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Compton's Legacy : Highlights from the Gamma Ray Observatory.

THE HIGHLY PRODUCTIVE AND LONG-LIVED COMPTON Gamma Ray Observatory mission - which documented a previously little-known world of gamma-ray bursts, unusual pulsars, and black-hole-fueled particle jets - has come to an end with the failure of one of the satellite's three gyroscopes. On March 24th NASA announced the decision to direct the satellite back to Earth. If all goes according to plan, Compton will glide into Earth's atmosphere during the first days of June, less than a month after this issue of Sky & Telescope goes to press. At 17 tons, Compton is too large to burn up entirely on reentry. Therefore it must be politely guided toward the isolated waters of the Pacific Ocean.

But what a trip it's been! Launched in 1991, Compton was the second of NASA's four planned Great Observatories, establishing new heights for the gamma-ray realm. What the Hubble Space Telescope (the first Great Observatory) and the Chandra X-ray Observatory (the third) are doing for optical and X-ray astronomy, Compton has done for gamma-ray astronomy. (The Space InfraRed Telescope Facility, or SIRTF, Part Four in the Great Observatory story, won't be launched until 2001.) Compton provided NASA with an unprecedented chunk of the electromagnetic pie, covering a broader range in energy than any other observatory - six orders of magnitude, nearly a million times wider than the visible portion of the spectrum.

Compton managed this feat with four main instruments (BATSE, OSSE, COMPTEL, and EGRET), each handling a different patch of the gamma-ray spectrum. Compton needed four instruments because the gamma-ray electromagnetic band is so broad. Also, gamma rays of different energies interact with matter in different ways, so different types of technology must be employed to collect them.

Compton needed to be in orbit because the Earth's atmosphere, which wonderfully protects our body tissue from harmful gamma-ray radiation, makes the gamma-ray light show impossible to see. Ground-based gamma-ray observatories can observe only the very most energetic cosmic gamma rays (those with energies measured in trillions of electron volts), and indirectly at that, via cascades of secondary particles created when those singular photons slam into the Earth's atmosphere (S&T: September 1995, page 20). Although imperative for certain types of science, ground-based observatories cannot witness anywhere near the range of gamma-ray activity that a satellite can.

Truth be told, Compton still had some "juice" left. All of its instruments functioned more or less perfectly. NASA's decision to deorbit was based mainly on the 1-in-1,000 chance of someone being injured should the spacecraft come down in a completely uncontrolled manner. "Plan A" called for a deorbit while there are still two working gyroscopes, for two were deemed necessary to steer the massive satellite safely. Deorbiting with one gyro, should the other fail, would be a bit riskier.

NASA space-flight engineers, an ever-resourceful and audacious breed, said they could safely steer the spacecraft back to Earth without any gyros. This group spent several months perfecting "Plan B" while some Compton scientists argued that the satellite could be useful for observing the now-ongoing solar-activity maximum. In the end, NASA didn't want to chance it.

A tough loss, yes. But the Compton team realizes that its satellite was wildly successful, a workhorse that lived many years longer than expected. Reflecting on the mission, the team boasts that Compton brought our understanding of gamma-ray bursts, quasars, and pulsars to new levels. Here we summarize but a few of the many "hits" from Compton's stellar career.

Gamma-Ray Bursts

COMPTON'S GREATEST ACHIEVEMENT, in many people's minds, was its work on gamma-ray bursts (GRBs). Although these bursts were discovered in 1967, Compton was the first satellite that truly enabled an in-depth study of the phenomenon.

GRBs are the most energetic events known in the universe, second only to the Big Bang in power. During a GRB's flash - as short as a few milliseconds or as long as a minute or more - the burst can outshine the rest of the gamma-ray universe. Then it disappears forever. The nature of the bursting objects remains unknown.

The 300-odd GRBs known prior to Compton were thought to be associated with neutron stars within the plane of the Milky Way. Compton's BATSE instrument, many assumed, would simply confirm this scenario, and GRBs would be relegated to an insignificant place in high-energy astrophysics research.

Instead, BATSE showed the GRB distribution to be isotropic (favoring no direction over another) and spatially limited (that is, the distribution has an outer edge). This rules out the galactic-plane hypothesis and favors the notion that GRBs originate at cosmological distances, vastly beyond our galaxy.

Two-thirds of the way through Compton's career, the Italian-Dutch BeppoSAX satellite discovered that many GRBs give off X-rays hours after the gamma-ray flash. Furthermore, BeppoSAX has been able to determine the positions of these X-ray afterglows with a precision of a few arcminutes - roughly 50 times more precise than BATSE. Follow-up observations at visual and radio wavelengths have nailed down the cosmological interpretation: redshifts for GRB counterparts range from roughly 0.4 to 4.0, implying distances measured in billions of light-years (S&T: February 1998, page 32). That's way out there.

