Recent results from the Milagro gamma ray observatory

Abstract

The Milagro gamma-ray observatory utilizes a large water Cherenkov detector to observe TeV extensive air showers produced by high energy particles impacting the Earth's atmosphere. Milagro is different from Atmospheric Cherenkov Telescopes that are used to study TeV gamma-rays in that it views a wide field (2 steradian over-head sky) and it continuously operates (>90% live time). These factors give Milagro the potential for discovery of new sources with unknown positions and times, such as gamma-ray bursts, flaring AGN, and observation of diffuse extended sources like the Galactic plane or large supernova remnants. The Milagro detector consists of a 4800 m2 pond instrumented with 723 8" PMTs which detect Cherenkov light produced by secondary air-shower particles. A sparse array of 175 4000l water tanks surrounding the central pond detector has recently been added which will extend the physical area of Milagro to 40,000 m2 and substantially increase the sensitivity of the instrument. Based on three years of operation, Milagro has established its sensitivity through the detection of the Crab plerion and active galaxy Markarian 421. A summary of the recent results from the Milagro collaboration is presented with a focus on the first observation of the galactic plane in the TeV range and evidence for two newly observed TeV sources: diffuse emission from the Cygnus Region, and evidence for an extended TeV hot spot near the EGRET unidentified 3EG J0520+2556. © 2005 Published by Elsevier B.V.