Very high energy gamma ray astronomy probes some of the most extreme environments in the known universe. Known sources of TeV gamma rays contain compact objects (black holes or neutron stars) with intense gravitational and/or magnetic fields. Known sources of TeV gamma rays include supernova remnants within our galaxy and active galactic nuclei outside of our galaxy. Other possible sources include gamma-ray bursts, micro-quasars, diffuse emission from cosmic rays interacting with matter in our own galaxy, and more exotic objects such as primordial black holes and weakly interacting massive particles. Six sources of TeV photons have been detected in the northern hemisphere. The paucity of known sources is due to both the available instrumentation and the sources. The combination of transient sources and narrow field instruments makes the detection of new sources difficult. Milagro is the first detector capable of continuously monitoring the entire overhead sky at energies above a few hundred GeV. Milagro is a water Cherenkov extensive air shower detector located near Los Alamos, NM at 2630m above sea level, consisting of a ~5,000 m2 central (pond) detector surrounded by an array of 175 instrumented water tanks, (outriggers) that span an area of roughly 40,000 m2. The Milagro pond is a 6-million gallon water reservoir, which measures 80m x 60m x 8m deep and is covered with a light-tight cover. The reservoir is instrumented with 723 20-cm PMTs on a 2.8m x 2.8m grid. The PMTs are deployed in two layers. The top layer of 450 PMTs is under 1.5 meters of water and the bottom layer of 273 PMTs is under 6 meters of water. Timing of the arrival of the shower particles is used to determine the direction of the primary air shower. During the three years we have been operating we have developed a robust method for reducing the cosmic ray background and used this rejection method to detect TeV gamma rays from the Crab Nebula, Mrk 421, and to see the first evidence for diffuse TeV emission from the galactic plane. In addition, we have monitored the northern hemisphere for both transient and steady sources of TeV photons. To date we have observed emission only from the three sources mentioned above. We have now set the best limits on TeV sources in the northern hemisphere over all timescales from 1 millisecond to 2.5 years. During the past year we have implemented a real-time analysis system that allows us to detect a gamma-ray burst within 4 seconds of its occurrence and to send an alert to other observatories. We have not yet detected significant emission from any region within our field of view on timescales from 1ms to 2hrs. During this period there were relatively few bursts detected by the HETE and INTEGRAL satellites. Later this year the launch of SWIFT will provide us with a rich dataset to search for coincident TeV emission from gamma-ray bursts. Just as Milagrito, our prototype, observed significant emission from one GRB detected by BASTE we expect the advent of SWIFT to prove a fruitful period for Milagro. This proposal will allow us to continue to operate Milagro for the next three years. During the past two years several improvements have been made to the detector. We have implemented an intelligent triggering system that has enabled us to substantially lower the trigger threshold of Milagro. This has increased the effective area at 100 GeV four-fold - critical for detecting gamma-ray bursts. Most importantly, we have just completed the construction of an array of 175 water tanks that surround the pond. These tanks improve the energy and angular resolution and the background rejection capabilities of Milagro. We have obtained a 3-fold increase in sensitivity now that these detectors are incorporated into our event reconstruction. Milagro data are easy to explain to students and the general public, who often have difficulty understanding how we claim to see particles that they don't. The collaboration has been very active in exposing students and the general public to the excitement of the particle astrophysics (PA) field. Each year we bring undergraduate students to work on Milagro and generally involve students in analysis. In addition, we give on average eight talks each year at high schools and at least two public lectures on the subject of PA. He also works with the US Physics Olympics team. In addition, he works with the Maryland program for 8th and 10th grade girls. He also started the physics "Question of the Week" (see it at www.physics.umd.edu) that is used by many teachers world-wide.
