Fermi Sends a Full Sky View 0f Gamma Rays

Fermi Telescope

Full sky view of the galactic gamma rays

Fermi gamma-ray image updates 'extreme Universe' view By Jason Palmer
Science and technology reporter, BBC News, Rome

The full sky is seen in the map from the Fermi telescope; gamma-ray sources abound along the galactic plane (central bar) but are scattered throughout the cosmos. The Fermi space telescope has yielded the most detailed gamma ray map of the sky - representing the Universe's most violent and extreme processes. The telescope's newest results, as well as the map, were described at the Third Fermi Symposium in Rome this week.
Gamma rays are the highest-energy light we know of, many millions of times more energetic than visible light. The Fermi collaboration will soon release a full catalogue of all the gamma ray sources discovered so far. The space telescope was launched in 2008, and the Rome meeting gathered together the hundreds of scientists who worked with the data it produces.

Every three hours, the telescope gathers up a full scan of the sky, spitting out 40 million bits of information each second that it beams back to the Earth. One of its two instruments, the Large Area Telescope (Fermi-Lat), has already identified some 1,400 gamma ray sources - a number that will jump significantly with the publication of the next catalogue. Meanwhile, its Gamma Ray Burst Monitor has caught hundreds of the bursts - occasional outpourings of gamma ray energy that can release in hours more energy than our Sun will ever produce.

"When you look at the Universe with gamma-ray eyes what you're seeing is the 'extreme Universe'," said Julie McEnery, Fermi project scientist. "You're looking at things where there's enormous acceleration, enormous energy. We see neutron stars, we see supermassive black holes, we see particles moving at close to the speed of light smashing into gas in our galaxy."
One topic of discussion at the meeting is the classification of various gamma ray sources. These can be so-called active galactic nuclei whose centres can contain black holes that spew out threads of gamma rays, sometimes pointing at the Earth. Similarly, they can be pulsars, the rapidly spinning neutron stars that rhythmically flash their radiation toward the Earth.

"We've seen a lot of what we expected to see, and some things we didn't expect to see," Dr McEnery said.
"We didn't expect that we'd see as many pulsars shining only in gamma rays, and we've been stunned to discover dozens of millisecond pulsars, that's been really astounding.  "And in some cases we haven't seen things we did expect to see, and that's interesting too. For example we haven't seen clusters of galaxies; you'd expect them to be gamma ray sources and by not seeing them, that means that some of the ideas people had about high-energy particles in galaxy clusters must not be true.
But lurking among the data Fermi has collected is the promise of new physics - there are certainly unidentified gamma ray sources that may represent new kinds of celestial objects.  And yet to come may be hints of the dark matter that is believed to make up the majority of the mass of the Universe.

"Dark matter is an excellent example of the kind of new physics that Fermi is sensitive to," said Steven Ritz, deputy principal investigator for Fermi-Lat. We know it must be a form of matter that is unlike the stuff we know about in our theories of particle physics - it must have different properties," he explained to BBC News.
"Theories that go beyond what we currently see tend very neatly to predict the existence of particles that... when they meet each other can undergo a process that generates gamma rays." Dr Ritz said that such "indirect" dark matter detections in far-flung parts of the cosmos could complement the kind of searches for never-before-seen particles that are going on at facilities such as the Large Hadron Collider.
In fact, the motions of charged particles in extreme magnetic fields that give rise to many of the gamma rays that Fermi sees are just like particle accelerators.

"It's one of the things I like about this field - the biggest things and the smallest things in the Universe are neatly tied together in surprising ways."

Active galactic nuclei are the most common Fermi gamma-ray sources.

 Fermi can lend its expert view to physics closer to home; several presentations at the meeting focused on gamma rays from the Sun that could shed light on events such as solar flares and coronal mass ejections. The shock wave that propagates outward from the Sun during such outbursts can also accelerate particles that can potentially endanger satellites and astronauts - but the details of such processes remain poorly understood.

"The point with Fermi is that it's so sensitive it's likely going to pick up events never seen before," said Gerald Share, a high-energy astrophysicist from the University of Maryland. "