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On the Advanced X-ray Astrophysics Facility
On the Advanced X-ray Astrophysics
Facility
On April 28, 1993, Space Studies Board Chair Louis J. Lanzerotti sent the
following letter to Dr. Wesley T. Huntress, Jr., associate administrator for NASA's
Office of Space Science.
In a letter to me dated September 15, 1993, from Mr. Joseph Alexander,
Assistant Associate Administrator for Space Science and Applications, NASA
requested that the National Research Council (NRC) conduct a scientific
evaluation of the restructured Advanced X-ray Astrophysics Facility (AXAF).
Working jointly with the NRC's Board on Physics and Astronomy, the Space
Studies Board established a Task Group on AXAF to perform this study. I am
pleased to enclose the report of this task group.
Please contact me if you have any questions about the report.
SCIENTIFIC ASSESSMENT OF THE RESTRUCTURED PROGRAM
FOR THE ADVANCED X-RAY ASTROPHYSICS FACILITY (AXAF)
April 28, 1993
Summary
The Task Group on AXAF (TGA), a joint panel of the Space Studies
Board and the Board on Physics and Astronomy, finds that the restructured
AXAF program-consisting of AXAF-I, to be launched into a high-Earth orbit in
1998, and AXAF-S, to be launched into a polar, low-Earth orbit in 1999-is fully
capable of meeting the primary scientific goals of the former AXAF program.
Although the need to reduce substantially the total cost of the program has led to
shorter mission lifetimes, the expected increase in operating efficiency partly
makes up for this shortfall. The TGA concludes that the revised AXAF program
continues to meet the scientific expectations set forth in previous NRC reports,
which have recommended AXAF as the highest-priority, new, large-scale
program in astronomy.
Thus the TGA urges NASA to proceed with the implementation of the
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On the Advanced X-ray Astrophysics Facility
restructured AXAF program and to make every effort to ensure the launch of both
AXAF-I and AXAF-S before the end of this decade.
Background
In a letter dated September 15, 1992, from Joseph K. Alexander,
Assistant Associate Administrator for Space Sciences and Applications, to Louis
J. Lanzerotti, Chair of the Space Studies Board, NASA asked the National
Research Council (NRC) to evaluate the scientific content and the expected
scientific return of the restructured AXAF program. In response to this request the
Space Studies Board and the Board on Physics and Astronomy jointly
established the Task Group on AXAF (TGA) as a subpanel of the newly formed
Committee on Astronomy and Astrophysics. Arthur F. Davidsen, of Johns
Hopkins University, was appointed Chair of the TGA. The full membership of the
task group is attached. The TGA was asked to carry out its review and evaluation
of the AXAF reconfiguration by the end of 1992.
The TGA held several meetings via teleconference during October and
November 1992 and developed a set of questions concerning the reconfigured
AXAF. These were addressed to AXAF program officials at NASA Headquarters
and AXAF project officials at the Marshall Space Flight Center. In addition, all
members of the AXAF Science Working Group were invited to provide to the TGA
information and comments concerning the revised program. Martin Weisskopf,
AXAF Project Scientist, provided an extensive written response comparing the
scientific capabilities of the original and revised AXAF programs, and Peter
Ulrich, AXAF Program Manager, provided written materials concerning the
programmatic aspects of the restructuring. The TGA discussed all the responses
in a teleconference on December 3, 1992, and held a meeting in Washington,
D.C., on December 10 and 11, 1992, at which it heard presentations concerning
the restructuring and had discussions with the several AXAF scientists and
managers who attended part of the meeting. This report presents the TGA's
conclusions and recommendations concerning the AXAF program.
This report was reviewed and discussed by the parent boards of the TGA,
the Space Studies Board and the Board on Physics and Astronomy, as well as by
the new joint Committee on Astronomy and Astrophysics of the two boards
(membership lists attached). Each of these reviews concurred fully with the
substance and findings of the report.
