Gamma-ray astronomy achieves.
Astronomers' eyes would glaze over with the mere mention of gamma rays and all that they entailed: matter-antimatter annihilation, particle decay, scintillation counters, and the like. Gamma rays were just junk radiation caused by cosmic-ray collisions. Few astronomers ever imagined their study would yield answers to the workings of the universe.
Physicists, on the other hand, saw little practical value for gamma rays in astronomy. There was professional respect but mutual avoidance, says Thomas L. Cline (NASA/Goddard Space Flight Center), who in 1961, as a doctoral candidate at Massachusetts Institute of Technology, published the first peer-reviewed report on the search for gamma rays from astrophysical origins. "In those days, astronomers and physicists came from different cultures," Cline says. "We were physicists doing physics experiments." MIT didn't even offer a course in astronomy when Cline studied there.
The field of gamma-ray astronomy slowly emerged during the 1960s. Because Earth's atmosphere blocks gamma rays from reaching the ground, it took the first rudimentary experiments on balloons and satellites to unveil the high-energy universe. Explorer 11 was the original gamma-ray satellite, providing "the first view of the universe at the shortest wavelength of the electromagnetic spectrum ... supplied by just 22 gamma rays." So wrote the satellite's creators, William L. Kraushaar and George W. Clark, in a 1962 Scientific American article. Explorer 11 collected data for four months during 1961.
Newly labeled gamma-ray astronomers marched forward, banded together in a tight-knit community, maintaining a precarious existence in the uncharted waters between the kingdoms of particle physics and traditional astronomy. Without a mature technology to capture and analyze gamma-ray photons, the outcast wavelength failed to raise many eyebrows.
The Compton Gamma-Ray Observatory (CGRO) changed all that. A sister to the Hubble Space Telescope that orbited from 1991 to 2000, CGRO raised the count of gamma-ray sources from 40 to 400 and detected more than 2,700 gamma-ray bursts, establishing the latter as the most powerful explosions in the universe (S&T: July 2000, page 48). Soon other scientists came to understand what gamma-ray astronomers had known all along: gamma rays offer unique insights to lingering mysteries, such as the history of star formation, the fate of matter around black holes, the creation of relativistic jets, and the nature of fundamental physical forces.
Now gamma-ray astronomy is embarking upon a golden age, with last October's successful launch of the European Space Agency's International Gamma-Ray Astrophysics Laboratory (Integral) and several more space-based missions to follow in the coming years. These spacecraft essentially build upon the legacy of CGRO--divvying up an enormous energy band nearly a million times wider than Hubble's window. In conjunction, upgraded ground-based telescopes will extend the view by indirectly detecting the highest-energy photons, which produce measurable particle showers upon impact with Earth's atmosphere. Gamma rays, it seems, aren't just for physicists anymore.
The Gamma-Ray Connection
Gamma rays are born from the most violent events in the universe. Those with the lowest energy, dubbed "soft" gamma rays, arise from the thermal emission of 100-million-degree gas. Matter swirling toward a black hole gets this toasty as it nears the event horizon. However, most gamma rays are produced nonthermally through collisions and radioactivity. A matter-antimatter collision, for example, is a pure transfer of matter into energy, producing gamma-ray photons and no ashes. A proton can make gamma rays when it smashes into a proton at near-light speed or bumps into a photon and boosts the photon's energy to the gamma-ray level.
Gamma rays are too energetic to focus; the photons fly right through mirrors made for visual or X-ray telescopes. "All you can do with gamma rays is measure their splash," says Charles Dermer (Naval Research Laboratory), who specializes in black-hole jets and other high-energy phenomena. Special techniques such as scintillators (depicted above) and Cherenkov telescopes (see the diagram on the facing page) have been devised to detect energy from this elusive wavelength by proxy.
Once they are captured, the opportunities to learn from gamma rays are immense. "Ah, where to begin?" asks Dermer. "Gamma-ray astronomy used to be a niche field. What's important these days is its connection with astronomy at other wavelengths."
