The Advanced X-ray Astrophysics Facility

by Fred Seward

AXAF, the Advanced X-ray Astrophysics Facility, is the U.S. follow-on to the Einstein
Observatory. Originally three instruments and a high-resolution mirror carried in one spacecraft,
the project was reworked in 1992 and 1993. The AXAF spacecraft will carry a high resolution
mirror, two imaging detectors, and two sets of transmission gratings. In addition, a cryogenic
spectrometer is being developed to fly on the Japanese Astro-E mission. 

AXAF

AXAF will be placed in an elliptical high-earth orbit allowing uninterrupted observing intervals
up to 24 hours in length. 

The AXAF telescope has been designed to have three times the area of the Einstein mirror at low
energies and to have considerable collecting area between 6 and 7 keV, the energy of the iron
lines strongly emitted by many astrophysical sources. Capability of the observatory goes far
beyond that of Einstein. There is an order of magnitude better angular resolution and imaging
sensitivity is two orders of magnitude higher. Relative increase in spectroscopy capability is even
greater.

The mirror consists of four pairs of nested reflecting surfaces, arranged in the usual Wolter type
1 geometry. The high energy response is achieved by use of relatively small reflection angles and
by coating the mirrors with iridium. Improvements in mirror technology since Einstein include
significant advances in grinding, polishing, alignment, and testing. Mirrors with a resolution of
0.3 arcseconds have been achieved. The combination of high resolution, large collecting area, and
sensitivity to higher energy X-rays will make it possible for AXAF to study extremely faint
sources, sometimes strongly absorbed, in crowded fields. 

There are two focal plane instruments. One is a high resolution camera (HRC). Smaller pore
size, larger microchannel plate (MCP) dimensions, lower background, charged particle
anticoincidence, and possible energy resolution are all advances over ROSAT. It will be used for
high resolution imaging, fast timing measurements, and for observations requiring a combination
of both.

The second instrument, the AXAF CCD imaging spectrometer (ACIS), is an array of charged
coupled devices. A two-dimensional array of these small detectors will do simultaneous imaging
and spectroscopy. Pictures of extended objects can be obtained along with spectral information
from each element of the picture. This was done with the Einstein IPC but in a primitive way
compared with this AXAF instrument. The new device combines the spatial resolution of the
Einstein HRI with the spectral resolution of the Einstein SSS, an order of magnitude
improvement over the IPC in both respects. ASCA (ASTRO-D) carries a similar CCD array but
the mirror limits the spatial resolution to ~2 arcminutes.

Two transmission grating spectrometers, formed by sets of gold gratings placed just behind the
mirrors, are being built for AXAF-I. One set is optimized for low energies (LETG) and the other
for high energies (HETG). Spectral resolving powers (E/deltaE) in the range 100-2000 can be
achieved with good efficiency. These produce spectra dispersed in space at the focal plane. Either
the CCD array or the HRC can be used to record data.

XRS

The X-ray spectrometer, the XRS, is a quantum calorimeter, a brand new development never
used in space, currently being designed and tested at the Goddard Space Flight Center. The
instrument will accurately measure the temperature rise due to energy deposited by a single
X-ray photon in a small crystal of silicon. An energy resolution of 10 eV, some 10 times better
than that of the Einstein SSS, is anticipated. The mirrors are being designed to concentrate
radiation from moderately bright, isolated sources. Spatial resolution will probably be about one
arcminute.

Science

AXAF will be capable of very sensitive observations. AXAF will be able to detect most of the
~300 stars in the Pleiades cluster. Individual O stars and RS CVn stars in the Magellanic Clouds
can be detected. High resolution grating spectra can be obtained for hundreds of stars. The goal is
to determine the temperature, extent, and density of the coronae, and the dependence of physical
parameters on stellar type. More detailed structure of coronae might be derived from spectral
observations of eclipsing binary systems.

Maps can be made of emission from SNR in our galaxy and spectral information will be obtained
for all features. Tycho's remnant, for example, can be mapped in the light of X-rays from Si
ions, from S ions, and even from Fe ions. Thus, the distribution of different elements within the
remnant can be measured. AXAF will be capable of detecting SNR in M31 and extended
remnants such as the Cygnus Loop, which, in M31, would be approximately 10" in diameter, will
be mapped. In M31 ~200 SNR should be accessible to AXAF.

Since higher energy X-rays are capable of penetrating gas and dust, emission from the center of
our Galaxy will be clearly seen. Sources in spiral arms on the other side of the center can also be
measured and some of their properties determined. The bright bulge sources in M31 are strong
enough so that light curves can be measured and a search for the eclipses which identify binary
systems will be possible. AXAF in one pointing will be able to monitor the emission of
approximately 50 bulge sources in M31.

Brighter binary sources in galaxies within the Virgo cluster can also be resolved and detected
individually, as obviously can sources in intermediate galaxies. Thus, the population of bright
X-ray sources in hundreds of galaxies can be determined. Since high-energy X-rays are
unaffected by obscuring material, luminosities of sources can be accurately measured and the
hypothesis that these sources, or a subset of them, are "standard candles" can be accurately tested.
If such "standard candles" are found, distances to nearby galaxies can be accurately determined.
These distances are a crucial step in the derivation of the Hubble Constant and the potential of
these measurements is truly exciting.

X-rays from distant clusters of galaxies (which are faint and small) can be imaged and spectra
measured as a function of position within the cluster. Since the emitting gas is quite hot (10**8
K), the high energy capability of AXAF gives a great sensitivity gain over that of Einstein and
ROSAT. Furthermore, since clusters emit the characteristic iron line, the redshift can be
measured directly. The spectral and spatial data combined delineate the gravitational potential.
The distribution of all matter, including dark matter, within distant clusters can be determined.
Data on the evolution of cluster structure with time will be collected.

AXAF is expected to detect quasars and active galaxies 100 times fainter than Einstein and can
thus look to significantly greater distances. This is unknown territory, except that the integrated
emission from many unresolved faint sources probably contributes most of the X-ray
background. Deep AXAF observations will come close to imaging this background and will
provide a sample of distant objects which record the state of the universe at early times.

Once launched, AXAF should enjoy a 5-10 year lifetime. The lifetime of the XRS instrument
will be limited to 3-5 years due to the finite amount of cryogen, necessary to cool the detector.
AXAF will operate as a true observatory, available through proposal, as is observing time at
ground-based facilities. 

rsimon@matisse.harvard.edu