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Julie McEnery: Fermi is an observatory designed to see the most extreme places in the
universe. We see gamma rays, which are are the highest-energy form of light,
and with each object that we see these gamma rays from, what we're doing is exploring some of the
places in the universe with most extreme environments. Matthew Wood: The kinds of
objects that it can study are pulsars and neutron stars, black holes, as well
as dark matter. The main challenge in
detecting gamma rays is that the Earth's atmosphere
is opaque to them, so to get around that we use satellites in space
to measure gamma rays. Julie: We don't have lenses
and mirrors because gamma rays would just go straight through them.
The main instrument on Fermi is the Large Area Telescope. It's a telescope
that detects gamma rays by converting them into electron-positron pairs.
Those are charged particles, so fundamentally our detector
is designed to measure the tracks of those charged particles moving through
and from that figure out where the gamma ray came from. The problem
is that in the environment of low Earth orbit
there's a very large number of charged particles. So for every gamma ray that we detect,
ten thousand charged particles are coming through our detector.
So we have to be able to tell the difference between that one
gamma ray from those ten thousand charged particles, and that's
the challenge. Often, they deposit a large amount of energy
and stuff splashes up in all directions and you end up with extra hits
in the tracking part of the detector that are not actually from the
original electron and positron, but rather from energy that's kicked
up when they interact further in the calorimeter.
Philippe Bruel: So, to analyze these events we have written a very long
and complex program that basically uses all the
information that was recorded by the instrument
and figures out what is the direction of the gamma ray,
its energy, and whether or not it's a real gamma ray
and not a charged cosmic ray. So, obviously,
software is really important for the LAT.
Matthew: The software that we use to analyze the LAT data has gone through many
revisions over the course of the mission, but Pass 8 is really the first
revision of the software where we took into account all the
experience that we gained from operating the LAT in its orbital
environment. Julie: Pass 8 has given us the equivalent
of an in-space hardware upgrade, but on the ground.
Matthew: We've increased the sensitivity of the LAT instrument
by 40 percent and this is roughly equivalent to launching another
LAT instrument and operating it as well for seven years.
So it's a fairly substantial improvement in the LAT performance.
Julie: The most immediate, kind of shocking, benefit of Pass 8
is our ability to view the sky at high energies
where we have particularly improved our angular resolution, so the
sharpness is very evident, and we've added lots more gamma rays,
so we filled in what was kind of a spotty
sky. Pass 8 has made everything better,
but one of the things that it's made better is that it's allowed us to open our
gamma-ray eyes to higher energies than before, so that's a completely new view,
and it's allowed us to open our gamma-ray energy eyes to, at energies lower than before,
so that's another completely new view. In addition to improving everything
across the entire energy range. We have a wider field of view.
We see more photons from any given point
in our detector than we did before. Matthew: The improvements that we've made to the
software retroactively apply to all the data that we've collected,
and so these improvements significantly enhance what we can do
with the data we already have, as well as the data that we'll collect in the
future. Julie: With Pass 8, we're able to go back to the sensor-by-sensor
information and we can reprocess that data into the
improved performance that we get with Pass 8, from the first
day of the mission, from August 2008, right the way up to the present day.
Philippe: So with Pass 8, we
use more completely and more efficiently all the information that is recorded
by the instrument. One thing that I like is that these
photons took a lot of time
to come to Earth, so that it's a pity to
just miss one of them. So I'm glad that with Pass 8 we're
more efficient and we record every gamma ray that passes through us.
Julie: Look, obviously I'm biased. You know, I do think it's really cool. And, you know, when I first
started working on Fermi, and you know, you start, you think
"Oh how does this detector work?" and then you think more and think more and think more about it
This is really cool! This is really cool even if you weren't doing any astrophysics with it. And then you add
the astrophysics and it's awesome.
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