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- This workshop was organized with the help
of:
- This workshop was organized with the help
of:
LIST of ABSTRACTS
| Scientific requirements and
prospects |
Gamma-ray Optics |
| Focal plane
instrumentation |
Ground facilities and
Flight systems |
Barret Didier
Focusing on X-ray binaries and microquasars
Gamma-rays above a few hundreds of keV carry unique informations on the thermal and non-thermal emission mechanisms at work around compact stars. In this talk, I will discuss the prospects of observing X-ray binaries, including microquasars, with a high-sensitivity focussing gamma-ray instrument. In particular I will emphasize the complementarity of gammay-ray observations with those performed in the classical X-ray band..
Giommi Paolo & Colafrancesco Sergio
Cosmological impact of gamma-ray observations of point-like and diffuse extragalactic sources: the cases of Blazars and clusters of galaxies
Gamma-ray observations can provide unique information on some crucial astrophysical and cosmological aspects of extragalactic sources. I will review the impact of gamma-ray observations of two specific classes of cosmologically relevant structures:
1) Blazars, which are the main extragalactic contaminants of the CMB temperature and polarization anisotropy spectrum (a fundamental tool to probe the physics of the early universe);
2) Galaxy clusters, which are the largest bound structures in the universe, and thus the largest containers of cosmic material (Dark Matter, baryons, cosmic rays), whose study might yield unique information of the cosmological parameters, on the nature of Dark Matter and on the origin of cosmic rays in the universe..Comastri Andrea
The sources of the hard X-ray / gamma-ray backgrounds
The fraction of the hard X-ray background (XRB) resolved into discrete sources strongly depends from the considered energy range While around a few keV deep Chandra and XMM surveys have essentially resolved the entire XRB flux above 7-8 keV no more than 50\% of the flux is due to resolved sources. At energies greater than 10 keV, where the bulk of the CXB energy density is produced, the resolved fraction is negligible, being strongly limited by the lack of imaging X--ray observations. Within the framework of AGN synthesis models the shape of the CXB spectrum and intensity in the 10--100 keV range is modeled assuming an important contribution from heavily obscured Compton thick ($N_H > 1.5 \times 10^{24}$ cm$^{-2}$) sources around 20--30 keV. Moreover a high energy cut--off ($E_{cut}$) usually parameterized as an exponential roll--over with an e--folding energy of the order of a few hundreds of keV has to be present in the high energy spectrum of all the sources in order not to overproduce the observed flux above 100 keV. The lack of a detailed knowledge of the absorption distribution at the highest column densities and of the distribution of exponential cut-off leaves a relatively wide portion of the parameter space unexplored. It can be shown that depending on the adopted high energy cut-off and the poorly known redshift evolution of the most heavily obscured sources the shape of the residual (not yet directly resolved) CXB spectrum is strongly dependent from the value assumed for the above mentioned parameters. An additional source of uncertainties comes from the contribution to the hard (around 100 keV) background from radio loud blazars which are known to provide a significant contribution to the gamma-ray background. I will discuss how high energy (above a few tens of keV) observations will improve our present understanding of the physical mechanisms associated to the AGN primary emission mechanism and the cosmological evolution of the most obscured sources.
Griffiths Richard E.
AGN at High Energy and the Prospects for 511 keV annihilation lines
The properties of known Active Galactic Nuclei are reviewed from the point of view of their high energy X-ray and gamma-ray spectra. The requirements on focussing telescopes in this energy range are then derived. Special emphasis is given to the possibility that AGN are also sources of 511 keV annihilation radiation. Nearby AGN such as M87 in Virgo, NGC 1275 and Cen A are excellent candidates for the detection of this emission.
Hernanz Margarita
Nucleosynthesis in nova explosions: prospects for their observation with focusing telescopes
Nova explosions are caused by the explosive burning of hydrogen in the envelope of accreting white dwarfs. During the thermonuclear runaway many radioactive isotopes are synthesized, which emit gamma-rays when they decay. The gamma-ray signatures of a nova explosion still remain undetected, because even the best instruments like SPI onboard INTEGRAL are not sensitive enough for the dim and broad lines emitted by novae at their typical distances. A very different situation is expected with a focusing telescope, like MAX. Prospects for detectability with a future gamma-ray lens telescope will be presented, with a special emphasis on the important information that gamma-rays would provide about the explosion mechanism and the underlying white dwarf star.
