A Search for TeV Emission from Active Galaxies using the Milagro Observatory

Abstract

Milagro is a unique instrument that observes very high energy gamma rays (100 GeV to 100 TeV) using the water-Cerenkov technique. The instrument has a large field of view, which covers the entire overhead sky (~2 sr). Located in northern New Mexico, Milagro observes most of the Northern Hemisphere over the course of a day. The high duty cycle (>90%) permits searches for TeV sources over a variety of time exposures.

This thesis presents the results of two separate searches of the Milagro data for TeV emission. A real-time search of the entire field of view has been running since 2002. The real-time search provides early notification of significant transient behavior for time integrations from two hours to one month. No new gamma-ray sources have been found although two known sources are detected at significant levels. A longer and more refined search is conducted of archived data to monitor a set of active galaxies selected as TeV candidates. Active galaxies have been observed to be highly variable at TeV energies. To test for episodic emission, a data set is constructed using observations taken from December 2000 through September 2003 and divided into six sets of shorter time integrations. No significant emission is detected, and upper limits are set on the maximum time-averaged flux from each object for each time integration. The flux limits are also calculated to include the effects of absorption of TeV gamma rays by extragalactic background light. Predictions for the spectrum are used to set flux limits that can be compared to predictions for the flux from each source. The Milagro flux limits constrain some predictions for the TeV emission from the sources. The amount of absorption expected for two of the selected sources does not explain the discrepancy between the predictions and the flux upper limits. The implications of the constraints are discussed.