Previous NRC Recommendations for AXAF
The AXAF mission has been anticipated and endorsed consistently by the
decadal studies of astronomy and astrophysics carried out under the NRC's
Board on Physics and Astronomy by the Bahcall committee and by the Field
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On the Advanced X-ray Astrophysics Facility
committee before that, and by several reports of the Space Studies Board (and
its predecessor, the Space Science Board) and its committees:
In 1979 (just prior to the launch of the Einstein satellite), the
Committee on Space Astronomy and Astrophysics of the Space Science Board,
in their document entitled A Strategy for Space Astronomy and Astrophysics for
the 1980's (National Academy of Sciences, 1979), envisioned and recommended
"a semipermanent (several-decade) national observatory facility . . . open to all
astronomers and with instrument-changing possibilities . . . . More than an order-
of-magnitude improvement in sensitivity over HEAO-2 (Einstein) is required to
allow high-resolution spectroscopy and in-depth studies of specific objectives
such as clusters of galaxies and active galaxies. This can be achieved by a
combination of greater telescope size, better optical surfaces, improved focal-
plane instrument sensitivity, and longer mission duration compared with HEAO-
2." (p. 13)
The Field Committee report (Astronomy and Astrophysics for the
1980's, Volume I, National Academy Press, 1982) identified four key programs of
critical importance for the advancement of astronomy and astrophysics in the
1980s. That committee's top priority was AXAF, which was envisioned as "a
permanent national observatory in space, to provide x-ray pictures of the
Universe comparable in depth and detail with those of the most advanced optical
and radio telescopes. . . . [T]his facility will combine greatly improved angular and
spectral resolution with a sensitivity up to one hundred times greater than that of
any previous x-ray mission." (p. 15)
In the report Long-Lived Space Observatories for Astronomy and
Astrophysics (National Academy Press, 1987), the Space Science Board's
Committee on Space Astronomy and Astrophysics stated that it "concurred with
the recommendations of the Astronomy Survey Committee (1980), which urges
the construction of AXAF . . . . [It] will play a fundamental role in the future
progress of astronomy and astrophysics." (p. 2)
In Space Science in the 21st Century (National Academy Press,
1988), the Space Science Board's Task Group on Astrophysics and Astronomy
found that "[t]he powerful capabilities of AXAF and the wealth of fundamental
problems it can address suggest that this facility will advance research [in x-ray
astronomy] for a long time to come." (p. 27)
The Astronomy and Astrophysics Survey Committee of the Board on
Physics and Astronomy (The Decade of Discovery in Astronomy and
Astrophysics, National Academy Press, 1991) found that AXAF "will return the
United States to preeminence in x-ray astronomy . . . [and] have a major impact
on almost all areas of astronomy . . . ." That committee reaffirmed the Field
Committee decision making AXAF "the highest-priority large program" of the
1990s. (pp. 64-65)
The TGA finds that the scientific performance of AXAF that was
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On the Advanced X-ray Astrophysics Facility
anticipated by these previous studies will still be achieved by the restructured
program. It is obvious, however, that the authors of these earlier reports
envisioned AXAF as a permanent or at least semipermanent x-ray observatory,
with an associated program of maintenance that would include new focal-plane
instrumentation. However, because the costs associated with such a program are
too high to sustain in the current budget environment, NASA and the AXAF
Science Working Group have decided that a pair of limited-life missions is a
preferable scenario for accomplishing the scientific goals of the AXAF program.
The TGA endorses this view and believes that the revised AXAF program will
satisfy the scientific expectations encompassed by previous NRC committee
reports, even though it will not provide a permanent x-ray observatory in space.
The new program should be designed to ensure that the capabilities of AXAF will
still be made available to the broad astronomical community through a vigorous
guest observer program.
Recent Developments in X-ray Astronomy
Since the AXAF program was first conceived in the 1970s, the field of x-
ray astronomy has progressed considerably. The TGA finds that recent
developments have only strengthened the arguments in previous NRC reports
supporting the need for the enhanced imaging and spectroscopic capabilities that
the AXAF program can provide. A few examples are cited below.
Several important results have recently come from the imaging detectors
on ROSAT. A prime example from galactic studies is the detection of multiple low-
luminosity x-ray sources in the cores of globular clusters. These may be the long-
sought cataclysmic variables (white dwarfs that have captured binary
companions in the dense cluster cores) and are only marginally resolved even
with the ROSAT High-resolution Detector. Thus, the much higher spatial
resolution of AXAF-I will be critical for more detailed studies.
There are many new extragalactic results: one is the detection of
extended x-ray emission around NGC 1068. Coupled with earlier work on NGC
4151, we now have solid evidence that a hot medium exists around the centers of
active galactic nuclei (AGNs) and that spatially resolved, moderate-resolution x-
ray spectroscopy will be an important tool for studying both the active nucleus
and the surrounding medium.
