Physics News Update 758

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Number 758, December 15, 2005 by Phil Schewe, Ben Stein, and Davide Castelvecchi

A Tera-Electronvolt Gamma Ray Originating in the Milky Way

The most energetic parcels of electromagnetic radiation -- Tera-electronvolt gamma rays -- ever determined to have originated in the plane of our home galaxy were observed recently by the Milagro detector, located at high mountain elevations in New Mexico. The potent photons are believed to have been part of the debris spawned when even more energetic cosmic rays struck the matter-dense heart of the Milky Way.

Photons in the TeV range arrive at the Earth very rarely, not often enough to attempt observation from a space-based gamma telescope: for a wristwatch-sized detector you'd have to wait about 10 years, on average. Therefore, terrestrial gamma observations are usually carried out by large-area-arrays attached to the ground.

Milagro, operated by scientists from nine institutions, records the arrival of energetic photons at Earth by observing the air shower of secondary particles generated when the gamma rays hit the atmosphere. These particles betray their presence by the light (Cerenkov radiation) emitted when the particles pass through a 6-million-gallon pond instrumented with photo detectors. This method of observation offers a rough ability to determine the direction of arrival.

For the Milagro experiment so far, 70,000 TeV photon events from within a region of the Milky Way plane were culled from an inventory of about 240 million TeV-level events seen so far seen from the same region. These numbers, says team member Roman Fleysher of New York University (roman.fleysher@physics.nyu.edu), are consistent with theoretical estimates for cosmic ray production.

And where do the cosmic rays get their 100-TeV-and-more energies? Fleysher recalled an analogy due to Enrico Fermi: imagine a pingpong ball trapped between two hole-filled walls zooming toward each other. The ball will ricochet many times, gaining energy, before eventually escaping out one of the holes. Ions in the interstellar medium, perhaps near a collapsed star or an active galactic nucleus (AGN), can get caught up by shock waves and accelerated to high energies like the pingpong ball.

Atkins et al., Physical Review Letters, 16 December 2005

Stargazing on Mars

Spirit and Opportunity, the two die-hard rovers that have been exploring the Martian surface for almost two years, are now offering some striking views of the planet's sky, including lunar eclipses, cosmic rays, and possibly shooting stars. The normally geology- and chemistry-oriented mission took on an astronomical task when NASA scientists realized they could use some of the spare power soaked up during the day by the rovers' solar panels to switch on the cameras at night and turn them skyward.

At a meeting of the American Geophysical Union (AGU) in San Francisco last week, Cornell scientist Jim Bell presented images of the Martian equivalent of a lunar eclipse. The pictures showed Phobos, one of Mars's two small moons, entering the planet's shadow on Nov. 27. As it happens to our own Moon during lunar eclipses, Phobos was not obscured completely, since some sunlight still reached it after being refracted by Mars's atmosphere. (See photos at Physics News Graphics.)

Bell also used Spirit to look for shooting stars. Like Earth, Mars can be hit by meteor showers, the streaks of light that cross the sky when scraps of meteors passing by enter the atmosphere and burn up. Scientists had predicted Martian meteor showers for the months of October and November. Lo and behold, the streaks showed up, just as the planet was crossing the path of Halley's comet, although Bell says it's possible that some of the streaks may be due to cosmic rays.

This would be the first clear detection of cosmic rays on the surface of another planet. "If you see this kind of dots and long streaks, they are most likely cosmic rays," says Insoo Jun of the Jet Propulsion Laboratory in Pasadena, Calif. Jun says these highly energetic, electrically charged particles coming from outer space can often hit the Martian surface directly, instead of disintegrating in the upper atmosphere and producing a shower of secondary particles as they do on Earth. That's because Mars's atmosphere is thin -- both less dense and shallower -- and also because Mars does not have a strong magnetic field that deflects charged particles.

Jun is helping design a planned follow-up mission called Mars Science Laboratory, which will carry a dedicated cosmic-ray detector. This Mini-Cooper-sized rover would land on the planet in 2010. Understanding the planet's radiation environment is especially important for the safety of future manned missions, he says. As of December 12, both Spirit and Opportunity have spent more than a full Martian year on the planet, thus experiencing a full cycle of its seasons, and they have covered about 6 kilometers (3.7 miles) each during their explorations.

Photos at Physics News Graphics
More from the Jet Propulsion Laboratory

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