Connections with the Infrared
Infrared radiation produced by starlight-warmed interstellar dust is the telltale signature of stellar activity. Vast amounts of energetic gamma rays readily interact with this diffuse infrared background to from electrons and positrons. That is, light smacks into light to form matter, and gamma rays disappear. Scientists can reconstruct the history of star formation by calculating how many gamma-ray photons are lost as they pass through diffuse infrared glows on their way toward Earth.
"The effect is like looking at a light at the other end of a smoke-filled pool hall," says Floyd W. Stecker (NASA/GSFC), who has provided much of the theoretical groundwork for exploiting this phenomenon. "Early in the evening there is less smoke, so more light gets through. Likewise, in the early universe there were fewer stars producing a haze of starlight. So more gamma rays get through."
Blazars serve as the gamma-ray sources. A blazar is a quasar flush with gamma rays whose powerful jet of relativistic particles is beamed directly at us. With a thousand or so blazars, Stecker says, one can map their gamma-ray flux as a function of redshift to see how far back in time the infrared background formed. CGRO began the effort, locating some 70 blazars. An upcoming NASA mission called the Gamma-Ray Large Area Space Telescope (GLAST), scheduled for launch in 2006, is expected to continue the hunt. It should attain high-resolution spectra of enough blazars to track the great gamma-ray disappearing act--chronicling the universe's star-formation history in the process.
Connections with the Visible and Ultraviolet
Gamma-ray bursts (GRBs) briefly light up regions more than 10 billion light-years away (greater than a redshift of 4.5 and conceivably as far as redshift 10). The most distant bursts likely signal the explosion of a star 10 to perhaps 100 times as massive as the Sun. Finding where bursts occur "may offer an opportunity to see the very first stars as they were forming and dying," says theorist Bohdan Paczynski(Princeton University).
This is straightforward in principle but not so easy in practice, since GRBs occur randomly and the gamma-ray portion of the burst fades within seconds, literally in a flash. Fortunately, most bursts have afterglows caused by shock waves sweeping up matter in the vicinity of the explosion. The afterglow lingers in visible and ultraviolet light and in X-rays for hours, sometimes days, offering a means to study the explosion.
The trick is pinpointing the location of the flash. The High-Energy Transient Explorer (HETE 2) helps point the way. HETE 2 detects GRBs and relays their locations within seconds to telescopes worldwide via the Gamma-Ray Burst Coordinates Network, an automated alert system. On October 4, 2002, a HETE 2-detected burst was viewed by both amateur and professional astronomers around the globe (January issue, page 22). A more recent burst seen on March 29, 2003--the brightest ever recorded--has provided a crucial link between these enigmatic events and supernovae (July issue, page 18).
Cosmologists and astronomers eagerly await December's launch of NASA's Swift satellite, so named because it will swiftly relay precise burst locations. A considerably beefed-up version of HETE 2, Swift is expected to lead to hundreds of well-studied bursts per year. Quick and accurate burst locations, Paczynski says, will allow powerful ground-based visible-light telescopes, such as Keck Observatory's 10-meter twins, to map the first generation of stars in regions otherwise too faint to attract attention.
Connections with X-Ray and Radio
The region around a black hole is an electromagnetic frenzy. Regardless of the size --be they the supermassive cores of galaxies or stellar-mass loners created by collapsing stars--black holes are gravitational sinks that pull in all matter that ventures too close. Matter swirls around a black hole in an accretion disk like water going down a drain. The commotion produces heat and releases radiation across the entire electromagnetic spectrum.
It is nothing less than a paradox how a black hole, notorious for pulling matter in and trapping it forever, also accelerates matter away in collimated jets moving near light speed. Scientists see jets from supermassive black holes (quasars and blazars) and from the stellar-mass variety closer to home (microquasars and microblazars). But the physics behind these outflows remains shrouded in mystery. "We don't even know the basics about jets," says Lynn Cominsky (Sonoma State University), a member of the GLAST team.