While BATSE provides only crude coordinates for GRBs, it relays them to the astronomical community in near-real time. The idea is that someone, somewhere will record the burst with something before it fades from view. On January 23, 1999, BATSE helped a robotic camera catch the first visible-light GRB counterpart seen to flare at the same time as the gamma-ray flash. The burst briefly reached 9th magnitude and would have been visible with good binoculars (S&T: May 1999, page 54).

BATSE may be in ashes, but there is gold in its treasure chest of data. Characteristics of the bursts' light curves may enable astronomers to determine GRB distances, allowing the bursts to be used as cosmological probes even when no visible-light counterpart is seen. This and other archival uses of Compton's data may someday help astronomers figure out the ultimate cause of GRBs.

Blazers

QUASARS ARE THE EXTRAORDINARILY bright cores of very distant galaxies, and they often are visible at radio and X-ray energies as well as in ordinary light. This emission is likely produced by a supermassive black hole accreting copious amounts of interstellar gas (S&T: May 1999, page 40). Along with gamma-ray bursts, quasars are among the most distant objects known to science.

When Compton was launched, quasars were not well understood. The only quasar seen in gamma rays before Compton was 3C 273, detected in 1976 by the European Space Agency's COS-B satellite. When Compton's EGRET instrument stared at 3C 273 in 1991, it found another quasar in the same field of view. This other quasar, named 3C 279, was many times brighter than 3C 273 to EGRET's "eyes." It just so happened that 3C 279 was undergoing a flare that made it one of the brightest sources of high-energy gamma rays in the entire sky at the time, despite its distance of 4 billion light-years.

Quasars visible at gamma-ray energies are now called blazars, and EGRET established them as a class of astronomical objects. The Third EGRET Catalog contains 66 high-confidence blazars and 27 lower-confidence ones. Blazars represent the largest well-defined class of nontransient gamma-ray sources.

As with quasars, each blazar likely harbors a central supermassive black hole with a pair of relativistic jets emanating in opposite directions. The bright, highly variable emission characteristic of blazars can be seen when the observer looks almost along one of the jets - that is, down the barrel of the gun.

Many of the "unidentified" gamma-ray sources EGRET found at high galactic latitudes may be blazars. A multitude of blazars also may account for a diffuse, isotropic, high-energy, gamma-ray background first observed by NASA's SAS-2 (Small Astronomy Satellite 2) in the 1970s and subsequently confirmed by EGRET at photon energies of 30 million electron volts or more.

Supernova Remnants

Supernovae reveal themselves to Compton in a unique way - through gamma rays created by the radioactive decay of trace elements in their fiery ejecta.

A supernova occurs after a massive star exhausts its nuclear fuel, allowing the core of the star to collapse suddenly, then explode. The exploding star's outer layers are thrown off into the interstellar medium and the mix is visible as a supernova remnant (SNR). For a few days to a couple of weeks, a single supernova can outshine its host galaxy.

On February 24, 1987, Supernova 1987A went off in the Large Magellanic Cloud (LMC). The Solar Maximum Mission saw the radioactive decay of cobalt-56 (56Co) six months later. This was the first direct confirmation of the widely held belief that heavy chemical elements are forged by supernovae. Shortly after Compton's launch in 1991, the OSSE instrument detected gamma rays from SN 1987A-spawned cobalt-57 (57Co), which has a half-life of 272 days. The data were used to determine that the abundance ratio of iron isotopes (56Fe/57Fe) in the LMC was 1.5 times that of our Sun, arguing against a much more substantial ratio, which had been reported previously on the basis of a less-reliable analysis.

Gamma rays from the decay of titanium-44 (44Ti) and aluminum-26 (26Al), both present in supernova remnants, are food for COMPTEL. Titanium-44 has a half-life of around 60 years; aluminum-26's is 700,000 years. As such, these isotopes serve as tracers for new supernovae and old ones, respectively. A prime example of a very young supernova is SNR GRO J0852-4642 near the Vela SNR. This remnant, approximately 680 years old and 650 light-years away, was discovered independently by COMPTEL and the X-ray-sensitive Rosat satellite. The extremely bright Vela SNR dominates the region and kept the younger, closer SNR hidden until recently. Why this supernova wasn't seen (or at least recorded) by astronomers 680 years ago is a mystery in its own right (S&T: April 1999, page 22).

