The Milagro Gamma-Ray Observatory Milagro is a water Cherenkov extensive air shower (EAS) detector located near Los Alamos, NM at 2630m above sea level, consisting of a ~5,000 m 2 central (pond) detector surrounded by an array of 175 instrumented water tanks, (outriggers) that span an area of roughly 40,000 m 2. The Milagro detector has 723 photomultiplier tubes (PMTs) submerged in a 24 million liter water reservoir. The PMTs are arranged in two layers, each on a 2.8 x 2.8 m grid. The top layer of 450 PMTs (under 1.4 m of water) is used primarily to reconstruct the direction of the air shower. By measuring the relative arrival time of the air shower across the array, the direction of the primary cosmic ray can be reconstructed with an accuracy of roughly 0.75 o.The bottom layer of 273 PMTs(under 6 m of water) is used primarily to discriminate between gamma-ray Initiated air shower and hadronic air showers. The sides of the reservoir are sloped (2:1) so that the area of the bottom of the reservoir is smaller than the top, leading to the smaller number of PMTs in the bottom layer. The Milagro pond with the cover inflated for servicing. Arial view of the Milagro detector. The Shadow of the Moon as observed by Milagro. The shadow of the Moon in cosmic rays can be used to determine the performance characteristics of Milagro. At TeV energies the Moon’s shadow is offset from the actual position of the Moon because the cosmic rays are bent in the earth’s magnetic field. From the position and shape of the observed shadow one can determine the angular resolution of the detector and the absolute energy response of the detector. Milagro, the Spanish word for miracle, is a new type of astronomical telescope. Like conventional telescopes, Milagro is sensitive to light but the similarities end there. Whereas "normal" astronomical telescopes view the Universe in visible light, Milagro "sees" the Universe at very high energies. The "light" that Milagro sees is in the TeV Range. What is Milagro? Why Milagro? When one views the heavens in the TeV range the picture is quite different from what we see when we look up at the night sky. The number of objects we see are much fewer and much more "extreme". We see super massive black holes and neutron stars. Some of these sources are known to be highly variable, flaring on timescale of minutes to days. In addition we hope to discover new sources of TeV photons, possibly observe TeV emission from Gamma-Ray Bursts, discover primordial black holes, or discover completely new phenomena. Until the advent of Milagro there was no instrument capable of continuously monitoring the entire overhead sky in the TeV energy regime. The existing instruments had to be pointed at small regions of the sky (usually known sources) and could only look at a source during the time of year it was overhead at night. Even then they could only look at the source if the weather was good and the moon was set. Milagro is ideally suited to monitor the variable TeV Universe and discover new sources of TeV gamma rays. Cosmic Rays and Extensive Air Showers The Earth is immersed in a "sea" of high-energy nuclei known as cosmic rays. Cosmic rays are composed of all nuclei, from the simple hydrogen nucleus (a proton) to the iron nucleus and beyond (transuranic elements have been observed in cosmic rays). The energy spectrum of cosmic rays has been measured up to 10 9 TeV. When a high-energy cosmic ray enters the atmosphere it loses its energy via interactions with the nuclei that make up the air. At high energies these interactions create particles. These new particles go on to create more particles, etc. This multiplication process is known as a particle cascade. This process continues until the average energy per particle drops below about 80 MeV. At this point the interactions lead to the absorption of particles and the cascade begins to die. This altitude is known as shower maximum. The particle cascade looks like a pancake of relativistic particles traveling through the atmosphere at the speed of light. Though the number of particles in the pancake may be decreasing, the size of the pancake always grows as the interactions cause the particles to diffuse away from each other. When the pancake reaches the ground it is roughly 100 meters across and 1-2 meters thick. If the primary cosmic ray was a photon the pancake will contain electrons, positrons, and gamma rays. If the primary cosmic ray was a nucleus the pancake will also contain muons, neutrinos, and hadrons (protons, neutrons, and pions). The number of particles left in the pancake depends upon the energy of the primary cosmic ray, the observation altitude, and fluctuations in the development of the shower. Shadow of the Moon AGN Active galaxies emit radiation over the entire electromagnetic spectrum from radio waves to TeV gamma rays. Thermal emission emanates from the accretion disk (infrared to X-rays) and the torus (infrared). Non-thermal emission (radio and gamma rays) comes from the jets. One of the more exciting discoveries of the 1990s has been the observation of TeV emission from several AGNs. TeV emission has been observed from Mrk 421, Mrk 501, and 1ES , 1ES Mrk 501 and 1ES are the first gamma- ray sources discovered by ground-based instruments. Milagro data was used to study Mrk 421 while it was flaring during the period of January to April of 2001 and again in November of During the 2001 period we observed a 4.7  excess and during the 2002 flare a 3  excess. Mrk 421 during the 2001 flare. The Crab Nebula The Crab nebula was the first source convincingly detected in TeV gamma rays. Since the original detection in 1989 the Crab has become the standard candle of TeV astronomy. The luminosity of the Crab is constant (within the accuracy of the measurements made to date) at 2.68(±0.42 stat ±1.4 sys )x10 -7 (E/1TeV) m -2 s -1 TeV -1. As a standard candle it is useful for cross calibrating the sensitivity of different instruments. From the shadow of the Moon and Monte Carlo simulation of the detector the angular resolution of Milagro is 0.8 degrees. The square angular bin that maximizes the significance of a signal has a width 2.8 times the angular resolution of the detector. Therefore an angular bin of width 2.1 degrees is used in this analysis. Data taken in the Crab Nebula region with 6  in the position of the Crab. In a manner identical to that used to analyze data from the region of the Crab nebula, the entire sky is searched for excesses over the background cosmic rays using data from Dec. 15, 2000 to Dec. 15, The sky is binned into 0.1x0.1 degree bins and the expected background and actual number of events detected in each bin is determined. These small bins are then summed into larger bins, commensurate with the angular resolution of Milagro. The resulting sky map is shown in the Figure. The circles are drawn around 26 active galaxies identified in Costamante and Ghisellini 2002 as likely sources of TeV gamma rays, including the five, which have all been observed at TeV wavelengths by other observatories: the Crab nebula, Mrk 421, Mrk 501, 1ES , and 1ES The brightest point in the TeV sky over this time period was Mrk 421. Most of the observed signal in this data set came from an outburst that began in December of 2000 and lasted for several months. The next brightest point in the sky is not associated with any of the drawn circles and is to the northwest of the Crab. Map of the Northern sky in TeV gamma rays. The scale is the significance of each point in the sky. The circles mark the locations of AGN and known TeV sources. Mrk 421 is the brightest object in the sky over this data set. All-Sky Survey Other topics that we are currently studying include: the study of Gamma Ray Bursts; Solar physics; and Dark Matter. The Milagro collaboration consists of more than ten institutions. To date more than ten Ph.D. theses have been completed. James Linnemann and Aous Abdo Department of Physics and Astronomy, Michigan State University Bottom-layer tubes Top-layer tubes The Galactic Plane Diffuse emission from the galactic plane is the dominant source in the MeV gamma ray sky. Milagro detected, for the first time, the galactic plane in the TeV range. The emission seems to be concentrated in the Cygnus region