The Miracle Array

Milagro Yields the Unexpected

From 2000 to 2008, Milagro detected over 200 billion showers from both gamma rays and cosmic rays, collected at the rate of 1,700 showers per second. Each was recorded electronically, analyzed on the spot, and characterized by statistical features that were saved for further analysis.

The analysis produced the most-sensitive survey of the TeV sky to date and led to several surprises. First were the five bright sources of TeV gamma rays (in the figure above, the sources with dark centers), with average gamma-ray energies of 12 TeV, standing out loud and clear above the uniform background of cosmic rays, and four less-prominent candidates.

Several of these sources overlapped the much lower energy gamma-ray sources that were discovered 5 to 10 years earlier by satellite-borne gamma-ray detectors. Ground-based atmospheric Cherenkov telescopes had not seen them, despite having searched for TeV gamma rays in the directions of these known sources.

One explanation was that some Milagro sources are spatially extended, up to a few degrees in diameter, making them difficult to detect with narrow-view telescopes. Of these, some turned out to be nearby pulsar wind nebulae, exotic nebulae left over from supernovae that have at their center a neutron star pulsar creating a wind of high-energy particles (see opening illustration showing the Crab Nebula).

A follow-up search with longer observation times by the HESS atmospheric Cherenkov telescope array in Namibia resulted in detection of one of Milagro's sources. The HESS result not only confirmed Milagro's measurement of the total flux of TeV gamma rays but also determined that this source was unusually bright at the very highest energies at which Milagro has its greatest sensitivity.

Another major Milagro discovery was a surprisingly large number of TeV gamma rays coming from an extended region of the Milky Way known as Cygnus. The expected number is calculated by assuming the gamma rays are produced by the uniform background of cosmic rays impinging on the interstellar matter and radiation in the Cygnus region. However, Cygnus contains an unusually high number of objects (supernova remnants, young stellar clusters, and stars called Wolf-Rayet stars that emit large winds) that could be cosmic-ray sources. "The excess of gamma rays detected by Milagro could be the smoking gun for a recent cosmic-ray source, an explosion of a star within the region in the past 10,000 years," explains Sinnis.

Milagro has taught us to "expect the unexpected" says Dingus. "When we open a new window on the universe, the biggest discoveries are not the ones predicted."

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Gus Sinnis and Brenda Dingus, leaders of both Milagro and HAWC.