COMPTEL also detected 44Ti emission from the Cassiopeia A supernova remnant, enabling estimates of the isotope's yield. Cassiopeia A is likely about 300 years old, but the supernova that spawned this remnant also wasn't recorded by astronomers (if indeed it ever was visible to the eye).

In 1979, the HEAO (High Energy Astronomy Observatory) 3 spacecraft was the first to "see" gamma-ray emission due to nucleosynthesis: the radioactive decay of small quantities of 26Al produced by massive stars and expelled into the interstellar medium. Aluminum-26 emission traces the galaxy's recent star-formation history. COMPTEL mapped this emission with unprecedented angular resolution and found that it is indeed concentrated in regions of star formation. The data have been used to calculate that there are roughly 1 to 2 solar masses of 26Al in the galaxy.

Neutron Stars

COMPTON'S LEGACY IN the world of pulsars was the revelation that some may be pulsing primarily (if not only) in gamma rays, and not at the telltale radio wavelengths preferred by most pulsars. This revelation will radically alter today's pulsar census as tomorrow's high-resolution gamma-ray instruments find more of these gamma-ray pulsars.

A pulsar is a rotating neutron star with a strong dipolar magnetic field. Created during a supernova explosion, a neutron star packs a mass slightly greater than the Sun's into a sphere with a 10-kilometer radius. As it spins, the neutron star produces a beam of radiation from charged particles trapped in its intense magnetic field. An observer fixed in space sees pulses of this radiation as the beam periodically sweeps through his or her line of sight, hence the name pulsar.

Since their discovery in 1967, pulsars have been largely the domain of radio astronomers. There are hundreds now identified in our galaxy. Before Compton, only two - the Crab and Vela pulsars - were known to emit gamma rays, and they did so in conjunction with their radio pulses. Compton found five more gamma-ray-emitting pulsars. These objects tend to be young and rapidly rotating. The Crab pulsar, for example, rotates 30 times per second. Pulsars rotating at these speeds seem to efficiently accelerate particles to very high energies.

A mysterious object called Geminga (for Gemini gamma-ray source) has turned out to be a pulsar. Initially Geminga could be seen only at gamma-ray wavelengths, and in an Italian dialect the word has a second meaning: "It is not there." Eventually X-ray pulses were seen from this region, and EGRET found a pulse period identical to that "seen" in X-rays, confirming that Geminga is indeed a pulsar. Geminga has only recently been detected at radio wavelengths, where pulsars have been traditionally discovered (and some astronomers find the radio data unconvincing). Several of the 170 "unidentified" EGRET sources may turn out to be Geminga-like pulsars.

Soft Gamma Repeaters (SGRs) are another class of neutron stars that Compton has scrutinized profitably. SGRs sporadically emit short bursts of "soft" gamma rays, those with energies below 10,000 electron volts. Before Compton, many thought SGRs had something to do with neutron stars that have very strong magnetic fields, and perhaps even with GRBs. Compton helped demonstrate that the former belief was true (in spades) and that the latter belief was completely false.

The three SGRs known before Compton were all discovered in 1979. The March 5, 1979, outburst of SGR 0526-66 in the Large Magellanic Cloud released more energy in gamma rays in one-tenth of a second than the Sun has released at all wavelengths over the past 1,000 years. A powerful outburst from SGR 1900+14 briefly disrupted communications on and near Earth on August 27, 1998, even though the object lies roughly 20,000 light-years distant (S&T: January 1999, page 22).

BATSE discovered a fourth SGR in June 1998, now called SGR 1627-41. Observations from the Rossi X-ray Timing Explorer (RXTE) satellite and the so-called InterPlanetary Network (IPN) of spacecraft helped link this SGR to a supernova remnant called G337.0-0.1 for its galactic coordinates in the constellation Ara. Many now agree that SGR outbursts are due to "starquakes" on magnetars, neutron stars born with extremely strong magnetic fields (1014 gauss). SGRs also appear to be related to anomalous X-ray pulsars, or AXPs. But SGRs have nothing to do with GRBs, which are vastly more powerful events that take place in distant galaxies.

Solar Flares

SOLAR FLARES ARE EXPLOSIONS of energetic particles and electromagnetic radiation in the outer atmosphere of the Sun. Lessons learned from solar explosions apply to much larger explosions that we see elsewhere in the universe. Closer to home, these solar particles can cause communications and electrical problems on Earth (March issue, page 50).

