New window opens on high-energy universe
Première lumière du télescope spatial Fermi
Le 26 août, la NASA et le ministère américain de l’Énergie ont annoncé les résultats de la première lumière du télescope spatial à rayons gamma GLAST. À cette occasion, le télescope a été rebaptisé télescope spatial Fermi. Fruit d’une collaboration internationale entre astrophysiciens et physiciens des particules, le télescope spatial Fermi devrait ouvrir une nouvelle fenêtre sur l’univers à haute énergie et découvrir des milliers de nouvelles sources de rayons gamma, et donc traiter de nombreuses questions restées sans réponse dans les domaines de la physique, de l’astronomie et de la cosmologie.
On 26 August, NASA and the US Department of Energy announced the first-light results of the Gamma-Ray Large Area Space Telescope (GLAST). At the same time GLAST changed its name to the Fermi Gamma-ray Space Telescope. Built in an international collaboration of astrophysicists and particle physicists with important contributions from research institutions in France, Germany, Italy, Japan, Sweden and the US, Fermi is expected to discover thousands of new sources of different classes, thus tackling many unresolved questions of fundamental physics, astronomy and cosmology. The telescope is already detecting high-energy gamma-rays from a wealth of cosmic sources – including super-massive black holes in active galactic nuclei, supernova remnants, neutron stars, galactic and solar system sources, and gamma-ray bursts (GRBs) – with more than 30 times the sensitivity of its successful predecessor, the Energetic Gamma Ray Experiment Telescope (EGRET).
Gamma-rays are produced by the interaction of high-energy charged particles with local matter, magnetic fields or ambient photons, and thus give insight into the physical conditions prevailing in these exotic sources. The physics of the particle acceleration mechanisms believed to be operational in many of these objects was first proposed by Enrico Fermi, who is now honoured with the new name of the telescope. Through investigation of the most extreme places in the universe, Fermi will shed light on many fundamental physics questions, such as, the nature of the ubiquitous dark matter. Dark matter particles could decay or annihilate into gamma-rays and possibly give rise to unambiguous signatures in gamma-ray spectra, which could be used to infer or constrain the properties of the original particles. In understanding dark matter, observations with Fermi will therefore be an essential complement to searches for new particles at CERN.
The main instrument on board Fermi is the Large Area Telescope (LAT), which detects gamma-rays between 20 MeV and 300 GeV (Michelson et al. 2008). The addition of the secondary GLAST Burst Monitor (GBM) – an instrument primarily dedicated to the detection of GRBs between 8 keV and 30 MeV (von Kienlin et al. 2001) – gives Fermi a total coverage of seven decades in energy. The aspect ratio of the LAT allows for a large field of view, observing 20% of the whole visible sky at any instant, while the GBM provides complete sky coverage for the detection of GRBs.
Fermi was launched by NASA on 11 June from the Cape Canaveral Air Station in Florida, for a 5–10 year long mission (figure 1). The first 60 days of data taking constituted the commissioning phase, which went smoothly thanks to the thorough preparatory work undertaken by the whole international Fermi Mission team. During this period, teams undertook the calibration and verification of the performance of the different subsystems. Background rejection, a key element to the success of the mission, proved very satisfactory. Then, on 14 August Fermi entered the phase of nominal science operations, surveying the complete gamma-ray sky every three hours.
Figure 2 shows the LAT all-sky image released on 26 August. Created using only 95 hours of “first light” observations from the early commissioning phase, this corresponds in source sensitivity to a whole year of observations by EGRET (Thompson et al. 1993). The map shows gas and dust in the plane of the Milky Way emitting gamma rays owing to collisions with cosmic rays. Other clearly visible sources include the Crab, Geminga and Vela pulsars in our own Galaxy as well as the blazar 3C454.3, an active galaxy located 7.1 billion years away. This source appears particularly bright in the map as it was in a flaring state at the time of the acquisition, as the Fermi/GLAST collaboration announced through the Astronomer’s Telegram (Atel 1628 2008).
