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Julie McEnery: Fermi is an observatory designed to see the

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most extreme places in the universe. We see gamma rays, which are are the highest-energy

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form of light, and with each object that we see these gamma rays from, what we're doing is

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exploring some of the places in the universe with most extreme environments.

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Matthew Wood: The kinds of objects that it can study are pulsars and neutron

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stars, black holes, as well as dark matter.

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The main challenge in detecting gamma rays is that

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the Earth's atmosphere is opaque to them, so to get around that we

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use satellites in space to measure gamma rays. Julie: We don't have

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lenses and mirrors because gamma rays would just go straight

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through them. The main instrument on Fermi is the Large Area Telescope.

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It's a telescope that detects gamma rays by converting them into

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electron-positron pairs. Those are charged particles, so fundamentally

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our detector is designed to measure the tracks of those

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charged particles moving through and from that figure out where the gamma ray came from.

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The problem is that in the environment of

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low Earth orbit there's a very large number of charged particles. So for every

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gamma ray that we detect, ten thousand charged particles are coming

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through our detector. So we have to be able to tell the difference between

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that one gamma ray from those ten thousand charged particles,

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and that's the challenge. Often, they deposit a

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large amount of energy and stuff splashes up in all directions and you end up with

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extra hits in the tracking part of the detector that are not actually

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from the original electron and positron, but rather from

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energy that's kicked up when they interact further in the calorimeter.

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Philippe Bruel: So, to analyze these events we have written

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a very long and complex program that basically

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uses all the information that was recorded by the instrument

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and figures out what is the direction of the gamma ray,

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its energy, and whether or not it's

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a real gamma ray and not a charged cosmic ray.

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So, obviously, software is really important for the LAT.

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Matthew: The software that we use to analyze the LAT data

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has gone through many revisions over the course of the mission, but Pass 8 is

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really the first revision of the software where we took into account

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all the experience that we gained from operating

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the LAT in its orbital environment. Julie: Pass 8

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has given us the equivalent of an in-space hardware upgrade,

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but on the ground. Matthew: We've increased the sensitivity of the LAT instrument

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by 40 percent and this is roughly equivalent to

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launching another LAT instrument and operating it as well for seven years.

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So it's a fairly substantial improvement in the LAT performance.

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Julie: The most immediate, kind of shocking, benefit of

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Pass 8 is our ability to view the sky

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at high energies where we have particularly improved our

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angular resolution, so the sharpness is very evident, and we've added

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lots more gamma rays, so we filled in what was kind of

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a spotty sky. Pass 8 has

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made everything better, but one of the things that it's made better is that it's

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allowed us to open our gamma-ray eyes to higher energies than before, so that's a completely

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new view, and it's allowed us to open our gamma-ray energy eyes to, at energies

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lower than before, so that's another completely new view. In addition to

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improving everything across the entire energy range.

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We have a wider field of view. We see more photons from

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any given point in our detector than we did before. Matthew: The improvement

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that we've made to the software retroactively apply to all the data that we've

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collected, and so these improvements significantly enhance

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what we can do with the data we already have, as well as the data

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that we'll collect in the future. Julie: With Pass 8, we're able to go back to the

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sensor-by-sensor information and we can reprocess that data

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into the improved performance that we get with Pass 8,

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from the first day of the mission, from August 2008,

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right the way up to the present day. Philippe: So with Pass 8, we

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use more completely and more efficiently all the information

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that is recorded by the instrument. One thing that I like is

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that these photons took a lot of time

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to come to Earth, so that it's a pity to

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just miss one of them. So I'm glad that with Pass 8 we're

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more efficient and we record every gamma ray that passes

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through us. Julie: Look, obviously I'm biased. You know, I do think it's really cool.

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And, you know, when I first started working on Fermi, and

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you know, you start, you think "Oh how does this detector work?" and then you think more and think more

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about it. This is really cool! This is really cool even if you weren't doing any

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astrophysics with it. And then you add the astrophysics and it's awesome.

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