Fermi comes to CERN
In only 10 months of scientific activity, the Fermi space observatory has already collected an unprecedented wealth of information on some of the most amazing objects in the sky. In a recent talk at CERN, Luca Latronico, a member of the Fermi collaboration, explained some of their findings and emphasized the strong links between High Energy Physics (HEP) and High Energy Astrophysics (HEA).
The Fermi gamma-ray telescope was launched by NASA in June 2008. After about two months of commissioning it started sending significant data back to the Earth. Since then, it has made observations that are changing our view of the sky: from discovering a whole new set of pulsars, the greatest total energy gamma-ray burst ever, to detecting an unexplained abundance of high-energy electrons that could be a signature of dark matter, to producing a uniquely rich and high definition sky map in gamma-rays.
The high performance of the instrument comes as no surprise to the members of the collaboration. "We are proud that data confirms the good design of our instrument", says Luca Latronico, head of the group responsible for instrument calibration and performance parameters. "The Large Area Telescope (LAT) was designed to have a much larger acceptance than EGRET, Fermi’s predecessor. This is a key factor in detecting photons coming from a large fraction of the sky at any instant, therefore catching transient emission of sources. Their origins are quite frequently distant active galaxies. When we observe such a signal, we alert the whole scientific community through the Astronomical Telegrams (ATels). A wide coverage is similarly important for detecting gamma-ray bursts, which are isotropically distributed in the sky".
With an acceptance of this magnitude, Fermi is about 30 times more sensitive than EGRET. "The increase in sensitivity is obtained by a combination of advanced detector technologies and sheer geometry", explains Latronico. "The use of silicon-strip detectors in the tracker, for example, allows many efficient detection layers to be stacked in a small height, providing a large field of view to the instrument".
The performance of the LAT detector and the accurate modelling of it through complex MonteCarlo simulations are a direct HEP heritage. Latronico, like many other members of the collaboration, had previous experience in experiments at CERN and in other HEP Labs. Although Fermi is not the only example of a strong collaboration between the high-energy physics and the high-energy astrophysics communities, in this case the natural links lead to a dramatic enlargement of the mission’s scientific scope. "By merging scientific interests from HEA and HEP, the collaboration enlarged its physics menu a lot, and it now spans from traditional gamma-ray astronomy, like gamma-ray sky maps and catalogues, to modelling of cosmic accelerators, to fundamental physics like tests of Lorentz Invariance or dark matter searches", confirms Latronico.
Fermi is designed to be very sensitive to passing electrons and photons, two possible signatures of dark matter in the Universe. Although the nature of dark matter is still unknown, many theories postulate that dark matter would eventually produce high-energy electrons, positrons and gamma rays following annihilation or decay and the subsequent interactions with the interstellar medium or relic radiation like the Cosmic Microwave Background. The recent observation of an excess of electrons and positrons in the energy region up to 1 TeV with respect to existing models is one of the most striking results produced by Fermi. "Such a measurement can be ascribed to the presence of nearby sources of electrons of either astrophysical or dark matter origin", explains Latronico. "The potential of Fermi observations is that we can couple electron and gamma-rays observations. If the electron spectrum at high energy is ultimately connected to a nearby dark matter source, we should see some correlated signature in the diffuse gamma-ray emission. The shape of such a photon excess relative to astrophysical model predictions, if found, would help to constrain possible dark matter models".
What does it mean to go from designing detectors for particle physics to building an astrophysical observatory? "Working for a NASA mission meant learning a different way of working over the instrument, with a lot of emphasis on quality assurance, documentation and involvement of industrial partners", says Latronico. "I do not find the approach to science research different between collaboration members with HEP or HEA background, but we are somewhat culturally different in our idea of a collaboration. HEP people naturally work together in large groups, HEA people tend to work more individually or in small teams".
In about one month, in a truly collaborative spirit, data from Fermi will be made publicly available. "Together with photon data we will make available the appropriate science analysis tools for extracting correct science", anticipates Latronico. In addition, the collaboration will distribute a catalogue of the sources detected in the first year of activity as well as the diffuse galactic gamma-ray model, which is needed to perform source analysis. "Gamma-ray sources are so numerous and so dynamic that we can only benefit from having a larger number of users helping resolve the gamma-ray sky".
Did you know?
Astronomical telegrams (ATels) are messages distributed to the whole astrophysical community through a website accessible by any one: http://www.astronomerstelegram.org/
Only a restricted set of users can post the news items, which are then archived so that one can easily search for old messages. An RSS option is available.
The instrument
Fermi - formerly named GLAST - is a gamma-ray space telescope designed to explore some of the most energetic phenomena in the Universe. It consists of two main parts: the Large Area Telescope (LAT) that measures particles with an energy range of 20 MeV-300 GeV, and a Gamma-ray Burst Monitor made of detectors sensitive to 8 keV to 40 MeV energies. The LAT consists of a silicon strip tracker (80 m2 of silicon in total), a calorimeter (1536 crystals) and an anticoincidence detector. Compact electronics and light-weight stiff mechanics maximize the detector active surface within the dimensional tolerances dictated by the launch vehicle. Fast detectors and electronics provide small instrumental dead-time and allow the LAT to record variations in gamma-ray emissions in the sub-millisecond time scale.
Fermi is an international, multi-agency mission with important contributions from research institutions in France, Germany, Italy, Japan, Sweden and the US.
