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

Gamma-Ray Bursts

Solar Physics

 Active 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 (GRBs)

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.

The EGRET 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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Solar Physics

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.