Fermi has since witnessed the intrinsically dynamic nature of the gamma-ray sky with the detection of another three active galactic nuclei in a high flaring state (Atel 1650, 1701 and 1707 2008) and the detection of two GRBs with giga-electron-volt energy emission (GCN Circulars 8183 and 8246 2008). These bursts were detected by the LAT in coincidence with the GBM, which has also detected another 30, lower-energy bursts since its turn-on on 25 June.
The LAT is a pair-conversion telescope, which consists of an array of 4 × 4 towers, each comprising a precision converter/tracker and a calorimeter (figure 3). Each tracker module has 18 x-y tracking planes, which contain single-sided silicon strip detectors (400 μm thick with a 228 μm pitch) interleaved with a high-Z converter material (tungsten). The tracker has an active surface of 70 m2 (comparable to the Inner Tracker of the ATLAS detector at CERN, with just over 60 m2) and 900,000 digital channels.
Each calorimeter module consists of 96 CsI(Tl) crystals, which are 2.7 cm × 2.0 cm × 2.6 cm in size and are arranged in eight layers of 12 crystals, each forming a hodoscope (x-y) array. The total depth of the calorimeter is 8.6 radiation lengths (out of 10.1 radiation lengths for the whole instrument). The dimensions of the crystals are comparable to the CsI radiation length (1.86 cm) and Moliere radius for electromagnetic showers (3.8 cm). The segmentation allows for spatial imaging of the shower profile and accurate reconstruction of the shower direction, thus making possible the high energy reach of the LAT and improving background rejection.
The tracker is surrounded by an anticoincidence detector (ACD), consisting of 89 plastic scintillator tiles of different sizes, which are read out by wavelength-shifting fibres coupled to photomultiplier tubes. The ACD is used to reject charged cosmic rays and therefore must have a high efficiency for charged particle detection (<0.9997). The segmentation is optimized to limit the effect of “backsplash” (secondaries produced in the interaction of high-energy photons with the heavy calorimeter, giving a signal in the ACD), which reduced the efficiency of EGRET by at least a factor of two at energies above 10 GeV. The calibration of the LAT is based on a combination of in-orbit and ground-based cosmic-ray data, together with beam tests performed at CERN (at the PS and SPS) and GSI (Baldini et al. 2007) and Monte Carlo simulations using Geant 4.
The GBM, which is dedicated to the detection of GRBs, includes 12 sodium iodide (NaI) scintillation detectors and two bismuth germanate (BGO) scintillation detectors. The NaI detectors cover the lower part of the energy range, from a few kilo-electron-volts to about 1 MeV, and provide burst triggers and locations. The BGO detectors cover the energy range from about 150 keV to around 30 MeV, providing a good overlap with the NaI at the lower end, and with the LAT at the high end.
Within only a few days of turn-on, using data originally planned for observatory calibration, Fermi has already corroborated many of the great discoveries both of EGRET and of AGILE (Tavani et al. 2008). The LAT instrument is already finding new sources. Such spectacular results have only been achieved thanks to an advanced design for the observatory, which makes use of state-of-the-art particle-physics instrumentation that gives Fermi exceptional resolution and sensitivity. As a result, understanding of the high-energy universe is sure to grow tremendously, but even more exciting could be the unexpected, as history shows that opening new observational windows often yields completely unanticipated discoveries.
• The institutions participating in the collaboration built and qualified the LAT subsystems which then were integrated at SLAC. The detectors for the GBM were produced at the Max-Planck-Institute for Extraterrestrial Physics in Garching, and were integrated at the Marshall Space Flight Center in Huntsville, Alabama. Both instruments were integrated with the spacecraft at General Dynamics, in Phoenix, Arizona, to form the Fermi observatory. Environmental testing was then performed both at General Dynamics and at the Naval Research Lab in Washington DC.