Compton was launched just after the last peak in solar activity, or solar maximum, but fortunately the Sun was still active and Compton got itself a tan with several large flares in June 1991. In this regard, Compton was following in the footsteps of OSO (Orbiting Solar Observatory) 7 (1971-74) and the Solar Maximum Mission (1980-81 and 1984-89), the only other space missions to see gamma-ray emission lines from solar flares. Compton's spectrometer, OSSE, detected several nuclear emission lines from a solar flare on June 4, 1991, including those of iron, magnesium, neon, silicon, carbon, oxygen, and nitrogen. These give information about the abundances of elements in the ambient coronal gas. EGRET detected a high-energy afterglow from a solar flare on June 11, 1991. No spectral cutoff was detected, so presumably the flare produced photons with even higher energies than those picked up by EGRET.

For its part, COMPTEL detected neutrons from a solar flare on June 15, 1991. (The instrument is able to discern when neutrons, rather than gamma rays, have collided with its innards.) This resulted in the first particle image of any astrophysical object. The Sun may be the only object that will ever be imaged this way, since neutrons decay with a half-life of only five minutes when they are not bound up within atomic nuclei. COMPTEL also detected a gamma-ray afterglow from the same flare. In this case, the particles were likely accelerated not just during the impulsive phase at the beginning of the flare but over an extended period of time.

Many questions about solar flares remain unanswered. Too bad Compton will not be around to observe them during this solar maximum.

The Future

IT IS HOPED THAT SEVERAL UPCOMING missions will continue where the Compton Gamma Ray Observatory left off. Each zooms in on a particular swath of gamma-ray bandwidth that Compton had covered.

NASA's HETE (High-Energy Transient Explorer) 2 is a small Explorer-class mission that will localize gamma-ray bursts more precisely than BATSE and BeppoSAX and relay that information to the ground very quickly. Now scheduled for a July or August launch, HETE-2 will also monitor X-ray and gamma-ray flares from a variety of astrophysical sources.

HESSI, the High Energy Solar Spectroscopic Imager, will observe solar-flare gamma rays with better energy resolution than Compton could provide. With a launch once scheduled for July 2000, HESSI would have been perfect for studying the present solar maximum. However, the spacecraft suffered serious damage during vibration tests in March. The mission has been postponed for at least six months, making it all the more frustrating that Compton won't be in orbit anymore.

The European Space Agency's International Gamma-Ray Astrophysics Laboratory (INTEGRAL) will be launched in 2001. INTEGRAL will concentrate on the high-energy X-ray and low- and medium-energy gamma-ray bands, essentially replacing OSSE and COMPTEL. And NASA's Swift mission, scheduled for 2003, will search for gamma-ray bursts and other explosive phenomena, filling BATSE's shoes. The name Swift reflects this spacecraft's ability to rapidly locate bursts, relay this information to Earth, and follow up with its own ultraviolet and X-ray observations.

Finally, the Gamma-ray Large Area Space Telescope (GLAST), scheduled for a 2005 launch, is a collaboration among NASA, the U.S. Department of Energy, and international partners. GLAST will continuously probe the high-energy gamma-ray sky with 50 to 100 times the sensitivity of EGRET. GLAST's ability to study relativistic particle jets from black holes makes the mission particularly alluring to both astronomers and particle physicists.

Ending in a Blaze of Glory

COMPTON'S UNDENIABLE legacy is in the numbers. The mission observed approximately 400 gamma-ray sources (not including GRBs); before Compton, only about 40 were known. BATSE detected more than 2,600 GRBs; before Compton, only about 300 had been logged. Scientific journals publish roughly 180 Compton-specific articles per year, about one every other day.

The Compton era will have ended as it began, in suspense. The nail biting began back in April 1991, a few hours after Compton's release from the Space Shuttle Atlantis. Compton's high-gain antenna would not erect itself properly, and its loss would have crippled the mission. Astronauts Jerry Ross and Jay Apt performed an unscheduled space walk to physically shake it loose.

The nail biting resumed last December with the news that one of Compton's gyros had failed. Months of debate over Compton's fate culminated with the decision to "bring 'er on home." If the players follow the script, during the first week of June the mighty observatory will burn to silvery dust high over the Pacific, south of Hawaii, where it may well be visible to skywatchers along the path. A few stubborn chunks will sink quietly to the bottom of the ocean.

No Compton spacecraft will adorn the halls of the Smithsonian Institution's National Air and Space Museum. But the Compton legacy will remain on permanent exhibit in those minds graced with fantastic visions of gamma-ray bursts, black-hole jets, and all the other things that go boom - no, make that KABOOM - in the night.

PETER LEONARD and CHRISTOPHER WANJEK work for Raytheon Information Technology and Scientific Services in support of NASA space-science missions.
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Author:Leonard, Peter J.T.; Wanjek, Christopher
Publication:Sky & Telescope
Date:Jul 1, 2000
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