Space

August 1, 2014

NASA’s Fermi telescope reveals new source of gamma rays

These images show Fermi data centered on each of the four gamma-ray novae observed by the LAT. Colors indicate the number of detected gamma rays with energies greater than 100 million electron volts (blue indicates lowest, yellow highest).

Observations by NASA’s Fermi Gamma-ray Space Telescope of several stellar eruptions, called novae, firmly establish these relatively common outbursts almost always produce gamma rays, the most energetic form of light.

“There’s a saying that one is a fluke, two is a coincidence, and three is a class, and we’re now at four novae and counting with Fermi,” said Teddy Cheung, an astrophysicist at the Naval Research Laboratory in Washington, and the lead author of a paper reporting the findings in the Aug. 1 edition of the journal Science.

A nova is a sudden, short-lived brightening of an otherwise inconspicuous star caused by a thermonuclear explosion on the surface of a white dwarf, a compact star not much larger than Earth. Each nova explosion releases up to 100,000 times the annual energy output of our sun. Prior to Fermi, no one suspected these outbursts were capable of producing high-energy gamma rays, emission with energy levels millions of times greater than visible light and usually associated with far more powerful cosmic blasts.

Fermi’s Large Area Telescope scored its first nova detection, dubbed V407 Cygni, in March 2010. The outburst came from a rare type of star system in which a white dwarf interacts with a red giant, a star more than a hundred times the size of our sun. Other members of the same unusual class of stellar system have been observed “going nova” every few decades.

In 2012 and 2013, the LAT detected three so-called classical novae which occur in more common binaries where a white dwarf and a sun-like star orbit each other every few hours.

“We initially thought of V407 Cygni as a special case because the red giant’s atmosphere is essentially leaking into space, producing a gaseous environment that interacts with the explosion’s blast wave,” said co-author Steven Shore, a professor of astrophysics at the University of Pisa in Italy. “But this can’t explain more recent Fermi detections because none of those systems possess red giants.”

Fermi detected the classical novae V339 Delphini in August 2013 and V1324 Scorpii in June 2012, following their discovery in visible light. In addition, on June 22, 2012, the LAT discovered a transient gamma-ray source about 20 degrees from the sun. More than a month later, when the sun had moved farther away, astronomers looking in visible light discovered a fading nova from V959 Monocerotis at the same position.

Astronomers estimate that between 20 and 50 novae occur each year in our galaxy. Most go undetected, their visible light obscured by intervening dust and their gamma rays dimmed by distance. All of the gamma-ray novae found so far lie between 9,000 and 15,000 light-years away, relatively nearby given the size of our galaxy.

Novae occur because a stream of gas flowing from the companion star piles up into a layer on the white dwarf’s surface. Over time – tens of thousands of years, in the case of classical novae, and several decades for a system like V407 Cygni – this deepening layer reaches a flash point. Its hydrogen begins to undergo nuclear fusion, triggering a runaway reaction that detonates the accumulated gas. The white dwarf itself remains intact.

One explanation for the gamma-ray emission is that the blast creates multiple shock waves that expand into space at slightly different speeds. Faster shocks could interact with slower ones, accelerating particles to near the speed of light. These particles ultimately could produce gamma rays.

“This colliding-shock process must also have been at work in V407 Cygni, but there is no clear evidence for it,” said co-author Pierre Jean, a professor of astrophysics at the University of Toulouse in France. This is likely because gamma rays emitted through this process were overwhelmed by those produced as the shock wave interacted with the red giant and its surroundings, the scientists conclude.

NASA’s Fermi Gamma-ray Space Telescope is an astrophysics and particle physics partnership managed by the agency’s Goddard Space Flight Center in Greenbelt, Md. It was developed in collaboration with the U.S. Department of Energy, with contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.




All of this week's top headlines to your email every Friday.


 
 

 
Image courtesy of NASA/CXC/M. Weiss

NASA’s Chandra X-ray Observatory finds planet that makes star act deceptively old

Image courtesy of NASA/CXC/M. Weiss A new study from NASA’s Chandra X-ray Observatory shows that a giant exoplanet, WASP-18b, is making the star that it orbits very closely act much older than it actually is. This artist&...
 
 
NASA photographs by Tom Tschida

NASA Shuttle Carrier Aircraft 911 moves to final home

NASA photographs by Tom Tschida NASA 911, one of two retired Shuttle Carrier Aircraft that ferried NASA’s space shuttles across the country for three decades, is towed from NASA Armstrong’s Bldg. 703 on its final journey to...
 
 
Image courtesy of NASA/JPL-Caltech/Univ. of Arizona

NASA’s Mars Curiosity rover arrives at Martian mountain

NASA’s Mars Curiosity rover has reached the Red Planet’s Mount Sharp, a Mount-Rainier-size mountain at the center of the vast Gale Crater and the rover mission’s long-term prime destination. “Curiosity n...
 

 

NASA announces 2014 aeronautics scholarship recipients

NASA has selected 20 students from across the nation to receive the agency’s Aeronautics Scholarship for the 2014-2015 school year. This scholarship program, which is in its seventh year, is designed to assist undergraduate and graduate students enrolled in fields of study related to aeronautics. Recipients were selected from hundreds of applications to the program....
 
 
NASA photograph by Dan Casper

NASA’s Orion spacecraft nears completion, ready for fueling

NASA photograph by Dan Casper The Orion crew module, stacked atop its service module, moved out of the Neil Armstrong Operations and Checkout Building at NASA’s Kennedy Space Center in Florida on Sept 11. Orion was transporte...
 
 

NASA awards cross-track infrared sounder instrument for the JPSS-2

NASA has awarded a sole source contract modification to Exelis, Inc., Geospatial Systems, of Fort Wayne, Ind., for the Cross-track Infrared Sounder Instrument for flight on the Joint Polar Satellite System-2 mission. This is a cost-plus-award-fee modification in the amount of $221 million. This action extends the period of performance of the contract from November...
 




0 Comments


Be the first to comment!


Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>