4,777 research outputs found

### The IBIS view of the galactic centre: INTEGRAL's imager observations simulations

The Imager on Board Integral Satellite (IBIS) is the imaging instrument of
the INTEGRAL satellite, the hard-X/soft-gamma ray ESA mission to be launched in
2001. It provides diagnostic capabilities of fine imaging (12' FWHM), source
identification and spectral sensitivity to both continuum and broad lines over
a broad (15 keV--10 MeV) energy range. It has a continuum sensitivity of
2~10^{-7} ph cm^{-2} s^{-1} at 1 MeV for a 10^6 seconds observation and a
spectral resolution better than 7 % at 100 keV and of 6 % at 1 MeV. The imaging
capabilities of the IBIS are characterized by the coupling of the above quoted
source discrimination capability with a very wide field of view (FOV), namely 9
x 9 degrees fully coded, 29 x 29 degrees partially coded FOV. We present
simulations of IBIS observations of the Galactic Center based on the results of
the SIGMA Galactic Center survey. They show the capabilities of this instrument
in discriminating between different sources while at the same time monitoring a
huge FOV. It will be possible to simultaneously take spectra of all of these
sources over the FOV even if the sensitivity decreases out of the fully coded
area. It is envisaged that a proper exploitation of both the FOV dimension and
the source localization capability of the IBIS will be a key factor in
maximizing its scientific output.Comment: 5 pages, LaTeX, to be published in the 4th Compton Symposium
Conference Proceedings, uses aipproc.cls, aipproc.sty (included

### Low-energy gamma rays from Cygnus X-1

The Cyg X-1 was observed by the balloonborne telescope OPALE, in June 1976. The high energy spectrum of the source, which was in its superlow state, was seen to extend well beyond 1 MeV. The observed low energy gamma ray component of Cyg X-1 is compared with the predictions of recent models involving accretion onto a stellar black hole, and including a possible contribution from the pair annihilation 511 keV gamma ray line

### Contact Moishezon threefolds with second Betti number one

We prove that the only contact Moishezon threefold having second Betti number
equal to one is the projective space.Comment: 5 pages. v2: exposition improved as suggested by the referee. To
appear in Archiv der Mat

### An Estimate of the Spectral Intensity Expected from the Molecular Bremsstrahlung Radiation in Extensive Air Showers

A detection technique of ultra-high energy cosmic rays, complementary to the
fluorescence technique, would be the use of the molecular Bremsstrahlung
radiation emitted by low-energy electrons left after the passage of the showers
in the atmosphere. The emission mechanism is expected from quasi-elastic
collisions of electrons produced in the shower by the ionisation of the
molecules in the atmosphere. In this article, a detailed calculation of the
spectral intensity of photons at ground level originating from the transitions
between unquantised energy states of free ionisation electrons is presented. In
the absence of absorption of the emitted photons in the plasma, the obtained
spectral intensity is shown to be 5 10^{-26} W m^{-2}Hz^{-1} at 10 km from the
shower core for a vertical shower induced by a proton of 10^{17.5} eV.Comment: 16 pages, 6 figures, accepted in Astroparticle Physics. Compared to
v1 version: 1. Inclusion of ro-vibrational processes. 2. Use of more accurate
ionization potential values and energy distribution of the secondary
electron

### Compton telescope with coded aperture mask: Imaging with the INTEGRAL/IBIS Compton mode

Compton telescopes provide a good sensitivity over a wide field of view in
the difficult energy range running from a few hundred keV to several MeV. Their
angular resolution is, however, poor and strongly energy dependent. We present
a novel experimental design associating a coded mask and a Compton detection
unit to overcome these pitfalls. It maintains the Compton performance while
improving the angular resolution by at least an order of magnitude in the field
of view subtended by the mask. This improvement is obtained only at the expense
of the efficiency that is reduced by a factor of two. In addition, the
background corrections benefit from the coded mask technique, i.e. a
simultaneous measurement of the source and background. This design is
implemented and tested using the IBIS telescope on board the INTEGRAL satellite
to construct images with a 12' resolution over a 29 degrees x 29 degrees field
of view in the energy range from 200 keV to a few MeV. The details of the
analysis method and the resulting telescope performance, particularly in terms
of sensitivity, are presented

