An All-Sky Search for Intermediate-Scale Structure Using Milagro

Milagro is a TeV gamma-ray observatory with a ∼ 2 sr field of view and a > 90% duty factor. The large field of view and long observation time make Milagro ideal for surveying large regions of the Northern Hemisphere sky. A previous all-sky survey searched for point sources [4], but the analysis is easily adaptable to look for intermediate-scale sources (∼ 10) as well. A search on intermediate scales has been conducted, and 2 regions are seen with a statistical significance above 11σ. Analysis to determine the nature of these regions is underway. The Milagro Detector Milagro [4] is a water-Cherenkov detector at an altitude of 2650m capable of continuously monitoring the overhead sky. It is composed of a central 60m x 80m pond with a sparse 200m x 200m array of 175 “outrigger” tanks surrounding it. The pond is instrumented with two layers of photomultiplier tubes. The top “air-shower” layer consists of 450 PMTs under 1.4m of purified water, while the bottom “muon” layer has 273 PMTs located 6m below the surface. The air-shower layer allows the accurate measurement of shower particle arrival times used for direction reconstruction and triggering. The greater depth of the muon layer is used to detect penetrating muons and hadrons to help distinguish between gamma-rayand hadroninduced air showers. The outrigger array improves the angular resolution of the detector by providing a more accurate core location and a longer lever arm with which to reconstruct the events. Milagro’s large field of view (∼ 2sr) and high duty cycle (> 90%) allow it to scan the entire overhead sky continuously, making it well-suited for searching for new sources of TeV gamma rays, as well as monitoring known sources at higher energies. Previous surveys [4, 2] were optimized for sources smaller than Milagro’s ∼ 1.1◦ angular resolution. However, the analysis can easily be modified to search for larger sources. Analysis Method In the analysis, a signal map is made based on the arrival direction of each event. A background map is also created using a technique called “direct integration” [4], in which a two-hour time interval is used to generate the background. The accuracy of the background map depends on the assumption that the shape of the local cosmic ray flux is constant during the two hours. Since this time interval corresponds to the earth rotating 30, this analysis is relatively insensitive to features with an extent larger than ∼ 30◦ in Right Ascension. In the standard analysis, the signal and background maps are smoothed with a bin size that is optimal for Milagro’s angular resolution (PSF smoothing may be used instead), and then the maps are compared. In this analysis, however, a square bin of size 10.1◦ in Declination and 10.1◦/cos(δ) in Right Ascension is used to increase the sensitivity to larger features. Because a 2-hour (30 in R.A.) background generation interval is used, a bin size larger than 10◦ is not feasible, especially at higher declinations. The analysis was applied to 6.5 years of data, beginning in July 2000 and ending in January 2007. IC R C 2007 P rocedings P re-C onrence E dtion MILAGRO INTERMEDIATE-SCALE SURVEY