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Observational Astronomy

X-ray Observatories

The age of the x-ray observatory opened with the December 12, 1970 launch of Uhuru, known also as the Small Astronomical Observatory 1 (SAS-1). Before this satellite, astronomers only had glimpses of the x-ray universe from high-altitude balloons and sounding rockets. With Uhuru's pair of proportional counters, which were sensitive in the 2 to 20 keV energy range, the first comprehensive catalog of astronomical x-ray sources was created. Since that time, many x-ray observatories have been constructed and launched. The data from many of these observatories are collected at the NASA High Energy Astrophysics Science Archive Research Center (HEASARC).

Currently three observatories dedicated to observing x-ray sources outside of our solar system are in operation: the Rossi X-ray Timing Explorer (Rossi XTE), the Chandra X-ray Observatory (Chandra), and Newton X-ray Multiple Mirror Mission (XMM-Newton). The NASA Rossi XTE is the oldest of these three observatories, having been launched at the end of 1995; this observatory is not a telescope, but a set of detectors that are designed to observe the variability of x-ray sources over a broad range of x-ray energies. These features are valuable in studying binary systems containing a neutron star or a black hole candidate, which produce very broad spectra that vary as the compact object orbits its companion star and as the compact object itself rotates. These features have also been used to study the quasi-period flickering of some x-ray binary stars. While the observatory cannot produce images of the sky, it can monitor the sky, alerting observers to changes in the brightness of x-ray sources.

The remaining two observatories, NASA's Chandra and ESA's XMM-Newton, are grazing-incidence x-ray telescopes with imagers and high-resolution spectrometers sensitive to low-energy x-rays; these two observatories were placed into orbit in 1999. While the two imaging observatories have similar designs; they are not identical. The XXM-Newton observatory has three x-ray telescopes that provide six times the collecting area and a broader spectral range in its images than Chandra, and Chandra has a much finer spatial resolution and a broader spectral range in its high-resolution spectroscopy than does XMM-Newton. These different characteristics lend these instruments to different types of studies. The one type of study both satellites perform, and from which groundbreaking advances have occurred, is the measurement of x-ray lines from various x-ray sources. These are the first observatories that can resolve to high precision the many x-ray lines produced by highly ionized elements such as carbon, nitrogen, oxygen, and iron. From these observations, conditions within the x-ray emission regions of the x-ray sources can be measured.

The list of sources whose lines have been studied is very long. Both of these satellites have studied x-ray lines produced in the coronas of low-mass stars, in the winds of massive stars, in supernovae shocks, in the accretion disks and winds of compact binary systems, in the hot gas in clusters of galaxies, and in the gas at the cores of active galaxies. These observatories have also obtained the continuum spectrum of degenerate dwarfs and neutron stars.

Rossi XTE

Photograph the Rossi XTE Satellite.

The Rossi XTE under construction in the laboratory. (Courtesy NASA/GSFC)

The Rossi XTE was launched on December 30, 1995. While it is called an observatory, it does not resemble what we think an observatory should be: there is no telescope, but instead there are three self-contained instrument packages that perform specific types of observations.1 The satellite has an array of proportional counters that observes x-rays in the 2 to 60 keV energy band, a timing instrument that observes x-rays in the 15 to 250 keV energy band, and an all-sky monitor that observes x-rays in the 2 to 10 keV band. The timing experiment is designed to measure the variability of x-ray sources on time scales ranging from milliseconds to months. Both the proportional counter array and the timing experiment sit behind collimators that keep the instruments from seeing more than one square degree of the sky. These instruments have large collecting area, which make them sensitive to faint sources. The collecting areas are 6500 cm2 for the proportional counter array and 1600 cm2 for the timing experiment.



Photograph the Chandra Satellite.

Chandra shortly after release by the space shuttle. The end at the top contains the mirrors of the telescope. Above the mirrors is the rocket engine, colored gold, that boosted the satellite into its elliptical orbit. At the bottom of the picture are the instruments in the focal plane of the telescope. (Courtesy NASA/CXC/SAO)

The Chandra X-ray Observatory, which was known as the Advanced X-ray Astrophysics Facility (AXAF) before launch, is a grazing-incidence x-ray telescope. This is the third of the four satellites of the NASA great observatory program. This observatory was launched on July 23, 1999.

