Our galaxy in gamma rays

Europe's Integral space telescope investigates gamma-ray sources in the Milky Way.Amanda Gefter
By | Published: November 18, 2003
With a suite of four instruments, Integral will provide new insights into the most violent and exotic cosmic objects, such as black holes, neutron stars, active galactic nuclei, supernovae, and mysterious gamma-ray bursts (the most energetic phenomena in the universe).
ESA / D. Ducros
November 18, 2003
If our eyes were built to see light in gamma wavelengths, our universe would look drastically different. Vast stretches of foreboding darkness would be punctuated not by the twinkle of glowing stars, but rather by the blaze of violent eruptions more potent than any since creation itself.

Such is the universe that the European Space Agency’s Integral space telescope is now exploring. Launched in October 2002, Integral (International Gamma-Ray Astrophysics Laboratory) is taking the most sensitive gamma-ray survey of the Milky Way Galaxy to date, which may supply answers to many questions about the most extreme events in our galactic backyard.

Many celestial objects and physical events marked by extraordinarily high energy produce light at gamma-ray wavelengths. So when astronomers find gamma rays in the galaxy, they start looking for a source. This is where Integral comes in. Equipped to survey the sky in multiple wavelengths simultaneously, Integral is the first observatory that utilizes gamma rays, X rays, and visible light at once. The telescope also carries a gamma-ray spectrometer that looks for imprints of particular elements in light. Probing gamma-ray signatures is the most direct test of current theories of element formation. These theories are crucial to understanding how the solar system — and life — formed.

In its searches thus far, Integral has found the radioactive isotope aluminum-26 scattered throughout the galaxy. Astronomers believe aluminum-26 is produced in intensely energetic supernova explosions that accompany the deaths of massive stars. Integral seeks to confirm this hypothesis by searching for the signature of radioactive iron, which is produced only in supernovae. If Integral’s map of iron “hot spots” matches locations where the aluminum is found, scientists will be able to label supernovae as the culprit.

This false-color picture was taken by the spectrometer on board Integral (SPI) between December 2002 and March 2003. The yellow dots correspond to bright known gamma-rays sources, while blue areas indicate regions of low emission.
ESA / SPI Team
This kind of dual survey was never possible before, but with Integral’s unprecedented resolution — forty times better than earlier satellites — astronomers hope to solve this and other mysteries. For example, when aluminum-26 decays into magnesium, the energy released is large enough to create antimatter (looking-glass particles like positrons and antiprotons). Integral’s study of the turbulent heart of the Milky Way, where scientists suspect a black hole three million times the mass of our Sun resides, reveals a huge cloud of antimatter that seems too spread out to be from a single source.

In addition, data collected by Integral reveals that the abundance of antimatter exceeds the amount of radioactive aluminum, so the antimatter must be coming from something else. Possible sources include black holes, supernovae, jets from neutron stars, stellar flares, or other extreme phenomena. By analyzing gamma rays associated with the antimatter, and also by surveying relevant locations in multiple wavelengths, Integral should soon be able to pinpoint what is producing the antimatter, letting scientists know exactly what lurks in our galactic core.

Astronomers know that gamma rays mark the celestial grounds prowled by the universe’s exotic beasts. As Integral continues its gamma-ray studies, it may be able to hunt down these enigmatic objects once and for all.

“We have collected excellent data in the first few months,” states Integral Project Scientist Chris Winkler, “but we can and will do much more in the next year. Integral’s accuracy and sensitivity have already exceeded our expectations and, in the months to come, we could get the answers to some of astronomy’s most intriguing questions.”