The Physics of Milagro
Milagro is a fundamentally new type of detector. It is
the first instrument capable of continuously viewing the entire overhead
sky in the TeV energy regime. As such it is uniquely suited to study
the transient Universe and to discover new phenomena. Its physics reach
is large. On this page we will begin discussing some of the physics
topics that Milagro will address. As time permits new topics will be
added to this page.
Active Galactic Nuclei
Galactic Nuclei (AGN)
Active Galaxies are believed to be supermassive black holes
(100 million times the mass of the sun) surrounded by an accretion disk.
The accretion disk is composed of a hot gas made from stars that have
been torn apart by the tremendous tidal forces exerted by the black
hole. Through a poorly understood mechanism the black hole emits beams
of high energy particles along its rotation is, perpendicular to the
disk. On the left in the photograph one can see the beams of high energy
particles, known as jets. On the right is a high-resolution photograph
taken by the Hubble Space Telescope of the accretion disk.
Active Galaxies have been observed to emit trillion volt photons. In
fact the bulk of their energy output occurs at energies above a billion
volts. The mechanism by which the particles are accelerated to such
high energies are not completely understood. However, it is not surprising
that these objects are highly variable, they can change their energy
output (luminosity) by a factor of 10 in a few days.
The picture below is a cartoon of a model of particle acceleration
in an AGN.
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Gamma-Ray Bursts were discovered by scientists at Los Alamos
National Laboratory in the 1960's. The Vela satellite was put in orbit
to monitor Soviet nuclear tests. When the satellite was over the United
States the scientists looked upward, towards the heavens. What they
saw were mysterious flashes of x-rays coming from random points in the
sky, with no point ever repeating itself.
The picture shown here is from the BATSE
instrument on the Compton
Observatory. The direction of over 1000 bursts is plotted in galactic
coordinates. If the bursts originated within the Milky Way one would
expect to see a horizontal clustering of points in the middle of the
graph. No such clustering is evident. At present there is no agreed
upon model for what causes these bursts. In fact scientists even disagree
over where the bursts are coming from. A few think they may originate
within our solar system, some believe that they are associated with
a large extended halo around the Milky Way, and others believe that
they are very distant objects.
instrument (also on the Compton Observatory) has detected photons with
energies up to 20 billion volts (GeV) from gamma-ray bursts. Thus there
is a good chance that gamma-ray bursts emit particles energetic enough
for Milagro to detect. If so Milagro may be able to estimate the distance
to these objects. This can be accomplished by studying the shape of
the energy spectrum at energies near 1 trillion volts. If they are distant
objects then these photons will be absorbed by interactions with starlight
as they travel towards the earth. In this case we will not see many
photons above 1 trillion volts. However if they are "close"
to earth, that is within our galactic neighborhood, we should see photons
with energies above a trillion volts.
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While it may appear that our Sun is a relatively quiet star, in fact
the surface of the Sun can be violent. As we enter a period of increased
solar activity (known as solar maximum), the violence on the Sun's surface
is increasing. In some cases, huge quantities of plasma are ejected
through the corona at speeds ranging from ~10 to ~2000 km/s. These events
are known as coronal mass ejections, or CMEs. Due to driven shocks and
reconfigurations of the solar magnetic field structure, these events
are often associated with energetic particle acceleration. When the
charged particles reach the earth they interact high in our ionosphere
and can wreak havoc on our power grid, communications, and satellites.
While the exact mechanism is not completely understood, the particles
that accompany a CME can be very energetic. To date the highest energy
particles observed from the Sun have had energies of several GeV (billion
electron-Volts). Follow this link for a
slideshow of a CME by P. Charbonneau and O.R. White of the High
Altitude Observatory (NCAR) in Boulder Colorado. This series of pictures
below of a CME is taken from their slide show.
Operating in a different mode than that normally used to detect air
showers (Milagro can and does operate in both modes simultaneously)
Milagro can have significant sensitivity to protons and neutrons with
energies as low as 4 GeV. We call this mode of operation "scaler
mode". Here we simply count how many times all of the PMTs in the
pond are struck by light each second. Thus, even if only a single particle
from an air shower reaches the pond it can be detected. This is how
we gain sensitivity at such low energies. The drawback of this technique
is that we can not reconstruct the direction from which the particle
arrived. So our "background" (the constant data rate over
which signal must be observed) rate is quite high, being integrated
over the entire overhead sky. Nonetheless, Milagro is the world's most
sensitive detector of multi-GeV particles from the Sun.