The facts astronomers do know lie in the intimate relationship among radio, X-ray, and gamma-ray emission. Jets produce lobes of radio waves as they ram into interstellar gas, and radio observatories such as the Very Large Array in New Mexico offer exquisite resolution to witness the collision. X-rays emerge from the innermost ring of the accretion disk--the point closest to the event horizon where temperature and velocity peak. The jets are likely powered by the accretion disk's large rotational energy and magnetic fields.
Gamma rays are right in the middle of the action, where matter either pours in or shoots away. Guided by the finer resolution afforded by radio and X-ray instruments, GLAST will detect radiation from the jets' base, allowing astronomers to deduce the jets' overall shape, the types of particles they accelerate, and how they accelerate those particles. Integral joins in by observing the inner accretion disk--an area so hot that the gas emits soft gamma rays along with X-rays. Only through multiwavelength observations such as these can astronomers identify all the pieces in the puzzle.
The Gamma Ray's Solo Act
The gamma-ray realm is vast, and much of it remains inadequately explored. "We haven't seen a lot of this stuff yet," says Neil A. Gehrels (NASA/GSFC), who led the CGRO mission.
Supernova remnants (SNRs) are one known source for gamma rays. Our galaxy is transparent to gamma rays from particle decay, and SNRs both young and ancient reveal themselves through the radioactive decay forged in their explosions. Specific elements are seen at specific gamma-ray energies--a telltale signature of newly minted elements. Two isotopes particularly common to SNRs are titanium-44, with a 60-year half-life, and aluminum-26, with a 700,000-year half-life. Titanium-44 is a sign of a brand-new supernova--one that is perhaps hidden and enshrouded in dust that blocks all but the most penetrating photons. Aluminum-26 speaks of supernovae from millennia past.
Integral should detect these and other isotopes, uncovering supernova remnants and pinpointing their ages through a technique analogous to carbon-14 dating. Integral will also provide the most detailed map of the Milky Way's nucleosynthesis regions, where lighter elements are forged into oxygen, carbon, and the other atoms needed for life.
The most important radioactive-decay lines of astrophysical interest lie on the lower edge of Integral's energy range, between 0.5 and 5 million electron volts (MeV), according to James D. Kurfess (Naval Research Laboratory). He says that exploring this realm is crucial for understanding novae (flush with gamma rays from sodium-22 and beryllium-7) and supernovae (with nickel-56 and cobalt-56). The most distant blazars are best observed at this energy range, too. New types of cosmic objects likely await discovery in the realm above 30 MeV, says Gehrels. CGRO's Energetic Gamma Ray Experiment Telescope (EGRET) detected 271 sources in this slice of the spectrum, and more than half have no counterparts in other wavelengths. GLAST, at least 30 times more sensitive than EGRET, is expected to uncover thousands of sources.
Cutting-Edge Physics with Gamma Rays
Cosmic gamma rays may take us beyond the Standard Model of particle physics. One candidate for dark matter is the WIMP, short for a Weakly Interacting Massive Particle. And one type of WIMP --the lightest supersymmetric particle (LSP)--may have formed in the Big Bang and since decayed like a radioactive particle, leaving gamma rays. The ground-based Very Energetic Radiation Imaging Telescope Array System (Veritas), along with GLAST, will look for these enigmatic objects. Solid evidence for LSPs would help provide observational support for supersymmetry, a prerequisite for string theory.
GLAST also might see signs of quantum gravity. According to this theory, high-energy photons move more slowly through space than low-energy photons, delayed by interactions with gravity particles called gravitons. The effect is subtle; one needs great distances and energetic gamma rays to measure any delay--there would be only a 10-millisecond pause between two photons arriving across a distance of 3 billion light-years. One photon would arrive at 10 MeV, the second at 1,000 MeV. Photons from exceedingly distant GRBs could do the trick--they would be outside Swift's sensitivity range but at the upper limit of GLAST's ability.
