is required. At least two of
these sources, but most likely the majority of this sample, have the maxima of
their gamma-ray luminosities in this energy band. The sources have rather soft
EGRET spectra (average photon index: 2.72), and seem to spatially cluster in
the inner Galaxy. Variability analyses revealed 11 out of the 22 sources to be
significantly variable. Object classes proposed as possible counterparts for
the unidentified EGRET sources are discussed in the light of these additional
constraints.Comment: 9 pages including 4 figures; A&A accepte
Book Review: The Invasion of Indian Country in the Twentieth Century: American Capitalism and Tribal Natural Resources
Book Review (Submitted by Jeffrey S. Hermsen) - The Invasion of Indian Country in the Twentieth Century: American Capitalism and Tribal Natural Resources, by Donald L. Fixic
COMPTEL observations of the blazars 3C 454.3 and CTA 102
We have analyzed the two blazars of 3C 454.3 and CTA 102 using all available COMPTEL data from 1991 to 1999. In the 10â30 MeV band, emission from the general direction of the sources is found at the 4Ï-level, being consistent with contributions from both sources. Below 10 MeV only 3C 454.3 is significantly detected, with the strongest evidence (5.6 Ï) in the 3â10 MeV band. Significant flux variability is not observed for both sources, while a low emission is seen most of the years in the 3â10 MeV light curve for 3C 454.3. Its time-averaged MeV spectrum suggests a power maximum between 3 to 10 MeV
COMPTEL observations of the Virgo blazars 3C 273 and 3C 279
We report the main MeV properties (detections, light curves, spectra) of the Virgo blazars 3C 273 and 3C 279 which were derived from a consistent analysis of all COMPTEL Virgo observations between 1991 and 1997
COMPTEL Observations of the Gamma-Ray Blazar PKS 1622-297
We report results of observations and analyses on the gamma-ray blazar PKS
1622-297, with emphasis on the COMPTEL data (0.75 - 30 MeV) collected between
April 1991 and November 1997. PKS 1622-297 was detected as a source of
gamma-rays by the EGRET experiment aboard CGRO in 1995 during a gamma-ray
outburst at energies above 100 MeV lasting for five weeks.
In this time period the blazar was significantly (~ 5.9 sigma) detected by
COMPTEL at 10-30 MeV. At lower COMPTEL energies the detection is marginal,
resulting in a hard MeV spectrum.
The combined COMPTEL/EGRET energy spectrum shows a break at MeV energies. The
broad-band spectrum (radio - gamma-rays) shows that the gamma-ray emission
dominates the overall power output. On top of the 5-week gamma-ray outburst,
EGRET detected a huge flare lasting for > 1 day. Enhanced MeV emission (10 - 30
MeV) is found near the time of this flare, suggesting a possible time delay
with respect to the emission above 100 MeV. Outside the 5-week flaring period
in 1995, we do not detect MeV emission from PKS 1622-297.Comment: 10 pages including 9 figures, accepted for publication in A&
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