The Chandra X-ray Observatory

Until the Chandra X-ray Observatory was launched, the best X – ray telescopes were only as capable as Galileo’s best optical tele­scope, with limited collecting area and very poor angular resolu­tion. With Chandra, X-ray astronomers gained several orders of magnitude of sensitivity, and the ability to make images as sharp as a medium-size optical telescope. Chandra was the third of NASA’s four “Great Observatories.” The others are the Hubble Space Tele­scope, launched in 1990 and still doing frontier science, the Comp­ton Gamma Ray Observatory, launched in 1991 and deorbited in 2000 after a successful mission, and the Spitzer Space Telescope, launched in 2003 and currently in the final “warm” phase of its mission since its liquid helium coolant ran out in 2009 (plate 17).

It took a while to open the X-ray window on the universe be­cause the Earth’s atmosphere is completely opaque to X-rays. In the 1920s, scientists first proposed using versions of Robert God­dard’s rocket to explore the upper atmosphere and peer into space. However, this idea wasn’t realized until 1948, when a re-purposed V2 rocket was used to detect X-rays from the Sun.21 The next few decades saw the development of imaging capabilities for X-rays and new detector technologies, and X-ray astronomy tracked the maturation of the space program. The first X-ray source beyond the Solar System, Scorpius X-1, in the constellation of Scorpius, was detected by physicist Riccardo Giacconi in 1962.22 This in­tense source of high-energy radiation is a neutron star, the end result of the evolution of a massive star where gravity crushes the remnant to a state as dense as nuclear matter. The intense X-rays result from gas being drawn onto the neutron star from a com­panion and being heated violently enough to emit high-energy radiation.

Giacconi is another “giant” in astrophysics—as leading scientist for X-ray observatories from Uhuru in the 1970s to Chandra in the 1990s, first director of the Space Telescope Science Institute, and winner of the Nobel Prize in Physics in 2002. This last and ultimate accolade fittingly came almost exactly a century after the award of the first Nobel Prize in Physics for the discovery of X – rays to Wilhelm Rontgen. Born in Genoa, Italy, his early life was disrupted by the Second World War; as a high school student, he had to leave Milan during allied bombing raids. He returned to complete his degree and started his life as a scientist in the lab, working on nuclear reactions in cloud chambers. With a Fulbright Fellowship he moved to the United States and forged his career there. He had his hand in all the pivotal discoveries of X-ray as­tronomy: the identification of the first X-ray sources, character­ization of black holes and close binary systems, the high-energy emission that emerges from the heart of some galaxies, and the nature of the diffuse X-rays that seem to come from all directions in the sky.23

The growth in the number of celestial X-ray sources gives a sense of how each new mission has advanced the capabilities: 160 sources in the final catalog of the UHURU satellite in 1974, 840 in the HEAO A-1 catalog in 1984, nearly 8,000 from the com­bined Einstein and EXOSAT catalogs of 1990, and about 220,000 from the ROSAT catalog of 2000. Chandra has had a wider-field but lower sensitivity counterpart in the European XMM-Newton mission, also launched in 1999. These two X-ray satellites have detected a total of over a million X-ray sources.

Chandra was launched by the Space Shuttle Columbia into a highly elliptical orbit that takes it a third of the way to the Moon. At five tons, it was the most massive payload launched up to that time by the Space Shuttle. The elongated orbit gives it lots of “hang time” in the perfect vacuum of deep space and lets science be done for 55 hours of the 64-hour orbit. This comes at the expense of the spacecraft being unserviceable by the Shuttle when it was still flying, so the facility has to work perfectly. In fact, the only tech­nical problem was soon after launch when the imaging camera suffered radiation hits during passage through the Van Allen radia­tion belts; it is now stowed as the spacecraft passes through those regions. The spacecraft had a nominal five-year mission at time of launch but it’s producing good science well into its second decade and is expected to last at least fifteen years.24

One of two different imaging instruments can be the target of incoming X-rays at any given time. The High Resolution Camera uses a vacuum and a strong electric field to convert each X-ray into an electron and then amplify each one into a cloud of electrons. The camera can make measurements as quickly as 100,000 times per second, allowing it to detect flares or monitor rapid variations. Chandra’s workhorse instrument is the Advanced Camera for Im­aging Spectroscopy. With 10 CCDs, it has one hundred times bet­ter imaging capability than any previous X-ray instrument. Either of these cameras can have one of two gratings inserted in front of it, to enable high – and low-resolution spectroscopy. Spectroscopy at X-ray wavelengths is a bit different from optical spectroscopy. The spectral lines seen by Chandra are usually very high excitation lines of heavy elements like neon and iron, coming from gas that’s kept highly agitated by high-energy radiation or violent atomic collisions.25

There are three major differences between optical and X-ray detection of sources in the sky. The first is the way the radiation is gathered. X-rays falling directly on silvered glass have such high energy that they penetrate the surface and are absorbed, like tiny bullets. X-ray telescopes use a shallow angle of incidence, so that the photons bounce off the mirror like a stone skimming off water. Chandra uses a set of four concentric mirrors, six feet long and very slightly tapered so they almost look like nested cylinders. This method of gathering radiation makes it difficult to achieve a large collecting area. The second difference is the much higher energy of X-rays. Chandra measures photons in a range of energy from 0.1 to 10 keV, or 100 to 10,000 electron volts, which is a standard unit of measure for photons. On the electron volt energy scale, two numbers that bracket this range are 13.6 eV, the modest energy required to liberate an electron from a hydrogen atom and 511 keV, the rest mass energy of an electron. For reference, photons of visible light have wavelengths 10,000 times longer and energies 10,000 times lower.

With each photon packing such a punch, a typical astronomical source emits far fewer X-ray photons than visible photons, so each is very valuable. The goal in X-ray astronomy is to detect every photon individually. Very few photons are required to detect a source (this is helped by the fact that the “background rate” is low; X-rays are not created by miscellaneous or competing sources, so the X-ray sky is sparsely populated). In some of the deepest ob­servations made by Chandra, two or three photons collected over a two-week period is enough evidence to declare that a source has been detected. There are papers in the research literature with more authors than photons!

Chandra unlocks the violent universe because celestial X-rays have high energy and can only be produced by extreme physical processes.26 The Sun and all normal stars are very weak X-ray sources because their cool outer regions produce thermal radia­tion peaking at visible wavelengths. It would take a gas at hun­dreds of thousands or millions of degrees to emit copious X-rays; diffuse gas with this very high temperature is distributed between galaxies. Another way X-rays can be made is when particles are accelerated to extremely high energies; they release the energy in a smooth spectrum that extends to X-rays and even gamma rays.27 Despite the million plus X-ray sources that have been cataloged, there are thousands of times more optical sources, so the X-ray sky is relatively quiet. But many of those X-ray sources are extremely interesting because they’re situations where matter has been sub­ject to extreme violence.