355 research outputs found
LS 5039 - the counterpart of the unidentified MeV source GRO J1823-12
The COMPTEL experiment on CGRO observed the gamma-ray sky at energies from
0.75 MeV to 30 MeV between April 1991 and June 2000. COMPTEL detected many
gamma-ray sources, among them an unidentified one labeled GRO J1823-12, which
is positionally consistent with the prominent high-mass X-ray binary LS 5039.
Because LS 5039 was established as gamma-ray emitter during recent years, whose
gamma-radiation radiation is modulated along its binary orbit, we reanalysed
the COMPTEL data of GRO J1823-12 including an orbital resolved analysis. We
find a significant MeV source, showing evidence for a modulated MeV flux
corresponding to the orbital period of LS 5039 of about 3.9 days. We show that
its MeV emission is stronger at the orbital part around the inferior conjuction
than at the part of the superior conjunction, being in phase with X-rays and
TeV gamma-rays, however being in anti-phase with GeV gamma-rays. We conclude
that the COMPTEL source GRO J1823-12 is the counterpart of the microquasar
candidate LS 5039, at least for the majority of its MeV emission. The COMPTEL
fluxes, put into multifrequency perspective, provide new constraints on the
modelling of the high-energy emission of LS 5039.Comment: accepted by Astronomy & Astrophysics; 11 pages, 9 figure
A model for the high-energy emission of Cyg X-1
We construct a model of Cyg X-1 which describes self-consistently its
emission from soft X-rays to MeV gamma rays. Instead of a compact
pair-dominated gamma-ray emitting region, we consider a hot optically thin and
spatially extended proton-dominated cloud surrounding the whole accretion disc.
The gamma-ray emission is due to the bremsstrahlung, Comptonization, and
positron annihilation, while the corona-disc model is retained for the X-ray
emission. We show that the Cyg X-1 spectrum accumulated by OSSE, BATSE, and
COMPTEL in 1991--95, as well as the HEAO-3 gamma1 and gamma2 spectra can be
well fitted by our model. The derived parameters are in qualitative agreement
with the picture in which the spectral changes are governed by the mass flow
rate in the accretion disc. In this context, the hot outer corona could be
treated as the advection-dominated flow co-existing with a standard thin
accretion disc.Comment: 5 pages including 2 figures, latex, aipproc.sty, aipproc.cls,
epsfig.sty. To be published in Proc. 4th Compton Symp., 1997 (27-30 April,
Williamsburg, Virginia
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
An Unidentified Variable Gamma-Ray Source near the Galactic Plane Detected by COMPTEL
We report the detection of an unidentified gamma-ray source near the Galactic plane by the COMPTEL experiment aboard the Compton Gamma-Ray Observatory. The source is detected at a significance level of ~ 7.2 sigma in the energy range 1-3 MeV and at ~ 4.6 sigma in the lower 0.75-1 MeV band in the time period March to July 1995. At energies above 3 MeV are only marginal hints or upper limits obtained. The MeV spectrum has a soft shape. Strong flux variability is found within one year at energies below 3 MeV. Possible counterparts of galactic and extragalactic nature are discussed
The synchrotron peak shift during high-energy flares of blazars
A prediction for the energy shift of the synchrotron spectrum of
flat-spectrum radio quasars (FSRQs) during high-energy flares is presented. If
the -ray emission of FSRQs is produced by Comptonization of external
radiation, then the peak of the synchrotron spectrum is predicted to move to
lower energies in the flare state. This is opposite to the well-known broadband
spectral behavior of high-frequency peaked BL-Lac objects where the external
radiation field is believed to be weak and synchrotron-self Compton scattering
might be the dominant -ray radiation mechanism. The synchrotron peak
shift, if observed in FSRQs, can thus be used as a diagnostic to determine the
dominant radiation mechanism in these objects. I suggest a few FSRQs as
promising candidates to test the prediction of the external-Comptonization
model.Comment: 9 pages, including 2 figures; uses epsf.sty, rotate.sty; accepted for
ApJ Letters; minor revision
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