ROSAT has detected substructure in all clusters of galaxies observed.
With detectors that provide both imaging and spectroscopic information, AXAF
will measure the mass of galaxy groupings within a cluster and trace out the
mass distribution. AXAF will provide a consistency check for the assumption of
hydrostatic equilibrium, since x-ray-emitting shock waves should be present if
hydrostatic equilibrium does not apply.
ROSAT has shown that many, if not most, AGNs are strongly absorbed
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On the Advanced X-ray Astrophysics Facility
below 2 keV. With its higher-energy imaging capability, AXAF will not be impeded
in its search for distant objects by the opacity of the circumstellar medium of an
AGN. ROSAT, with an energy range below 2 keV, has resolved a large fraction of
the x-ray background into discrete objects. AXAF, having significantly more
sensitivity and angular resolution than ROSAT, should more completely resolve
the x-ray background, if it is indeed entirely composed of discrete sources.
Furthermore, AXAF results will apply to energies above 2 keV.
The ROSAT all-sky survey has yielded a total of more than 50,000
objects that can be studied in depth with AXAF's broad range of spectroscopic
capabilities. The ROSAT catalog is expected to be publicly available by the time
AXAF is operating.
The Japanese x-ray satellite Ginga detected 6- to 7-keV x-ray lines from
nearby AGNs. This implies that iron lines, probably broadened fluorescence lines
from circumnuclear material, are common emission features in AGN spectra, and
that AXAF will therefore have the capability to measure redshifts of distant AGNs.
A very recent result from the Broad-band X-ray Telescope (BBXRT)
confirms the existence of an x-ray absorption line in the spectra of BL Lac
objects. The greater sensitivity and spectral resolution of AXAF is needed to
extend this search to other objects and other lines. The detection of other x-ray
absorption lines will resolve ambiguities that currently plague the interpretation of
these features.
The premier astronomical event of the 1980s was the occurrence of
SN1987a, the closest supernova explosion in 400 years. During its planned time
in orbit AXAF may have the opportunity to observe an extraordinary phase in the
evolution of SN1987a. The expanding shell of debris from the explosion will
collide with a slow-moving ring of matter ejected by the star prior to its death. The
best estimate for the time when collisions will begin is about the year 2000. Not
only will the event be spectacularly bright in x-rays, but it will also be highly
variable in intensity and in its spectral line distribution. The resulting display will
provide the best determination of the abundances of newly synthesized matter.
As collisions of different clumps of ejecta occur, SN1987a will reveal the
composition of different parts of the supernova ejecta. AXAF will also have the
ability to locate the positions of the discrete clumps as they are heated to
temperatures at which x-rays are emitted. By observing how stars make
elements, we will better understand how galaxies evolve. Such a direct
observational test of nucleosynthesis theory will allow us to apply these models
with confidence to abundance patterns in galaxies at high redshift.
Comparison of the Original and Revised Programs
The restructuring of the AXAF program splits the original, single facility
(AXAF-O), a low-Earth-orbit serviceable mission, into two nonserviceable simpler
missions: one devoted principally to imaging (AXAF-I), which will be launched
into a high, elliptical orbit, and one devoted principally to spectroscopy (AXAF-S),
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which will be in a low, polar orbit. AXAF-I will carry four of the original six mirror-
shell pairs that made up the AXAF-O telescope, two imaging cameras-the AXAF
CCD Imaging Spectrometer (ACIS) and the High-resolution Camera (HRC)-and
two spectrometers-the Low- and the High-energy Transmission Grating
Spectrometers (LETGS and HETGS, respectively). AXAF-S will carry a lower-
resolution, shorter-focal-length, foil-mirror telescope and the x-ray Spectrometer
experiment (XRS). AXAF-O was designed to be serviced at five-year intervals for
a total lifetime of fifteen years. The design lifetime of AXAF-I is five years, and the
design lifetime of AXAF-S is three years.
Since the technical aspects of the various instrument designs remain
almost entirely unchanged, the scientific performance of the unified AXAF
program is largely preserved. The principal differences are associated with the
changes in the telescope complement and in the mission profile. Specifically:
The reduction in the number of mirror-shell pairs in the AXAF-I
telescope (from six to four) diminishes by about 40% the effective area of the
system at low energies. However, this effect is largely offset by the increase in
the observing efficiency of the mission brought about by the change to high-Earth
orbit. In particular, the number of observations that can be accomplished at fixed
sensitivity over an extended period of time is very nearly the same for the original
and the revised AXAF missions. At high energies, the effective area of the
system has actually been improved, due to the introduction of high-reflectivity
iridium coatings in place of the nickel and gold coatings planned for the original
telescope's outer and inner mirror shells, respectively.
