Working Papers Astronomy and Astrophysics Panel Reports (1991) / Chapter Skim
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High Energy from Space
High Energy from Space
Pages 129-152
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From page 129... ...
Correlative observations from the Bubble Space Telescope (MST) and eventually the Space Infrared Telescope Facility (SIRTF)
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From page 130... ...
In summary, our vision of a vigorous program for high energy astrophysics in the next decade includes · Launch and extended flight operations of AXAF and GRO, and wide community involvement in their observing programs · At least two new Explorer-class missions in addition to XTE, to address particularly exciting opportunities in X- and y-ray astronomy · Exploitation of smaller, less expensive space missions for important specialized problems · An ambitious program of technology development in optics, detectors, and related hardware · Changes in selected programmatic approaches that affect the research base in our field This program can in the coming decade challenge scientific issues as diverse as stellar chromospheres, relativistic stars, the intergalactic medium, dark matter, the energetics of active galactic nuclei, and the large scale structure of the Universe. Observations of high energy phenomena from space will be fundamental to the astrophysics of the 1990s.
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From page 131... ...
Compact objects are the degenerate end-points of stellar evolution, and include white dwarfs, neutron stars, and stellar-mass black holes. Supernovae are the most visible and violent manifestation of the death of stars, and include both the gravitational collapse of massive stars and the nuclear detonation of white dwarf stars.
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From page 132... ...
For example, neutron stars commonly have surface magnetic field strengths ~ 10~2 G 106 times stronger than the strongest magnetic fields that have been produced on Earth.
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From page 133... ...
However, no totally convincing proof of the existence of black holes, or method of unambiguously differentiating them from neutron stars, is available. Progress thus far has been made primarily through a combination of soft X-ray observations, and optical observations of the source counterparts.
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From page 134... ...
Within our own Galaxy, accretion fuels the most powerful high energy stellar sources, the X-ray binary systems. As noted above, on a much larger scale, accretion onto massive black holes is believed to provide the energy source for nearly all varieties of active galactic nuclei, the most intrinsically luminous cosmic sources currently known.
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From page 135... ...
Elemental abundances can also be determined in this way. Improved hard X-ray spectroscopy yields detections of cyclotron absorption and emission features originating in the magnetospheres of accreting neutron stars.
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From page 136... ...
Pioneering studies with the Einstein Observatory demonstrate that X-ray observations can make a unique and important contribution to this question. The reason is that observations of hot gas in many galaxies and in most or all clusters and groups of galaxies traces the gravitational potential of the dark matter with a precision that far exceeds that of any other method.
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From page 137... ...
The range of astrophysics objectives targeted by GRO is extremely diverse, and includes high-energy phenomena in the vicinity of neutron stars and stellar-mass black holes, such as accretion processes, pair-plasmas, magnetic fields, and particle acceleration; determination of the origin of ,-ray bursts; the energetics and emission mechanisms of AGN; nucleosynthesis in massive stars, novae, and supernovae; energetic particle interactions in molecular clouds and the ISM; Gray line and continuum emission from solar flares; and the diffuse cosmic y-ray background. It is essential for the scientific success of GRO and for the future health ~y-ray astronomy that this mission, as part of the Great Observatory program, enjoy widespread participation from the astronomical community.
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From page 138... ...
The XTE program has been started, and should be completed in a timely fashion. There are good scientific reasons to expect that a full understanding of processes occurring near the event horizons of black holes and the surfaces of neutron stars will require instruments with even better sensitivity and higher time resolution than XTE.
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From page 139... ...
The mission has great generality, devoted to pointed observations of X-ray sources with all four imaging spectrometers coaligned; the angular resolution of the telescopes is ~ 2' EWHM, which is constant over the entire 30' FOV, with energy resolution ~ 100200 eV over the range 0.1-12 keV. After an initial proprietary season of about nine months for the PI teams, US guest investigators will be entitled to 15% exclusive use of the facility, and another 25% in collaboration with Japanese investigators.
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From page 140... ...
possibility of switching to a high throughput concept unconnected to the Station, should this become necessary to accomplish the science in a timely and economic manner. Major Missions for the 1990s The highest priority major programs in high energy astrophysics from space are the rapid completion and flight of the Advanced X-ray Astrophysics Facility, together with integration of the AXAF Science Center into the worldwide astronomical community as a smoothly functioning entity; and the successful flight and reduction of data from the Gamma Ray Observatory.
