The Gigantic Particle Accelerator Of Outer Space

The Fermi Gammaray space telescope and the mysteries it seeks to solve

Ever wonder what you would see if you could look into other regions of the electromagnetic spectrum, if you could look beyond the narrow region of visible light into infrared, ultraviolet, X-rays, even gamma rays? That is the question scientists from across the world are seeking to answer. Three hundred and fifty miles above us, travelling at more than four miles per second, is the Fermi Gammaray Space Telescope, taking a picture of the entire sky in the gamma ray region every three hours. Fermi, formerly GLAST (Gamma-ray Large Area Space Telescope), is providing new data on the most extreme conditions and exotic phenomena in the universe. From supermassive black holes, emitting jets of particles travelling close to the speed of light, to dense neutron stars, some of which spin more than six hundred times per second, Fermi is pushing the outer limits of our view of the universe.

by ALEYA RIYAZ

Flight Center. The telescope represents a new frontier of physics, where particle physicists who work on subatomic scales and astrophysicists who observe the farthest reaches of the cosmos are coming together in search of answers to some of the deepest mysteries of the universe.

How it Works

Fermi functions through an application of Einstein’s famous energy-mass equality. When a gamma ray photon, a particle of light, enters the LAT, it passes through a series of thin tungsten sheets. Upon interacting with these sheets, the photon turns into an electron and a positron—the antimatter counterpart to the electron, having the same mass but opposite charge. This conversion from energy (the photon) to matter (the electron and positron) complies with the equation E=mc 2. These particles then pass through eight hundred thousand silicon detectors which are used to track the particles’ paths backwards and determine the original direction of the gamma ray. By adding each individual gamma ray, the telescope constructs a picture of the astrophysical source. The GBM, on the other hand, has an array of detectors all facing different directions. When gamma rays from a gamma-ray burst (GRB) reach the GBM, the detector facing the burst will detect more gamma rays than the others. Consequently, by comparing the data from the different detectors, one can determine the direction of the GRB.

Credit: sxc.hu

Launched on June 11th, 2008 from Cape Canaveral, Fermi represents the joint effort and collaboration of NASA, the U.S. Department of Energy and astrophysics and particle physics scientists from six countries and fourteen U.S. research institutions. The space telescope carries two instruments: the Large Area Telescope or LAT, whose operations center is located at the SLAC National Accelerator Laboratory, and the GLAST Burst Monitor, or GBM, whose base of operations is at NASA’s Marshall Space

The Gigantic Particle Accelerator of Outer Space

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The functions of these two instruments are complimentary. While the LAT has an energy range of around 20MeV to 300 GeV, the GBM’s energy range is approximately 8keV to 25MeV, allowing the telescope as a whole to record quite a large spectrum.

The Supermassive Black Holes at the Centers of Galaxies

Fermi’s main objects of study are active galactic nuclei (AGN). AGN are supermassive black holes that are actively accreting matter. Think of a drain pipe with water flowing nearby. The geometry of the drainpipe is such that as water gets closer, it starts swirling around the drain and, if it comes close enough, is sucked in. Similarly, the gravitational pull of a black hole is so strong that it collects nearby matter into a surrounding accretion disk. Beyond a certain radius, even light cannot escape. Most if not all galaxies are thought to host a supermassive black hole. However, not all of them, including the one at the center of the Milky Way, are active. AGN also emit an extremely large amount of radiation throughout the electromagnetic spectrum. In fact, the most luminous AGN

outshine their entire host galaxies. This is equivalent to an object of about the size of our solar system outshining over one hundred billion stars by a factor of one thousand. Even more mystifying is the discovery of two jets, perpendicular to the accretion disk, carrying high energy plasma away from AGN at more than ninety-nine percent of the speed of light. When they point close to the earth’s line of sight, AGN are known as blazars and their material appears to be moving even faster than the speed of light. Physicists do not yet know what causes these jets or why they remain highly collimated over thousands and thousands of light years.

The History and Cosmology of Our Universe

By studying the energy spectra and variability of gamma rays from AGN, the LAT will help determine the composition of the jets and thereby help determine the best of the current theoretical models for their existence. Moreover, the link between AGN and their surrounding environment should give clues as to their role in galaxy and nearby star formation.

‘Microquasars’ are black holes of the size of a star. This microquasar, named GRO, exists 3,600 light-years deep in our galaxy and is part of a much larger complex of black holes that form the core of the Milky Way. Credit: nasaimages.org

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Telescopes also act as a sort of time machine. Since the astrophysical objects being studied are so far way, it takes a long time for their light to reach us. Hence, Fermi is actually seeing what AGN were like in the distant past and thus obtaining important information about the high-energy history and evolution of the universe. According to Dr. Peter F. Michelson, Professor of Physics at Stanford and LAT principal investigator, by the end of its first year of operation Fermi may detect objects as far back as a redshift of six, which is around a billion years after the original Big Bang and birth of the universe. Considering the universe is around thirteen and a half billion years old, this figure represents a significant percentage of its history.

