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Dark matter at the Fermi Symposium

A prime motivator for building the Fermi Gamma-ray Space Telescope was to search for signals of dark matter. After one year of data collection, scientists have learned a lot from Fermi and are improving their models of the universe, which will eventually provide road maps to this elusive phenomenon. But as far as actually finding definite signals of the stuff: not yet.

Simona Murgia of the Kavli Institute for Particle Astrophysics and Cosmology at SLAC gave a presentation at this week's Fermi Symposium about the many ways Fermi is searching for dark matter, but reported that its initial explorations haven’t turned up any smoking guns.

But astrophysicists aren’t deterred. What was perhaps most promising was that some of the items that Murgia listed as needing improvement in order to advance the dark matter search were already being presented as new results by other scientists. Other speakers reported finding potential new sources of the diffuse gamma ray background or untangling the many gamma ray sources at the galactic center, where we expect to find a dense concentration of dark matter. The work of scientists in various sub-areas is clearly linked by a common need to understand our gamma-ray universe.

Murgia also made an announcement in relation to the dark matter search by Fermi’s predecessor EGRET (Energetic Gamma Ray Experiment Telescope). EGRET detected an excess of gamma rays in the GeV range during its nine years of observation. It was theorized that this detection might be evidence of some new phenomenon, possibly dark matter.

To determine if there is an excess of gamma rays, you first need to define what is normal. Scientists can use models to approximate the number of gamma rays we should see coming from individual point sources in the galaxy and the rest of the universe, and add them all up. When researchers subtracted that total from the EGRET data, they found that there were more gamma rays than they had accounted for. That immediately suggests that either an as-yet-unidentified source, possibly dark matter, is emitting gamma rays, or more likely that the model needs to be refined in any number of ways. Any claim of an excess relies heavily on the strength of the model, which relies on an ever-growing understanding of astrophysics and a growing library of observations.

Murgia announced that Fermi has not found the same excess reported by EGRET. This does not necessarily negate the EGRET result, but brings into question the models on which it was based. The final answer may lie somewhere in between.

“You make a model that includes dark matter, and if your observation doesn’t match your model then sometimes you really have to rethink your parameters,” said Murgia. “Maybe the right one is one we haven’t even thought of yet.”

The meeting did see a little commotion over the dark matter search. Last week, a paper by researchers outside the Fermi collaboration who analyzed the telescope's publicly available data  reported finding an excess of gamma rays between us and the galactic center. In their paper the group steered clear of claiming that this was a signal of dark matter; but after the symposium talk by Neal Weiner of New York University, the suggestion hung thick in the air (their paper also stirred up rumors of dark matter on the blogosphere). At the end of his talk, when the moderator asked if there were any questions, an audience member immediately went to the microphone and said the excess could be explained another way -- namely inverse Compton scattering (which is when energized electrons strike photons and boost their energies) created by a cosmic bubble very near our solar system. Weiner responded quite pleasantly, “Yes, and we’re looking forward to discussing that.”

The person who spoke up at the end of Weiner’s talk was referring to work  by Jean Marc Casandjian and Isabel Grenier of CEA Saclay, which was presented in a poster. Casandjian and Grenier, representing the Fermi collaboration, say the excess gamma rays fit the already known shape of the LOOP 1 cosmic bubble. LOOP 1 might be the remnants of an exploded supernova or other massive event that left a radiating bubble about 100 parsecs from the sun (we are surrounded by a similar bubble called the local bubble). LOOP 1 lies directly in our line of sight to the galactic center, and radiation from the bubble interferes with our view of the busy galactic plane. Casandjian hopes the analysis he is doing will allow researchers to subtract the effects of LOOP 1 and get a much clearer view of the Milky Way and the galactic center. There is more work to be done on their analysis before it is ready for publication.

In response to the notion that the gamma-ray excess could be something other than LOOP 1, Casandjian says other analyses did not take the time to extract the detailed structure of the gamma-ray emissions, but instead saw them as a general haze. From the detailed map that he and Grenier constructed, Casandjian said he believes the emissions are a very good fit to the known structure of LOOP 1, which can also be seen in radio frequencies.  Still, he acknowledged that he could not claim to understand everything going on with the gamma ray excess.