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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 improvement
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
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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