### EGRET Observations of the Diffuse Gamma-Ray Emission in Orion: Analysis Through Cycle 6

We present a study of the high-energy diffuse emission observed toward Orion
by the Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton
Gamma-Ray Observatory. The total exposure by EGRET in this region has increased
by more than a factor of two since a previous study. A simple model for the
diffuse emission adequately fits the data; no significant point sources are
detected in the region studied ($l = 195^\circ$ to $220^\circ$ and $b =
-25^\circ to -10^\circ$) in either the composite dataset or in two separate
groups of EGRET viewing periods considered. The gamma-ray emissivity in Orion
is found to be $(1.65 \pm 0.11) \times 10^{-26} {s sr}^{-1}$ for E > 100 MeV,
and the differential emissivity is well-described as a combination of
contributions from cosmic-ray electrons and protons with approximately the
local density. The molecular mass calibrating ratio is $N(H_2)/W_{CO} = (1.35
\pm 0.15) \times 10^{20} cm^{-2} (K km/s)^{-1}$.Comment: 16 pages, including 5 figures. 3 Tables as three separate files.
Latex document, needs AASTEX style files. Accepted for publication in Ap

### High-energy emission from the stellar wind collisions in gamma-2 Velorum

The binary system gamma-2 Velorum (WC8+O7.5) contains the nearest known
Wolf-Rayet star to the Sun, at a distance of 258$_{-31}^{+41}$ pc. Its strong
radio emission shows evidence for a partially absorbed nonthermal component,
which has been interpreted as synchrotron emission from electrons accelerated
in the colliding wind region. Inverse Compton cooling of these electrons in the
intense UV radiation field from the O-type companion star could produce a
significant hard X-ray and gamma-ray emission, whose flux depends on the ratio
of the energy densities of magnetic to seed photon fields. The Vela region was
observed with the INTEGRAL satellite in 2003, as part of the Core Programme. No
signals from gamma-2 Vel are detected in the images obtained with the
IBIS/ISGRI coded aperture instrument in the energy ranges 20-40 and 40-80 keV.
From the derived 3$\sigma$ upper limits, we show that the average magnetic
field near the region of stellar wind collision should be relatively high,
greater than about 1 G. The high-energy emission of gamma-2 Vel might be
detected with the forthcoming GLAST experiment.Comment: 4 pages, 3 figures. To appear in the Proceedings of the 5th INTEGRAL
Workshop: "The INTEGRAL Universe", February 16-20, 2004, Munich, German

### Imaging extended sources with coded mask telescopes: Application to the INTEGRAL IBIS/ISGRI instrument

Context. In coded mask techniques, reconstructed sky images are
pseudo-images: they are maps of the correlation between the image recorded on a
detector and an array derived from the coded mask pattern. Aims. The
INTEGRAL/IBIS telescope provides images where the flux of each detected source
is given by the height of the local peak in the correlation map. As such, it
cannot provide an estimate of the flux of an extended source. What is needed is
intensity sky images giving the flux per solide angle as typically done at
other wavelengths. Methods. In this paper, we present the response of the
INTEGRAL IBIS/ISGRI coded mask instrument to extended sources. We develop a
general method based on analytical calculations in order to measure the
intensity and the associated error of any celestial source and validated with
Monte-Carlo simulations. Results. We find that the sensitivity degrades almost
linearly with the source extent. Analytical formulae are given as well as an
easy-to-use recipe for the INTEGRAL user. We check this method on IBIS/ISGRI
data but these results are general and applicable to any coded mask telescope.Comment: 9 pages, 6 figures, Accepted for publication in A&

### Maximum solutions of normalized Ricci flows on 4-manifolds

We consider maximum solution $g(t)$, $t\in [0, +\infty)$, to the normalized
Ricci flow. Among other things, we prove that, if $(M, \omega)$ is a smooth
compact symplectic 4-manifold such that $b_2^+(M)>1$ and let
$g(t),t\in[0,\infty)$, be a solution to (1.3) on $M$ whose Ricci curvature
satisfies that $|\text{Ric}(g(t))|\leq 3$ and additionally $\chi(M)=3 \tau
(M)>0$, then there exists an $m\in \mathbb{N}$, and a sequence of points
$\{x_{j,k}\in M\}$, $j=1, ..., m$, satisfying that, by passing to a
subsequence, $(M, g(t_{k}+t), x_{1,k},..., x_{m,k})
\stackrel{d_{GH}}\longrightarrow (\coprod_{j=1}^m N_j, g_{\infty},
x_{1,\infty}, ...,, x_{m,\infty}),$ $t\in [0, \infty)$, in the $m$-pointed
Gromov-Hausdorff sense for any sequence $t_{k}\longrightarrow \infty$, where
$(N_{j}, g_{\infty})$, $j=1,..., m$, are complete complex hyperbolic orbifolds
of complex dimension 2 with at most finitely many isolated orbifold points.
Moreover, the convergence is $C^{\infty}$ in the non-singular part of
$\coprod_1^m N_{j}$ and
$\text{Vol}_{g_{0}}(M)=\sum_{j=1}^{m}\text{Vol}_{g_{\infty}}(N_{j})$, where
$\chi(M)$ (resp. $\tau(M)$) is the Euler characteristic (resp. signature) of
$M$.Comment: 23 page

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