The use of the low-resolution foil telescope for AXAF-S affects both
the spatial resolution and the effective area of the XRS investigations. Although
some capability for spatially resolved high-resolution spectroscopy still exists with
this experiment, measurement of spectral variations on fine angular scales is no
longer possible. The foil telescope has very high throughput, and so the net
effective area is comparable to that for AXAF-O at high energies and is down by
a factor of only about 2 at low energies. Most importantly, the XRS is likely to be
more productive during its design lifetime on the AXAF-S mission than it would
have been on AXAF-O, simply because it can be operated continuously, thereby
utilizing its limited supply of cryogen more efficiently.
Another advantage of the restructured program will be the opportunity
to conduct simultaneous observations with the two missions. This can be
extremely useful for complementary measurements of time-variable sources. As
an example, for many sources such as active galactic nuclei, x-ray binaries, and
stellar flares, AXAF-S can be used to obtain high-resolution spectra of the Fe K
complex near 6 keV (E/ E ~ 500), while at the same time the HETGS
experiment on AXAF-I is used to make high-resolution observations of the Fe L
complex near 1 keV (E/ E ~ 1000). The comparison of Fe K to Fe L line fluxes
and profiles will prove very useful for constraining plasma conditions in these
sources.
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On the Advanced X-ray Astrophysics Facility
Finally, elimination of the servicing aspect of the program reduces net
observing time by a factor that is less than 2, since in the restructured mission,
AXAF-S and AXAF-I will be operated independently. Perhaps the most serious
loss in this regard involves the capability of fielding new instrumentation that
might have capitalized on future technological advances or been designed
specifically to follow up earlier AXAF discoveries. It seems likely, however, that
alternative, post-AXAF mission scenarios could prove equally effective as
platforms for fielding new instrumentation, perhaps even in a more cost-effective
manner.
The restructured AXAF mission maintains essentially all of the
outstanding scientific capabilities of the baseline mission. The angular resolution
of AXAF-I is more than an order of magnitude better than that offered by any
other mission under development or even in the planning stages. The U.S.
investment in high-precision x-ray optics makes AXAF-I unique in its capabilities
to undertake x-ray investigations on the largest scales and at the earliest epochs
of the universe.
Similarly, the broad-band, nondispersive spectroscopy enabled by the
development of the micro-calorimeter (the XRS) is maintained in the restructured
mission. AXAF-S will provide a combination of high sensitivity and high spectral
resolution in the important energy region above 4 keV that is unavailable with any
other planned missions. Its capabilities for high-resolution spectroscopy of
extended sources are particularly notable and unique in comparison with those of
dispersive spectrometers.
The restructured AXAF program continues to provide unmatched angular
resolution, spectral resolution, and sensitivity that will make it the centerpiece of
international efforts in x-ray astronomy for the foreseeable future. When the
AXAF-I and AXAF-S spacecraft are launched at the end of this decade, they will
provide unique capabilities permitting major advances in our understanding of the
universe.
References
1. Committee on Space Astronomy and Astrophysics, Space Science
Board, A Strategy for Space Astronomy and Astrophysics for the 1980s, National
Academy of Sciences, Washington, D.C., 1979.
2. Astronomy Survey Committee, Astronomy and Astrophysics for the
1980's, Volume I, National Academy Press, Washington, D.C., 1982.
3. Committee on Space Astronomy and Astrophysics, Space Science
Board, Long-Lived Space Observatories for Astronomy and Astrophysics,
National Academy Press, Washington, D.C., 1987.
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4. Task Group on Astronomy and Astrophysics, Space Science Board,
Space Science in the 21st Century, National Academy Press, Washington, D.C.,
1988.
5. Astronomy and Astrophysics Survey Committee, Board on Physics and
Astronomy, The Decade of Discovery in Astronomy and Astrophysics, National
Academy Press, Washington, D.C., 1991.
