Detection Of Multi-photon Events With The Fermi-lat

One of the striking improvements in performance of the Fermi-LAT over previous gamma-ray missions is its "shutter speed". When viewed as a camera, the LAT has a shutter speed approximately equal to its trigger window width (600 ns) and a frame advance time set by the readout dead time (26.5 us).

The combination of speed and large effective area suggests the possibility of recording simultaneous photons. It has been indeed suggested that some astrophysical sources could produce coherent high-energy gamma rays. In addition, extraordinarily bright, short bursts from, for example, the evaporation of black holes could result in such multi-photon events.

However, searches for such exotic events are not possible with the current reconstruction algorithms. More specifically, the lack of calorimeter clustering and a background rejection tuned on single-photon events kills almost completely any sensitivity the LAT might have to see such events. We are addressing both of these deficiencies with the re-design of the LAT reconstruction software currently underway. The new calorimeter clustering algorithm recognizes and separates distinct energy depositions within it and this, coupled with the new tracker pattern recognition, will enable a search for multi-photon events.