The telescope's optics are four pairs of cylindrical mirrors. Each pair of mirrors, with one sitting behind the other, produces two reflections of an x-ray that sends it to the an instrument at the focal point. The sets of mirrors are nested one inside the next. The most energetic x-rays only reflect off of the innermost pair of mirrors, where the size of the deflection is small. For the larger mirror pairs, where the deflection of an x-ray is larger, the maximum energy of an x-ray that can be deflected is smaller. The consequence is that the telescope has a larger collecting area for low-energy x-rays than for high-energy x-rays. At 0.25 keV, the effective collecting area is 800 cm2, while at 5 keV the effective collecting area is 400 cm2. The telescope has a field of view of 30 arc minutes diameter.

Four different instruments can be placed at the focal plane of the telescope.2 These are a CCD imaging array, a CCD spectroscopic array, a high-resolution imaging camera, and a high-resolution spectroscopic camera. The two spectroscopic detectors are used in conjunction with the two spectroscopic gratings.

The CCD arrays have an range of 0.2 to 10 keV, and the high-resolution cameras have an energy range of 0.1 to 10 keV. The imaging camera has a spatial resolution of 0.5 arc seconds. The spectroscopic gratings give the observatory its high spectral resolution: the high-energy grating combined with the CCD spectroscopic array give spectra with a resolution of 1.7% to 0.1% of the x-ray energy over the 0.5 to 10 keV energy range, and the low-energy grating combined with the high-resolution spectroscopic camera give spectra with a resolution of 3.3% to 0.05% over the 0.08 to 6 keV energy range.

Chandra is in a highly-elliptical orbit that permits observations for periods of up to 44 hours.


Photograph of part of XMM-Newton.

The mirror assembly of one telescope, showing the 58 mirror pairs. (Courtesy Dornier Satellitensysteme GmbH and ESA)

ESA's XMM-Newton observatory was launched on December 10, 1999. It comprises three identical, coaligned telescopes, each with an effective area of 1500 cm2 at 1 keV, for a total effective area of 4500 cm2, which is considerably larger (better) than that for Chandra; the spatial resolution of the telescopes is 6 arc seconds, also considerably larger (worse) than that for Chandra. The telescopes have an energy range of 0.1 to 15 keV. As with Chandra, a telescope of XMM-Newton consists of pairs of nested cylindrical mirrors, but unlike Chandra and its 4 mirror pairs, each of the telescopes of XMM-Newton have 58 mirror pairs.

Under each telescope is mounted a CCD imaging camera.3 Two of these cameras are of identical design, using metal-oxide-silicon CCDs that give a field of view of 33 by 33 arc minutes over an energy range of 01. to 15 keV. Each of these cameras have an effective area of 922 cm2 at 1 keV. The third camera of pn CCDs gives a field of view of 27.5 by 27.5 arc minutes over the same energy range; it has an effective area of 1227 cm2 at 1 keV.

As with Chandra, the groundbreaking instrument on XMM-Newton is the spectrometer. On this satellite, high-resolution spectra are provided by a reflection grating spectrometer. Reflection gratings mounted in the mirror assemblies of two of the telescopes disperse 40% of the x-rays collected by these telescopes onto the CCDs of the spectrometer. The spectrometer produces spectra with a resolution of 0.5% to 0.125% of the x-ray energy over a spectral interval of 0.35 to 2.5 keV.

The observatory also carries a 30cm optical and ultraviolet telescope that is coaligned with the x-ray telescopes.

This unusual design permits all instruments on the observatory to operate simultaneously. The observatory is in a highly-elliptical orbit that permits continuous observations of up to 40 hours.

1 Detailed information about Rossi XTE can be found in the RXTE Technical Appendix

2 Detailed information about Chandra can be found in the Chandra Proposer's Guide.

3 Detailed information about XMM-Newton can be found in the XMM User's Handbook

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