Rotating Neutron Stars: Lighthouses of Outer Space

Another class of objects that is of great interest to astrophysicists is pulsars. A pulsar (pulsating star) is a type of neutron star that emits narrow radio beams as it rotates at a speed anywhere between a few times to a few hundred times a second. A neutron star forms when a massive star not big enough to form a black hole undergoes gravitational collapse, ending in a supernova explosion, at the end of its lifespan. Such stars, as their name implies, are composed almost entirely of neutrons. These stars are so dense that one teaspoon would weigh about a billion tons, and a typical neutron star is around ten kilometers across. Neutron stars also have humongous magnetic fields, about a trillion (1,000,000,000,000 = 1012) times stronger than the magnetic field of the earth. These magnetic fields cause most of the light and radiation that the neutron star emits to be concentrated into narrow beams. Because the neutron star is spinning, if the Earth happens to lie in the path of the beams, we see a pulse of light each time a beam sweeps across the earth like the beacon of a lighthouse.

Many pulsars cannot be detected because of their narrow beams. However, these beams are much broader in the gamma ray spectrum. Fermi will consequently provide a much more complete sample of the neutron star population in the Milky Way galaxy, which in turn will advance our understanding of the life cycle of stars. By monitoring millisecond pulsars, which rotate hundreds of times per second, Fermi may also discover previously unknown effects caused by the special theory of relativity, which may be distorting what we see of pulsars. By separating these effects from what is actually happening, Fermi’s observations may potentially revolutionize what we know of pulsars and how they work. The LAT may even uncover the process of how pulses are actually produced.

First Gamma-ray-only Pulsar Detection

Theories of pulsars and star formation may already be changing due to Fermi’s observations. According to Michelson, Fermi is seeing “a large number of gammaray pulsars that only emit pulsed energy in gamma rays and not in radio or X-rays.” The first such gamma-ray-only pulsar was detected in a supernova remnant known as CTA 1. Astronomers believe that this may be the first of a new class of astrophysical objects.

Chacra nebula, an X-ray pulsar detected by the NASA in 1999. It has been theorized that this pulsar was caused by the explosion of a star. Credit: nasaimages.org

Telescopes also act as a sort of time machine. Since the astrophysical objects being studied are so far way, it takes a long time for their light to reach us. volume VIII 35

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The second instrument aboard Fermi, the GLAST Burst Monitor, together with the LAT and ground-based telescopes, will hopefully uncover the cause of GRBs, intense explosions of gamma rays that last anywhere from a few milliseconds to several minutes. GRBs were originally discovered in the 1960’s by American surveillance satellites looking for signs of covert Soviet nuclear tests. Instead, they found brief and intense bursts of gamma rays coming from random directions. Scientists have postulated that GRBs may be shock waves from the death of a massive star, or the result of the collision of two neutron stars or of a neutron star and a black hole. Fermi may provide the answer to these and many more questions.

Looking Ahead

The Fermi Gamma-ray Space Telescope is expected to be in orbit for another five to ten years, depending on the consistency and quality of the results obtained. What is next for gamma-ray astronomy? According to Michelson, scaling up sensitivity in the field is difficult. He suggests that “in the shorter term, the most likely way progress will be made is from the ground. There are instruments that actually use the earth’s atmosphere as a detector.” This process involves very high energy gamma rays, in the energy range even higher than

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“In the shorter term, the most likely way progress will be made is from the ground. There are instruments that actually use the earth’s atmosphere as a detector.” - Peter Michelson that detected by Fermi, striking the earth’s atmosphere and generating a shower of particles. Michelson goes on to explain that “because they are moving faster than the speed of light in the medium of the atmosphere . . . they radiate electromagnetic radiation called Cherenkov radiation.” This pattern of radiation can be imaged by ground-based telescopes to determine the direction of the gamma rays. Gamma rays represent some of the most violent and extreme events in the universe, conditions which cannot be reproduced on earth. The Fermi Gamma-ray Space Telescope thus provides a new means of studying phenomena in high energy situations. Together with experiments on the ground, such as the new Large Hadron Collider at CERN in Switzerland, Fermi may be radically changing our understanding of the universe and how it works.

To Learn More

For more information and pictures, visit NASA’s image archive at www.nasaimages.org

Credit: sxc.hu

Ideally, the LAT will provide scientists with the data to help solve some of the fundamental problems of physics. Among these is the search for dark matter, the unidentified component of the universe that makes up around twenty two percent of the universe’s total energy. Additionally, scientists hope Fermi will reveal the origin of cosmic rays, particles from space that bombard Earth’s atmosphere, causing showers of secondary particles that can be detected on the surface.