305 research outputs found
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
Scientific objectives and first results from COMPTEL
The imaging Compton telescope (COMPTEL) is the first imaging telescope in space to explore the MeV gamma ray range. At present it is performing a complete sky survey. In later phases of the mission, selected celestial objects will be studied in more detail. Targets of special interest in the COMPTEL energy range are radio pulsars, X-ray binaries, novae, supernova remnants, molecular clouds, and the interstellar medium within the Milky Way, as well as the nuclei of active galaxies, supernovae, and the diffuse cosmic background radiation in extragalactic space. The first four months of operation demonstrated that COMPTEL basically performs as expected. The Crab is clearly seen at its proper position in the first images of the anticenter region of the Galaxy. The Crab pulsar lightcurve was measured with unprecedented accuracy. The quasar 3C273 was seen for the first time at MeV-energies. Several cosmic bursts within the COMPTEL field of view could be located to an accuracy of about 1 degree. On June 9, 11, and 15, 1991 COMPTEL observed gamma ray (continuum and line) emission from three solar flares. Neutrons were also detected from the June 9 flare. At the present state of analysis, COMPTEL achieves the prelaunch predictions of its sensitivity within a factor of 2. Based on the present performance of COMPTEL, the team is confident that COMPTEL will fulfill its primary mission of surveying and exploring the MeV sky
The Spectral Variability of Cygnus X-1 at MeV Energies
In previous work, we have used data from the first three years of the CGRO
mission to assemble a broad-band -ray spectrum of the galactic black
hole candidate Cygnus X-1. Contemporaneous data from the COMPTEL, OSSE and
BATSE experiments on CGRO were selected on the basis of the hard X-ray flux
(45--140 keV) as measured by BATSE. This provided a spectrum of Cygnus X-1 in
its canonical low X-ray state (as measured at energies below 10 keV), covering
the energy range from 50 keV to 5 MeV. Here we report on a comparison of this
spectrum to a COMPTEL-OSSE spectrum collected during a high X-ray state of
Cygnus X-1 (May, 1996). These data provide evidence for significant spectral
variability at energies above 1 MeV. In particular, whereas the hard X-ray flux
{\it decreases} during the high X-ray state, the flux at energies above 1 MeV
{\it increases}, resulting in a significantly harder high energy spectrum. This
behavior is consistent with the general picture of galactic black hole
candidates having two distinct spectral forms at soft -ray energies.
These data extend this picture, for the first time, to energies above 1 MeV.Comment: 5 pages, 4 figures, to be published in AIP Conf. Proc., "The Fifth
Compton Symposium
Contribution of point sources to the soft gamma-ray Galactic emission
The nature of the soft gamma-ray (20-200 keV) Galactic emission has been a
matter of debate for a long time. Previous experiments have tried to separate
the point source contribution from the real interstellar emission, but with a
rather poor spatial resolution, they concluded that the interstellar emission
could be a large fraction of the total Galactic emission. INTEGRAL, having both
high resolution and high sensitivity, is well suited to reassess more precisely
this problem. Using the INTEGRAL core program Galactic Center Deep Exposure
(GCDE), we estimate the contribution of detected point sources to the total
Galactic flux.Comment: Proceedings of the 5th INTEGRAL Workshop, Munich 16-20 February 2004.
ESA SP-55
SPI Measurements of the Diffuse Galactic Hard X-ray Continuum
INTEGRAL Spectrometer SPI data from the first year of the Galactic Centre
Deep Exposure has been analysed for the diffuse continuum from the Galactic
ridge. A new catalogue of sources from the INTEGRAL Imager IBIS has been used
to account for their contribution to the celestial signal. Apparently diffuse
emission is detected at a level ~10% of the total source flux. A comparison of
the spectrum of diffuse emission with that from an analysis of IBIS data alone
shows that they are consistent. The question of the contribution of unresolved
sources to this ridge emission is still open.Comment: Proceedings of the 5th INTEGRAL Workshop, Munich 16-20 February 2004.
ESA SP-552. Reference to Terrier et al. (2004) updated to include astro-ph
versio
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