F. W. Samuelson

Discovery of TeV gamma-ray emission from the Cygnus region of the galaxy

The diffuse gamma radiation arising from the interaction of cosmic ray particles with matter and radiation in the Galaxy is one of the few probes available to study the origin of the cosmic rays. Milagro is a water Cherenkov detector that continuously views the entire overhead sky. The large field-of-view combined with the long observation time makes Milagro the most sensitive instrument available for the study of large, low surface brightness sources such as the diffuse gamma radiation arising from interactions of cosmic radiation with interstellar matter. In this paper we present spatial and flux measurements of TeV gamma-ray emission from the Cygnus Region. The TeV image shows at least one new source MGRO J2019+37 as well as correlations with the matter density in the region as would be expected from cosmic-ray proton interactions. However, the TeV gamma-ray flux as measured at {approx}12 TeV from the Cygnus region (after excluding MGRO J2019+37) exceeds that predicted from a conventional model of cosmic ray production and propagation. This observation indicates the existence of either hard-spectrum cosmic-ray sources and/or other sources of TeV gamma rays in the region.

TeV Gamma-Ray Survey of the Northern Hemisphere Sky Using the Milagro Observatory

Milagro is a water Cerenkov extensive air shower array that continuously monitors the entire overhead sky in the TeV energy band. The results from an analysis of ~3 yr of data (2000 December-2003 November) are presented. The data have been searched for steady point sources of TeV gamma rays between declinations of 11 and 80°. Two sources are detected, the Crab Nebula and the active galaxy Mrk 421. For the remainder of the northern hemisphere, we set 95% confidence level (CL) upper limits between 275 and 600 mcrab (4.8 × 10-12 to 10.5 × 10-12 cm-2 s-1) above 1 TeV for source declinations between 5° and 70°. Since the sensitivity of Milagro depends on the spectrum of the source at the top of the atmosphere, the dependence of the limits on the spectrum of a candidate source is presented. Because high-energy gamma rays from extragalactic sources are absorbed by interactions with the extragalactic background light, the dependence of the flux limits on the redshift of a candidate source are given. The upper limits presented here are over an order of magnitude more stringent than previously published limits from TeV gamma-ray all-sky surveys.

Evidence for TeV Gamma-Ray Emission from a Region of the Galactic Plane

Gamma-ray emission from a narrow band at the Galactic equator has previously been detected up to 30 GeV. We report evidence for a TeV gamma-ray signal from the Galactic plane by Milagro, a large field of view water Cherenkov detector for extensive air showers. An excess with a significance of 4.5 standard deviations has been observed from the region of Galactic longitude between 40 and 100 deg and latitude |b|<5 deg. Under the assumption of a simple power law spectrum, with no cutoff, in the EGRET-Milagro energy range, the measured integral flux is phi(>3.5TeV) = (6.4 +/- 1.4 +/- 2.1) 10^{-11} cm^{-2}s^{-1} str^{-1}. This flux is consistent with an extrapolation of the EGRET spectrum between 1 and 30 GeV in this Galactic region.

The High-Energy Gamma-Ray Fluence and Energy Spectrum of GRB 970417a from Observations with Milagrito

Evidence of TeV emission from GRB 970417a has been previously reported using data from the Milagrito detector. Constraints on the TeV fluence and the energy spectrum are now derived using additional data from a scaler system that recorded the rate of signals from the Milagrito photomultipliers. This analysis shows that if emission from GRB 970417a has been observed, it must contain photons with energies above 650 GeV. Some consequences of this observation are discussed.Evidence of TeV emission from GRB970417a has been previously reported using data from the Milagrito detector. Constraints on the TeV fluence and the energy spectrum are now derived using additional data from a scaler system that recorded the rate of signals from the Milagrito photomultipliers. This analysis shows that if emission from GRB970417a has been observed, it must contain photons with energies above 650 GeV. Some consequences of this observation are discussed.

A SURVEY OF THE NORTHERN SKY FOR TeV POINT SOURCES

A search for steady TeV point sources anywhere in the northern sky has been made with data from the Milagrito air-shower particle detector. Over 3 × 109 events, collected from 1997 February to 1998 May, have been used in this study. No statistically significant excess above the background from the isotropic flux of cosmic rays was found for any direction of the sky with declination between -5° and 717. Upper limits are derived for the photon flux above 1 TeV from any steady point source in the northern sky.

Limits on Very High Energy Emission from Gamma-Ray Bursts with the Milagro Observatory

The Milagro telescope monitors the northern sky for 100 GeV-100 TeV transient emission through continuous very high energy (VHE) wide-field observations. The large effective area and ~100 GeV energy threshold of Milagro allow it to detect VHE gamma-ray burst (GRB) emission with much higher sensitivity than previous instruments and a fluence sensitivity at VHE energies comparable to that of dedicated GRB satellites at keV-MeV energies. Even in the absence of a positive detection, VHE observations can place important constraints on GRB progenitor and emission models. We present limits on the VHE flux of 40 s-3 hr duration transients near Earth as well as sensitivity distributions that have been corrected for gamma-ray absorption by extragalactic background light and cosmological effects. The sensitivity distributions suggest that the typical intrinsic VHE fluence of GRBs is similar to or weaker than the keV-MeV emission, and we demonstrate how these sensitivity distributions may be used to place observational constraints on the absolute VHE luminosity of GRBs for any GRB emission and progenitor model.