How was this Fermi telescope experiment done?

[thenewmans]

I’m interested in how they did this experiment. They determined that two 7 billion year old gamma photons arrived at nearly the same time. But how do they know they left at the same time? If I had to, I would guess that they continually measure the light at a variety of frequencies, find the time of the peak of the burst at each frequency and then see how far apart those peaks are. But there’s no way for me to tell if it was done anything like that from these articles.

http://www.dailymail.co.uk/sciencetech/article-1223853/Nasa-Fermi-telescope-uncovers-proof-Einsteins-space-time-theory.html

http://www.newscientist.com/article/dn18068-universes-quantum-speed-bumps-no-obstacle-for-light.html

http://www.popsci.com/node/40163

http://www.nytimes.com/2009/10/29/science/space/29light.html