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[Music]

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Julie McEnery: Fermi is an observatory designed to see the most extreme places in the

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universe. We see gamma rays, which are are the highest-energy form of light,

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and with each object that we see these gamma rays from, what we're doing is exploring some of the

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places in the universe with most extreme environments.  Matthew Wood: The kinds of

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objects that it can study are pulsars and neutron stars, black holes, as well

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as dark matter. The main challenge in

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detecting gamma rays is that the Earth's atmosphere

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is opaque to them, so to get around that we use satellites in space

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to measure gamma rays. Julie: We don't have lenses

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and mirrors because gamma rays would just go straight through them.

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The main instrument on Fermi is the Large Area Telescope. It's a telescope

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that detects gamma rays by converting them into electron-positron pairs.

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Those are charged particles, so fundamentally our detector

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is designed to measure the tracks of those charged particles moving through

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and from that figure out where the gamma ray came from. The problem

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is that in the environment of low Earth orbit

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there's a very large number of charged particles. So for every gamma ray that we detect,

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ten thousand charged particles are coming through our detector.

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So we have to be able to tell the difference between that one

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gamma ray from those ten thousand charged particles, and that's

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the challenge. Often, they deposit a large amount of energy

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and stuff splashes up in all directions and you end up with extra hits

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in the tracking part of the detector that are not actually from the

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original electron and positron, but rather from energy that's kicked

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up when they interact further in the calorimeter.

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

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and complex program that basically uses all the

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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 a real gamma ray

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and not a charged cosmic ray. So, obviously,

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software is really important for the LAT.

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Matthew: The software that we use to analyze the LAT data has gone through many

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revisions over the course of the mission, but Pass 8 is really the first

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revision of the software where we took into account all the

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experience that we gained from operating the LAT in its orbital

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environment. Julie: Pass 8 has given us the equivalent

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of an in-space hardware upgrade, but on the ground.

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Matthew: We've increased the sensitivity of the LAT instrument

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by 40 percent and this is roughly equivalent to launching another

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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 Pass 8

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is our ability to view the sky at high energies

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where we have particularly improved our angular resolution, so the

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sharpness is very evident, and we've added lots more gamma rays,

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so we filled in what was kind of a spotty

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sky. Pass 8 has made everything better,

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but one of the things that it's made better is that it's allowed us to open our

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gamma-ray eyes to higher energies than before, so that's a completely new view,

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and it's allowed us to open our gamma-ray energy eyes to, at energies lower than before,

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so that's another completely new view. In addition to improving everything

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across the entire energy range. We have a wider field of view.

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We see more photons from any given point

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in our detector than we did before. Matthew: The improvements that we've made to the

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software retroactively apply to all the data that we've collected,

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and so these improvements significantly enhance what we can do

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with the data we already have, as well as the data that we'll collect in the

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future. Julie: With Pass 8, we're able to go back to the sensor-by-sensor

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information and we can reprocess that data into the

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improved performance that we get with Pass 8, from the first

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day of the mission, from August 2008, right the way up to the present day.

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Philippe: So with Pass 8, we

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use more completely and more efficiently all the information that is recorded

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

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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 through us.

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Julie: Look, obviously I'm biased. You know, I do think it's really cool. And, you know, when I first

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started working on Fermi, and you know, you start, you think

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"Oh how does this detector work?" and then you think more and think more and think more about it

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This is really cool! This is really cool even if you weren't doing any astrophysics with it. And then you add

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the astrophysics and it's awesome.

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[Music]

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[Silence]

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[Beeping]

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[Beeping]

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[Beeping]