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).
Image Credit:
NASA/DOE/Fermi LAT Collaboration
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 (LAT) 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.
The white dwarf star in V407 Cygni, shown here in an artist's concept, went nova in 2010. Scientists think the outburst primarily emitted gamma rays (magenta) as the blast wave plowed through the gas-rich environment near the system's red giant star.
Image Credit:
NASA's Goddard Space Flight Center/S.
Wiessinger
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 typically originate in binary systems
containing sun-like stars, as shown in this artist's rendering. A nova in a
system like this likely produces gamma rays (magenta) through collisions among
multiple shock waves in the rapidly expanding shell of debris.
Image Credit:
NASA's Goddard Space Flight Center/S.
Wiessinger
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, Maryland. 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.
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Guillermo Gonzalo Sánchez Achutegui
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