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From page 141... ...
While this may be sufficient, for example, for currently achievable sensitivities of 10-7 photons cm~2 s~i keV~i at 1 MeV, with the order-of-magnitude improvement in sensitivity expected for post-GRO instruments, angular resolution of considerably better than 1° is needed to avoid severe source confusion. Similarly, while 1° resolution is sufficient for determining the spectra of bright y-ray emitting objects, detection and identification of fainter objects will require arc minute resolution or better.
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From page 142... ...
In the case of accretion onto magnetic neutron stars, phase-resolved spectroscopy of cyclotron line emission will be critical in determining the magnetic field and plasma characteristics of the accretion column. No high sensitivity survey of hard X-ray and soft y-ray emitting objects currently exists.
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From page 143... ...
The detection of nuclear lines, if accomplished, would provide significant new information on the energetic particle environment in AGN. Similar studies of galactic compact objects, in particular neutron stars and stellar-mass black holes, would yield correspondingly important measurements of the plasma and energetic particle environment around these objects.
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From page 144... ...
. Following the GRO mission, there will be a need for high-energy -ray observations with sufficient sensitivity and angular resolution to accurately locate sources, and to define detailed spatial features of emission regions such as molecular clouds, Galactic arms, and nearby galaxies, as well as to measure variations of compact sources.
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From page 145... ...
It is remarkable that for a total mass of 100 kg, including both instrumentation and platform, one can cover the UV, X-ray, and ~y-ray bands simultaneously, with sensitivity and angular resolution almost surely sufficient to make very significant, and quite possibly definitive, progress on the issue of the nature of y-ray burst sources. As a specific example of future possibilities, we note that recent innovations in X-ray optical design raise the possibility of a Wide Field X-ray Telescope in this small mission category.
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From page 146... ...
CCDs certainly present an attractive option for future missions because of the combination of very high spatial resolution coupled with moderate energy resolution that they provide. The use of CCDs for X-ray astronomy has consequently received much attention in this field, both in the US and abroad.
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From page 147... ...
ultra-thin windows improved reflection gratings · improved transmission gratings · various types of polarimeters y-ray Astronomy VI-19 It is likely that the scientific objectives of y-ray astrophysics in the 21st century will be addressed by multiple missions, involving a mix of platforms for scientific observations. Examples include: a broadband high-sensitivity, high-spectral resolution mission in the 5 keV to 10 MeV energy range with angular resolution of 1' or better, a high-sensitivity, high-energy mission in the energy range 30 MeV to 100 GeV, also with angular resolution better than 1', and a mission capable of high-sensitivity, high-spectral resolution observations of -ray bursts.
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From page 148... ...
We also urge investigation of other changes in management style for space missions, such as less concentration on formal documentation and management oversight, and more direct management responsibility for involved scientists. The very successful example of the Japanese X-ray astronomy program, which features a fixed budget on a fixed schedule, where scientists and their small management teams make all the tradeoff decisions, is an interesting paradigm.
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From page 149... ...
In the past, each of these initiatives has been handled as a special case, often with extraordinary effort required from NASA Headquarters and the investigators. The NASA Astrophysics Division has attempted unsuccessfully to obtain a regularly recurring budget line item specifically to fund US participation in foreign space missions.
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From page 150... ...
Theoretical Programs Theoretical research in support of space missions has always been crucial for advances in high energy astrophysics, and there are firm grounds to believe it will become yet more vital to the success of the missions of the 90s. As both X- and ~y-ray astronomy shift an increasing fraction of their observations from imaging to spectroscopy, far more sophisticated theoretical models will be required to interpret the data.
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From page 151... ...
The new program for the 1990s which we have described here is of modest incremental cost, as can be seen from the estimates in Table 1, but we believe will lead the discipline into the new millenium with an exciting array of new data, a technology base needed for future experiments, and an infrastructure of both scientists and facilities essential for leadership in the field.
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From page 152... ...
HALL, University of California, Berkeley CYRUS M HOFFMAN, Los Alamos National Laboratory EDWARD KOLB, Fermi National Accelerator Laboratory LAWRENCE M
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