NASA’s Fermi Gamma-ray Space Telescope has produced a catalog of 1873 persistent gamma-ray sources detected over the entire sky. The nature of each object has been determined, when possible, and it turns out that a majority of them are very bright active galaxies. However, it wasn’t possible to conclusively identify about 30% of the sources.
Astronomers delight in the possibility of finding new types of gamma-ray-emitting objects within the “unassociated sources” that constitute roughly a third of the catalog. But Fermi’s LAT is revealing gamma-rays from an increasing — and sometimes, surprising — variety of astronomical objects.
One thing that astronomers and astrophysicists do a lot of is compile catalogs and censuses of various kinds of astronomical objects – stars, extrasolar planets, galaxies, or whatever. Since the universe is a pretty big place, these catalogs can involve “astronomical” numbers of objects. (Although, in the case of extrasolar planets, we are able to detect only a minuscule proportion at this time.) Earlier this year NASA released a new catalog comprising only 1873 objects altogether – but they’re pretty spectacular objects, namely objects that consistently emit gamma rays over long periods of time. (That leaves out such things as gamma-ray bursts and gamma-ray flares from the sun and other stars.)
Since gamma rays are the very most energetic form of electromagnetic radiation, it takes a pretty spectacular source to be able to generate significant quantities of gamma rays over an extended period of time – from years to a hundred million years or more. The latest catalog has attempted to identify the nature of each object in the census, and as expected, a majority of objects are active galaxies – that is, galaxies containing a supermassive black hole that is feeding on immense amounts of infalling matter and spitting out jets of ultra-high energy particles and radiation.
Exactly what sorts of energies are we talking about here? The detector used to compile the catalog is called the Large Area Telescope, and it is sensitive to gamma rays with energies up to 300 GeV – 3×1011 eV (electron-volts). (Gamma rays can be generated in some astrophysical processes up to at least 30 times that energy, but such are pretty rare.) To put that energy in perspective, 300 GeV is the kinetic energy of a 5 ounce baseball moving at the grand speed of about 3 meters per hour (if my calculations are correct). Although that doesn’t sound too impressive at first, the point is that one single 300 GeV gamma-ray photon has as much energy as that baseball’s energy of motion.
Active galaxies are capable of converting matter to pure energy much more efficiently than any star, and the most luminous examples can convert as much matter into energy in one year as is contained in a couple of solar mass stars (which normally shine for 10 billion years or so). That’s pretty intense, so it’s not surprising that part of that energy output is in the form of high-energy gamma rays – and that can go on for tens of millions of years.
NASA’s catalog of 1873 persistent gamma-ray sources attempts to identify the nature of each source, and active galaxies make up more than half the total, 58% to be exact. Another 10% is made up of pulsars and supernova remnants (like the Crab Nebula) together. The interesting thing is that the nature of most of the remaining 30% is unknown.