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Gamma-Ray Astronomy

The Astroparticle physics group at the CENBG studies the sky as seen in GeV gamma rays. From 2003 through 2008, we prepared the LAT (“Large Area Telescope”) for what was then called the GLAST satellite but is now called Fermi. It was launched by NASA on 11 June 2008. The 2014 NASA senior review commended Fermi for its excellent ongoing scientific production and recommended that the mission continue through 2018. The instrument team involves laboratories from the USA, France, Italy, Japan and Sweden but the collaboration analysing the data comes from nearly everywhere.

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Fermi’s >1 GeV gamma-ray sky after six years on orbit
The bright horizontal band is the Milky Way: cosmic rays collide with interstellar dust and gas to produce pions, which then yield gamma-rays. One also see individual point sources. In the plane, they are mostly pulsars, with some supernova remnants and pulsar wind nebula. Off the plane, they are mostly AGNs (’blazars’) but there are also some millisecond pulsars.

Click here for a brief history of the CENBG Astroparticle Group.

Click here for a brief overview of Fermi in France and abroad.

Gamma rays are detected when they convert to an electron-positron pair in the LAT. One of the world’s largest silicon tracker measures their direction, with an on-axis resolution between 30 MeV and 10 GeV of 0.6/E0.8 degrees (E in GeV), a few-fold improvement as compared to the previous mission (EGRET on the Compton Gamma-Ray Observatory was de-orbited in 2000). The CsI(Tl) calorimeter measures energies up to 300 GeV, with an effective area 10 times greater than EGRET’s. The overall sensitivity will be 25 times greater than for EGRET. Hence both the number of gamma-ray emitters that can be detected and the ability to associate them with objects known at other wavelengths will be greatly enhanced.

The CENBG’s contribution to the instrument has been to study the energy response of the CsI crystals. A calorimeter module was taken to the GSI, Darmstadt to determine its response to relativistic heavy ions [1]. As preparation, a smaller version with superior electronics was built at the CENBG and taken to beams at the CERN SPS and GANIL. A ‘calibration unit’ consisting of two silicon trackers and three calorimeters will spend several weeks in the CERN PS and SPS beams this summer. CENBG personnel also worked on the calorimeter calibration using atmospheric muons while the LAT was being assembled at Stanford University during 2005. The final energy calibration will be performed on-orbit using cosmic ray ions.

With launch only a year away, efforts are shifting towards physics analysis. The group is concentrating on two topics: ‘blazars’ and ‘pulsars’. A ‘blazar’ is an active galactic nucleus harboring a supermassive black hole generating a jet pointing high energy particles towards Earth, similar in many respects to the better-known quasars. A major discovery by EGRET was to learn that the high energy sky is dominated by these extremely distant particle accelerators, and thus extremely powerful. Fermi has detected thousands of blazars, allowing major steps forward in their understanding.

Click here for a detailed list of Pulsar activities at the CENBG.

Pulsars are rapidly rotating neutron stars that sweep the earth with an intense beam, much like a lighthouse flashing an observer. EGRET detected fewer than 10 gamma-ray pulsars, while Fermi has detected about two hundred [2]. The end-point of the life cycle of many stars, an accurate census of gamma-ray pulsars is lacking. Furthermore, 40 years after their discovery, models of high energy particle acceleration by pulsars have many flaws which new observations should resolve. Gamma-ray pulsar studies benefit greatly from accurate radio timing, and our group is working with the 100 meter radio telescope in Nançay, France, to track over a hundred gamma-ray candidates. Ten years after the birth of the Astroparticle group at the CENBG, most of which time was spent building and running the CELESTE atmospheric Cherenkov telescope, the group is about to share in what will probably be the most significant mission in high energy astrophysics for the decade to come.

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References :

[1] B. Lott et al., Nucl. Instr. Meth. A560 (2006) 395
[2] D. Smith et al., Int. Conf. Astronomical Union, Prague (2006)