Knodlseder Jürgen
Prospects in Space-Based Gamma-Ray Astronomy
At the uppermost part of the electromagnetic spectrum, observations of the gamma-ray sky reveal the most powerful sources and the most violent events in the Universe. While at lower wavebands the observed emission is generally dominated by thermal processes, the gamma-ray sky provides us with a view on the non-thermal Universe, where particles are accelerated by still poorly understood mechanisms to extremely relativistic energies, and nuclear interactions, reactions, and decays are organising the basic elements of which our world is made of. Cosmic accelerators and cosmic explosions are the major science themes that are addressed in this waveband.
With the unequalled INTEGRAL observatory, ESA has provided a unique tool to the astronomical community that has made Europe the world leader in the field of gamma-ray astronomy. INTEGRAL provides an unprecedented survey of the soft gamma-ray sky, revealing hundreds of sources of different kinds, new classes of objects, extraordinary views of antimatter annihilation in our Galaxy, and fingerprints of recent nucleosynthesis processes.
While INTEGRAL provides the longly awaited global overview over the soft gamma-ray sky, there is a growing need to perform deeper, more focused investigations of gamma-ray sources, comparable to the step that has been taken in X-rays by going from the ROSAT survey satellite to the more focused XMM-Newton observatory. Technological advances in the past years in the domain of gamma-ray focusing using Laue diffraction techniques have paved the way towards a future European gamma-ray mission, that will outreach past missions by large factors in sensitivity and angular resolution. Such a future Gamma-Ray Imager will allow to study particle acceleration processes and explosion physics in unprecedented depth, providing essential clues on the intimate nature of the most violent and most energetic processes in the Universe.Leising Mark
We discuss the contributions to our understanding of supernova physics that a high-sensitivity focusing gamma-ray instrument can uniquely provide. For a number of objectives we outline the instrument parameters, especially sensitivity and energy resolution, required. We discuss the number of supernovae that can plausibly be expected to be studied in detail with such an instrument considering realistic observing constraints and the likely efficiency of supernova discoveries in the future.
Olive Jean-François
Focusing on pulsars
Energetic and young pulsars accelerate electrons and positrons to very high energies in their magnetosphere. The location of the accelerating regions is still being debated. The particle outflows can interact with the surrounding medium to produce synchrotron nebulae. Our understanding of the pulsars and their nebulae has increased dramatically in the past few years, with the high resolution imaging X-ray telescopes. These observations have revealed the complex and time-varying structures of the centermost regions with rings, jets and hot spots. In this talk, the advantage to observe such systems with a sensitive focusing gamma-ray telescope will be presented. In particular, I will discuss the instrument parameters in terms of energy and angular resolution that will allow us to infer very important informations on the particle winds and the pulsating neutron stars themselves.
Sazonov Sergey
Annihilation of positrons in the Galaxy
The Galactic Center is a site of copious production of positrons, whose origin is not clear - the hypotheses range from stellar nucleosynthesis to annihilating dark matter. INTEGRAL has made the most precise measurements of the GC annihilation spectrum, sheding some light on the properties of the ISM where positrons are annihilating. The measured width of the 511 keV line is 2.37+/-0.25 keV, while the strength of the ortho-positronium continuum suggests that most positrons (94+/-6 per cent) form positronium before annihilation. These spectral parameters can be explained by a warm (7000-40000 K) gas with degree of ionization larger than a few per cent. One of the widespread ISM phases - warm (8000 K) and weakly ionized - satisfies these criteria. The observed spectrum can also be explained by annihilation in a multiphase ISM. We discuss how future telescopes with higher angular resolution and lower background could improve our understanding of the GC 511 keV emission.
Smith David
Puzzles and Potential for Gamma-ray Line Observations of Solar Flare Ion Acceleration
The acceleration and interaction of high-energy electrons in solar flares can be studied via the copious bremsstrahlung x-rays they produce and their synchrotron emission in the radio and microwave. To study flare-accelerated ions at the Sun, we use gamma-ray lines from positron annihilation, nuclear de-excitation, and neutron capture. Recent data from RHESSI have shown that the positron-annihilation line is often surprisingly broad, offering a unique probe into the temperature, density and ionization state of the flaring atmosphere. The Doppler profiles of the de-excitation lines provide information on the direction and angular distribution of the accelerated ions. But these observations have only been made on the very brightest flares due to sensitivity limitations. In addition to reviewing the state of our knowledge and what we'd like to observe next, I will give sensitivity requirements and discuss the required field of view and pointing issues. I will close by touching briefly on the need for hard x-ray imaging instrumentation to study electron acceleration in microflares and nanoflares.
Update : September 2005
Questions and comments : Peter von Ballmoos