687 research outputs found
Centaurus A at Hard X-rays and Soft Gamma-rays
Centaurus A, at a distance of less than 4 Mpc, is the nearest radio-loud AGN.
Its emission is detected from radio to very-high energy gamma-rays. Despite the
fact that Cen A is one of the best studied extragalactic objects the origin of
its hard X-ray and soft gamma-ray emission (100 keV < E < 50 MeV) is still
uncertain. Observations with high spatial resolution in the adjacent soft X-ray
and hard gamma-ray regimes suggest that several distinct components such as a
Seyfert-like nucleus, relativistic jets, and even luminous X-ray binaries
within Cen A may contribute to the total emission in the MeV regime that has
been detected with low spatial resolution. As the Spectral Energy Distribution
of Cen A has its second maximum around 1 MeV, this energy range plays an
important role in modeling the emission of (this) AGN. As there will be no
satellite mission in the near future that will cover this energies with higher
spatial resolution and better sensitivity, an overview of all existing hard
X-ray and soft gamma-ray measurements of Cen A is presented here defining the
present knowledge on Centaurus A in the MeV energy range.Comment: 8 pages, 5 figures, article for conference proceedings "The Many
Faces of Centaurus A"; accepted for publication in PAS
Hadronic-Origin TeV gamma-Rays and Ultra-High Energy Cosmic Rays from Centaurus A
Centaurus A (Cen A) is the nearest radio-loud AGN and is detected from radio
to very high energy gamma-rays. Its nuclear spectral energy distribution (SED)
shows a double-peak feature, which is well explained by the leptonic
synchrotron + synchrotron self-Compton model. This model however cannot account
for the observed high energy photons in the TeV range, which display a distinct
component. Here we show that ~ TeV photons can be well interpreted as the
neutral pion decay products from p-gamma interactions of Fermi accelerated high
energy protons in the jet with the seed photons around the second SED peak at
~170 keV. Extrapolating the inferred proton spectrum to high energies, we find
that this same model is consistent with the detection of 2 ultra-high-energy
cosmic ray events detected by Pierre Auger Observatory from the direction of
Cen A. We also estimate the GeV neutrino flux from the same process, and find
that it is too faint to be detected by current high-energy neutrino detectors.Comment: Phys. Rev. D in press, This article supersedes arXiv:1007.045
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
COMPTEL upper limits for Seyfert galaxies
The gamma‐ray emission of Seyfert galaxies has fallen far short of pre‐GRO expectations. No single object of this class has been detected by either COMPTEL or EGRET, and OSSE has detected only a fraction of the Seyferts expected. To derive a more stringent upper limit to the emission from these objects in the energy ranges 0.75 to 1 and 1 to 3 MeV, we have summed a large number of COMPTEL observations acquired during Phase 1 of the GRO mission. From a total of 47 observations of 23 individual X‐ray selected Seyfert galaxies, we derive preliminary upper limits of 8×10−8 photons/(cm2 s keV) in the 0.75‐1 MeV band and 1×10−8 photons/(cm2 s keV) in the 1‐3 MeV band
COMPTEL observations of the quasar PKS 0528+134 during the first 3.5 years of the CGRO mission
The COMPTEL observations of the blazar-type quasar PKS 0528+134 in the energy
range 0.75 MeV to 30 MeV carried out between April 1991 and September 1994 have
been analyzed. During the first two years PKS 0528+134 was most significantly
detected at energies above 3 MeV. During the last year there is only evidence
for the quasar at energies below 3 MeV indicating a spectral change. The
time-averaged COMPTEL energy spectrum between 0.75 MeV and 30 MeV is well
represented by a power-law shape. Spectra collected from different
observational periods reveal different power-law shapes: a hard state during
flaring observations reported by EGRET, and a soft state otherwise. The
combined simultaneous EGRET and COMPTEL spectra indicate these two spectral
states as well. During low intensisty gamma-ray phases no spectral break is
obvious from the combined COMPTEL and EGRET measurements. For the gamma-ray
flaring phases however, the combined COMPTEL and EGRET data require a spectral
bending at MeV-energies. By fitting broken power-law functions the best-fit
values for the break in photon index range between 0.6 and 1.7, and for the
break energy between ~5 MeV and ~20 MeV. Because the flux values measured by
COMPTEL below 3 MeV in both states are roughly equal, the observations would be
consistent with an additional spectral component showing up during gamma-ray
flaring phases of PKS 0528+134. Such a component could be introduced by e.g. a
high-energy electron-positron population with a low-energy cutoff in their bulk
Lorentz factor distribution. The multiwavelength spectrum of PKS 0528+134 for
gamma-ray flaring phases shows that the major energy release across the entire
electro-magnetic spectrum is measured at MeV-energies.Comment: 10 pages, 8 postscript figures, latex, to appear in: A&A 328, 33
(1997
Recent results from COMPTEL observations of Cygnus X‐1
The COMPTEL experiment on the Compton Gamma‐Ray Observatory (CGRO) has now observed Cyg X‐1 on four separate occasions during phase 1 and phase 2 of its orbital mission (April, 1991 to August, 1993). Here we report on the results of the latest analysis of these data, which provide a spectrum extending to energies greater than 2 MeV. A spectral analysis of these data, in the context of a classical Comptonization model, indicates an electron temperature much higher than previous hard X‐ray measurements would suggest (200 keV vs 60–80 keV). This implies either some limitations in the standard Comptonization model and/or the need to incorporate a reflected component in the hard X‐ray spectrum. Although significant variability near 1 MeV has been observed, there is no evidence for any ‘MeV excess.
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