By Emily Levesque, University of Washington
By the first half of the 20th century, astrophysicists may not have known how to capture X-ray images, but they did know how to detect X-rays, using tools like chambers of inert gas that would produce electrons and ions when an X-ray passed through them. Yet, even with these detectors, X-ray astronomy didn’t begin in earnest until the early 1960s, thanks in large part to the efforts of Riccardo Giacconi.
Riccardo Giacconi was born in Genoa, Italy in 1931 and studied at the University of Milan before coming to the United States as a Fulbright Scholar in the 1950s, just as the American space program was taking off.
He focused his research on how to design an X-ray telescope that could actually capture focused images of X-rays, rather than simply detecting their presence. He knew that with the ability to capture X-ray images, astronomy could move from simply collecting high-energy photons to literally seeing the universe in a new light.
While working on an X-ray imaging detector, Giacconi also led a research group that flew Geiger counters atop a suborbital rocket, conducting a brief sweeping search across the sky to search for signs of X-rays from deep space. In June of 1962 they succeeded, detecting an influx of 100 X-ray photons per second coming from somewhere in the constellation Scorpius.
The source was eventually named Scorpius X-1, the first cosmic X-ray source ever discovered.
By 1970, Giacconi had upgraded from rockets to satellites, leading the launch of Uhuru, an astronomy satellite carrying X-ray counters. Named ‘Freedom’ in Swahili to recognize its launch site near Mombasa, Kenya, Uhuru was capable of detecting X-ray photons, measuring their energies, pinpointing where they were coming from, and studying how the flux of X-rays changed with time.
Still, Giacconi’s goal of building an imaging X-ray telescope remained.
Giacconi’s idea, first proposed with colleague Bruno Rossi in 1960, was elegantly simple and based on a design known as a grazing-incidence telescope, or Wolter telescope. A grazing-incidence telescope rearranges the traditional light-and-mirror set-up that is used in telescopes at longer wavelengths.
We know that an X-ray hitting a mirror at a right angle will travel straight through the mirror. However, an X-ray that merely grazes the surface of a mirror will bounce off that surface. It’s the photon equivalent of a rock that will plop into a lake when dropped from above but skip across the surface if thrown at an angle.
In other words, if mirrors are positioned so that X-rays can travel along their surface rather than through their surface, the X-rays will be reflected.
Working off of Wolter’s designs, Giacconi and Rossi imagined an optical system constructed from a series of parabolic segments, painstakingly curved and positioned such that grazing strikes from X-rays would ultimately end with a focused image.
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The Einstein Observatory
With this design, scientists could capture clear images of the X-ray sky for the first time. The first fully imaging X-ray telescope was launched in 1978, just 10 months after the International Ultraviolet Explorer telescope.
Dubbed the Einstein Observatory, it revolutionized our view of high-energy astronomy in just three and a half years, studying the energy and shape of supernova remnants like the Crab Nebula, mapping the X-rays emitted from gas churning around enormous clusters of galaxies, and discovering thousands of surprising previously-undetected X-ray sources across the sky.
Arthur B. C. Walker Jr
Meanwhile, another X-ray astronomer, Arthur B. C. Walker Jr, was also making major strides toward capturing X-ray images. He worked at the Space Physics Laboratory of the Aerospace Corporation, eventually directing the space astronomy program and focusing on the same technology that had launched George Carruthers and Giacconi’s early research, literally.
Walker was a pioneer of rocket-based astronomy and was particularly interested in studying the sun. His team launched payloads of more than a dozen telescopes on several sub-orbital rockets. These flights would only last about 14 minutes from beginning to end, and only five of those minutes were spent high enough above the Earth’s atmosphere to observe ultraviolet and X-ray light.
In spite of this, Walker and his team were able to use these brief flights to capture the first ultraviolet and X-ray observations of the Sun. These wavelengths offer astronomers a unique opportunity to study the sun’s hot outermost layers, known as the solar corona.
The Solar Corona
The solar corona is the bright white nimbus of light visible during a total solar eclipse, caused by the solar wind carrying a stream of plasma away from the sun and into the solar system. Observations of the corona using these short wavelengths help us to understand the complex physics at play.
While Walker was changing our view of our own sun, Giacconi was continuing to push the limits of X-ray astronomy beyond our solar system.
Chandra X-Ray Observatory
Two years before the Einstein Observatory was even launched, Giacconi was already hard at work on its successor, a larger and even more powerful X-ray observatory. Named for astrophysicist Subrahmanyan Chandrasekhar, a pioneer of theoretical astrophysics, the Chandra X-ray Observatory launched in 1999 and is still operating today.
Chandra has captured exquisitely detailed images of supernova remnants as well as colliding galaxies and enormous black holes. Chandra even studied the surrounding environment of Alpha Centauri, the closest star system to our Sun, to understand how X-ray light might impact the possibility of life flourishing in that system’s planets.
Giacconi’s lifetime of work on X-ray astronomy—from his first rocket observations to his ever-evolving series of orbiting X-ray observatories—earned him the Nobel Prize in physics in 2002.
Common Questions about Riccardo Giacconi
If mirrors are positioned so that X-rays can travel along their surface rather than through their surface, the X-rays will be reflected.
The solar corona is the bright white nimbus of light visible during a total solar eclipse, caused by the solar wind carrying a stream of plasma away from the sun and into the solar system.
The Chandra X-Ray Observatory was launched in 1999 and is still operating today.