PhD Dissertation · Michigan State University · 2007

Discovery of Localized TeV Gamma-Ray Sources and Diffuse Emission from the Galactic Plane with Milagro

Aous Ahmad Abdo  ·  Department of Physics & Astronomy

Very-high-energy gamma rays probe the most powerful objects in the universe — active galactic nuclei, supernova remnants, pulsar wind nebulae. This dissertation introduces a new background-rejection technique for the Milagro water-Cherenkov detector: the A₄ gamma-hadron separation variable, coupled with an event weighting analysis. Together they sharpened Milagro enough to make its first discoveries.

4localized TeV sources
3in the Cygnus region
~12σMGRO J2019+37
A₄the "Abdo variable"

What it found

Four localized sources of TeV gamma-ray emission were discovered — three in the Cygnus region of the Galaxy (led by MGRO J2019+37, ~80% of the Crab flux at 20 TeV) and one toward the inner Galaxy (MGRO J1908+06). Beyond the point sources, a diffuse TeV emission was detected from Cygnus whose flux at ~12 TeV exceeds conventional cosmic-ray production-and-propagation models — pointing to hard-spectrum cosmic-ray sources, or new gamma-ray sources, hidden in the region. Additional candidates above 4σ were seen along the Galactic plane.

Why A₄ mattered

Milagro saw the entire overhead sky continuously, making it uniquely suited to large, low-surface-brightness sources. But cosmic-ray hadrons outnumber gamma rays by orders of magnitude. The A₄ variable exploits a real physical difference: hadronic showers are clumpy, scattering penetrating particles far from the core, while gamma-ray showers are smooth. Reading that clumpiness — and optimally weighting events by their likelihood of being gamma rays — delivered the sensitivity leap behind every discovery above.

From the dissertation

The actual figures — the real data behind the interactive recreations elsewhere on this site.

Milagro TeV gamma-ray significance map of the Galactic plane with labeled sources and their significances
Figure 8.12.Milagro's TeV gamma-ray sources and source candidates, with the pre-trials significance for each. MGRO J2019+37, at 11.3σ, is the brightest — the anchor of the Cygnus detections.
TeV gamma-ray significance map of the northern sky in equatorial coordinates
Figure 7.8.The TeV gamma-ray sky over the northern hemisphere in equatorial coordinates. The most significant source is the Crab Nebula (RA 83.6°, Dec 22°) at 15.2σ; the Cygnus Region glows near RA 300°.
TeV gamma-ray significance map of the northern sky in Galactic coordinates
Figure 8.1.The same sky in Galactic coordinates. The large grey region is the part of the Galaxy obscured by the Earth from Milagro's latitude; the discoveries cluster along the Galactic plane.
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