If luck is with us, Integral might observe a nearby Type Ia supernova, which occurs about once every 500 years in our galaxy. This kind of supernova is produced by runaway thermonuclear reactions within a white dwarf, the result of an abundant stream of plasma from a companion star crashing down upon it. Type Ia supernovae are used as standard candles because their intrinsic luminosities are well known (S&T: November 2002, page 28). But, in general, "theory is way ahead of the observations for novae and supernovae," says Gerald J. Fishman (NASA/Marshall Space Flight Center), who headed CGRO's GRB instrument. A next-generation instrument would likely be needed, Fishman says, to probe galaxies in the Virgo Cluster, 50 million light-years away, to find enough supernovae for a respectable analysis.
Gamma-Ray Astronomy's Multifaceted Future
Gamma rays have catalyzed new collaborations among the traditionally isolated fields of radio, visual, and high-energy astronomy, and even particle physics. A September 2002 gamma-ray meeting in Rome attracted some of the biggest names in cosmology and "conventional" astronomy. "Old-school gamma-ray astronomers were the minority there," says Gehrels.
GRBs may very well be bread-and-butter astronomy for many observers and theorists in the years to come. Along with HETE 2 and Integral, the new missions--Swift, Italy's AGILE (Extremely Light Imager for Aamma Astronomy), and GLAST --all have detectors tied into the Gamma-Ray Burst Coordinates Network. The goal is to detect as many GRBs as possible and to use them as cosmological probes. With but a few GRBs in hand, Jay P. Norris (NASA/GSFC) has already uncovered several key properties, namely, that short and long bursts are different and that there appears to be a relationship between intrinsic luminosity and photon arrival time (S&T: April 2002, page 20).
Bradley E. Schaefer (University of Texas, Austin) holds out hope that GRBs will provide an independent measurement of the "dark energy" accelerating the universe's expansion. A graph plotting GRB distances versus their redshifts would complement visible-light searches for Type Ia supernovae and X-ray surveys of galaxy clusters. It would help to determine the distributions of both dark matter and dark energy. "When you have all these methods giving you the same answer, then you know you've got it iced," Schaefer says.
Bolstered by this long-overdue acceptance, gamma-ray astronomers have their eyes on bigger and better telescopes. First on the wish list--and rapidly becoming a reality, according to Kurfess--is the Advanced Compton Telescope (ACT). It will observe from 1 to 30 MeV, investigating the "hidden valley"--what is arguably the least-explored region of the entire electromagnetic spectrum. A proposed mission called the Energetic X-ray Imaging Survey Telescope (EXIST) will image soft gamma rays with unprecedented precision. Like Veritas and GLAST, EXIST was spotlighted in the 2000 decadal survey of US astronomers as one of the most important proposed missions of the coming decade (S&T: January 2001, page 38).
Studying black holes? Solving the mystery of dark matter? Testing quantum gravity? Not a bad turn of events for a field that once had more scientists than photons--a field that, as Cline says, started "as a physicist's hobby."
Launch Date: December 2003
Energy Range: 200 to 150,000 electron volts
Instruments: Burst Alert Telescope (BAT); X-Ray Telescope (XRT); Ultra-Violet/ Optical Telescope (UVOT)
Scientific Goals: To classify and determine the origin of gamma-ray bursts and to use gamma-ray bursts to study the early universe.
Organization: European Space Agency
Launch Date: October 17, 2002
Energy Range: 150,000 to 10 million electron volts
Instruments: Gamma-Ray Spectrometer (SPI); Gamma-ray Imager (IBIS); X-ray monitor (JEM-X); Optical Monitor (OMC)
Scientific Goals: To study black holes, active galactic nuclei, supernovae, gamma-ray bursts, and the formation of new chemical elements.
Launch Date: September 2006
Energy Range: 20 million to 300 billion electron volts
Instruments: Large Area Telescope (LAT); GLAST Burst Monitor
Scientific Goals: To understand the mechanisms behind active galactic nuclei, pulsars, and supernova remnants, to resolve the gamma-ray sky, to determine the mechanics behind gamma-ray bursts, and to probe for dark matter in the early universe.