Membership Lists
Last update 9/5/00 at 9:40 am
Site managed by Anne Simmons, Space Studies Board
The National Academies Current Projects Publications Directories Search Site Map Feedback
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On the Advanced X-ray Astrophysics Facility (Membership)
On the Advanced X-ray Astrophysics Facility
Membership Lists
TASK GROUP ON AXAF
ARTHUR F. DAVIDSEN, Johns Hopkins University, Chair
DAVID ARNETT, University of Arizona
HALE BRADT, Massachusetts Institute of Technology
ANNE P. COWLEY, Arizona State University
PAUL GORENSTEIN, Smithsonian Astrophysical Observatory
STEVEN M. KAHN, University of California at Berkeley
JAMES D. KURFESS, Naval Research Laboratory
ROBERT L. RIEMER, Senior Program Officer
COMMITTEE ON ASTRONOMY AND ASTROPHYSICS
MARC DAVIS, University of California at Berkeley, Chair
ARTHUR F. DAVIDSEN, Johns Hopkins University
SANDRA M. FABER, University of California at Santa Cruz
HOLLAND C. FORD, Space Telescope Science Institute
JONATHAN E. GRINDLAY, Harvard-Smithsonian Center for Astrophysics
DOYAL A. HARPER, JR., Yerkes Observatory
KENNETH I. KELLERMANN, National Radio Astronomy Observatory
RICHARD A. McCRAY, University of Colorado at Boulder
JEREMIAH P. OSTRIKER, Princeton University Observatory
BERNARD SADOULET, University of California at Berkeley
ANNEILA I. SARGENT, California Institute of Technology
ROBERT L. RIEMER, Senior Program Officer
BOARD ON PHYSICS AND ASTRONOMY
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CHARLES KENNEL, University of California at Los Angeles, Chair
LLOYD ARMSTRONG, Johns Hopkins University
DAVID E. BALDWIN, Lawrence Livermore National Laboratories
HOWARD C. BERG, Harvard University
WILLIAM F. BRINKMAN, AT&T Bell Laboratories
PRAVEEN CHAUDHARI, IBM T.J. Watson Research Center
FRANK DRAKE, University of California at Santa Cruz
ROBERT C. DYNES, University of California at San Diego
JEROME I. FRIEDMAN, Massachusetts Institute of Technology
MARTHA P. HAYNES, Cornell University
GILLIAN KNAPP, Princeton University
STEVEN E. KOONIN, California Institute of Technology
ALBERT NARATH, Sandia National Laboratories
GEORGE W. PARSHALL, E.I. du Pont de Nemours & Co., Inc.
JOSEPH M. PROUD, GTE Laboratories Inc.
DAVID N. SCHRAMM, University of Chicago
DAVID WILKINSON, Princeton University
SIDNEY WOLFF, National Optical Astronomy Observatories
DONALD C. SHAPERO, Director
SPACE STUDIES BOARD
LOUIS J. LANZEROTTI, AT&T Bell Laboratories, Chair
JOSEPH A. BURNS, Cornell University
JOHN A. DUTTON, Pennsylvania State University
ANTHONY W. ENGLAND, University of Michigan
JAMES P. FERRIS, Rensselaer Polytechnic Institute
HERBERT FRIEDMAN, Naval Research Laboratory
RICCARDO GIACCONI, European Southern Observatory
HAROLD J. GUY, University of California at San Diego
NOEL W. HINNERS, Martin Marietta Civil Space and Communications Co.
DAVID A. LANDGREBE, Purdue University
ROBERT A. LAUDISE, AT&T Bell Laboratories
RICHARD S. LINDZEN, Massachusetts Institute of Technology
JOHN H. McELROY, University of Texas at Arlington
WILLIAM J. MERRELL, JR., Texas A&M University
ROBERT H. MOSER, University of New Mexico
NORMAN F. NESS, University of Delaware
MARCIA NEUGEBAUER, Jet Propulsion Laboratory
SIMON OSTRACH, Case Western Reserve University
JEREMIAH P. OSTRIKER, Princeton University Observatory
CARLÉ M. PIETERS, Brown University
JUDITH PIPHER, University of Rochester
MARK SETTLE, ARCO Oil and Gas Company
WILLIAM A. SIRIGNANO, University of California at Irvine
JOHN W. TOWNSEND, JR., NASA (retired)
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FRED TUREK, Northwestern University
ARTHUR B.C. WALKER, JR., Stanford University
MARC S. ALLEN, Director
Return to Letter
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