THE GAMMA-RAY PRIMER
By Christopher Wanjek
How are cosmic gamma rays--the most highly penetrating photons--captured and analyzed? The highest-energy gamma rays, having a trillion (tera) electron volts (TeV), penetrate deep into Earth's atmosphere, collide with air molecules, and create showers of secondary particles and radiation. Ground-based observatories detect these air showers and enable astronomers to picture the gamma-ray collision. High-energy gamma rays, about 30 million (mega) to 30 billion (giga) electron volts (30 MeV to 30 GeV), interact with detectors, each producing an electron and a positron that reveal the direction and energy of the photon. Medium-energy gamma rays (1 MeV to 30 MeV) interact with detectors via Compton scattering, in which the gamma ray collides with an electron in the detector and imparts energy to it. Lower-energy gamma rays, which range from 30,000 electron volts (30 KeV) to 1 MeV, are measured via the photoelectric effect, in which the photon collides with an atom and is absorbed, while knocking loose one of the atom's electrons. Integral and Swift use coded aperture masks, which block some gamma rays from hitting the detector and create a pattern of shadows that can be analyzed to determine the location of the gamma-ray source.
OUR GAMMA-RAY SUN
By Robert P. Lin
Gamma-ray astronomy isn't limited to exotica like blazars and black holes on the other side of the galaxy or visible universe. The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) spacecraft was launched by NASA in February 2002 to study the brightest gamma-ray source in the sky, the Sun.
Solar flares typically occur near sunspots, where the Sun's magnetic field is strong, and they are the most powerful explosions in the solar system. Each of these blasts releases as much energy as a few billion 1-megaton hydrogen bombs. Most of this force appears to accelerate particles to high energies. They impact the solar atmosphere, produce soft gamma rays, and heat the gas to tens of millions of degrees. In addition, the now-superhot gas emits even more soft gamma rays on its own. RHESSI's goal is to understand how the Sun releases this energy, presumably stored in the magnetic fields of the corona, and to determine how it accelerates particles rapidly and efficiently.
Gamma-ray astronomy has never seen details as fine as RHESSI's 2-arcsecond resolution. The craft can also produce soft gamma-ray "movies" that follow the accelerated particles. These movies are then compared to visible, extreme-ultraviolet, and soft-X-ray images from ground-based observatories and spacecraft such as the Solar and Heliospheric Observatory (SOHO), the Transition Region and Coronal Explorer (TRACE), and the Geostationary Operational Environmental Satellites 12 (GOES 12).
RHESSI uses large detectors, cooled to -368[degrees]F (75[degrees]Kelvin), to measure the energies of the gamma rays with exquisite precision (within about 0.2 percent). This has allowed us to observe the Doppler shift of gamma-ray spectral lines for the first time. With such observations, RHESSI has revealed that accelerated solar-gas ions interact in a different region of the Sun's atmosphere than do the corresponding electrons. This suggests that strong electric fields may be separating the positively charged ions from the negatively charged electrons. Even at quiet times, RHESSI detects tiny microflares that are dominated by accelerated particles. These events may be important for heating the active corona. Moreover, RHESSI has seen the acceleration of electrons high in the corona at the very beginning of a large solar flare --possibly the signature of an impending energy release.
RHESSI's detectors also witness gamma rays from cosmic and terrestrial sources. One remarkable RHESSI finding is that the high-energy radiation from a gamma-ray burst is highly polarized--the gamma-ray waves preferentially wiggle in the same plane. This indicates that very strong magnetic fields must be involved in the GRB's explosion. Other extrasolar results include detecting the emission from the massive black hole Cygnus X-1 and from X-ray pulsars. RHESSI will also image the Crab Nebula's gamma rays with unprecedented angular resolution.
University of California, Berkeley, astrophysicist and solar-physics pioneer ROBERT P. LIN is the principal investigator for RHESSI.
CHRISTOPHER WANJEK, a writer for SP Systems, supports NASA's Structure and Evolution of the Universe theme. He is also the author of Bad Medicine (Wiley, 2002), a humorous take on health misconceptions. Beware, this has nothing to do with astronomy. The author thanks Chris Shrader for his input and guidance.
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|Publication:||Sky & Telescope|
|Date:||Aug 1, 2003|
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