50
percent
95
percent
TExaS SyMPOSIuM ON RElaTIvISTIC aSTROPhySICS, hEIDElBERG, GERMaNy, DECEMBER 6 – 10
ice content of
most of the
solar system
ice content of
saturn’s puzzling
ring system
Shocking signals from Crab nebula
Gamma-ray flares from supernova remnant baffle astronomers
AGILE team member Marco Tavani of
the INAF-IASF in Rome and the Uni-
versity of Rome Tor Vergata, who spoke
about the findings on December 6 and 7.
Fermi researcher Rolf Buehler joined
Tavani in a December 6 session to discuss
variable sources of energetic radiation
in the Milky Way. Tavani and Buehler
declined to talk to reporters because both
teams have submitted papers to Science.
In one widely accepted model, the
stage is set for any kind
of gamma-ray emis-
sion — steady or short-
lived — when electrons
hurled from the Crab’s
core run into strong
magnetic fields in sur-
rounding debris. The
electrons gyrate in the
magnetic fields and get
revved up to energies
high enough to emit
gamma rays.
By Ron Cowen
Radiation from the Crab nebula supernova remnant is believed to be so
constant that astronomers use it as a
standard candle with which to measure
the energetic radiation of other astronomical sources.
That’s why researchers are astounded
that two spacecraft recently recorded
giant gamma-ray hiccups from the Crab, the
remnants of a stellar
explosion 6,500 light-years from Earth.
For three days beginning September 19, the
intensity of the Crab’s
gamma-ray radiation
suddenly became two
to three times stronger, scientists with the
Italian Space Agency’s AGILE telescope
reported. The Fermi
The Crab nebula, shown in multiple wavelengths, has hosted some surprising amma-ray flares recently.
Gamma-ray Space Telescope detected an even
larger increase over roughly the same
time period.
Both teams also announced they had
found evidence of previous similar flares.
AGILE recorded an outburst in fall 2007,
and Fermi spotted one in February 2009.
The suspected source of the energetic
flares, along with steadier radiation emanating from the nebula, is blizzards of
electrons spit out by the Crab’s pulsar —
the rapidly rotating, exploded cinder of a
star that lies at the nebula’s center. Figuring out exactly how the electrons got
revved up to energies of at least 1015 electron volts — the most energetic charged
particles ever associated with a distinct
astrophysical object — for so short a time
has astronomers scratching their heads.
But the Crab’s
recently detected outbursts would seem
to pose problems for
that acceleration model. The brevity
of the flares suggests that the electrons
couldn’t have gyrated long enough
to produce the energetic radiation,
Buehler noted. And because electrons
accelerated to very high energies lose that
energy quickly, the magnetic field in the
inner region of the nebula might have to
be three to 10 times stronger than commonly assumed.
Finding the flares “was a shock,” said
The short duration may mean that the
gamma rays originated in a relatively
small part of the inner nebula. Astron-
omers are trying to pinpoint the exact
outburst region. Buehler suggested that
in addition to magnetic fields, the pulsar’s
electric field may have helped acceler-
ate the electrons in the inner part of the
nebula to energies high enough to emit
the gamma rays.
Beast at galaxy’s core lacks spin
the Milky Way center’s supermas-sive black hole barely spins, say
avery Broderick of the canadian
institute for theoretical astrophysics
in toronto and colleagues, including
avi loeb of Harvard university. the
scientists base those tentative findings on radio emissions from the
galactic center. the emissions are
believed to be generated by material
at the inner edge of the whirlpool
of matter that spirals into and fuels
the black hole. combining the radio
data with modeling of the expected
emissions suggests that the super-massive black hole spins slowly or
not at all, loeb reported december
6. — Ron Cowen
Particle collider in the sky
protons caught between two fierce
stellar furnaces are being boosted
to energies comparable to those
at the world’s most powerful atom
smasher, new observations suggest.
the two orbiting stars form the eta
carinae system, some 7,500 light-years from earth. every week, eta
carinae expels dense winds of material equivalent to about an earth
mass. nasa’s fermi Gamma-ray
space telescope recently recorded
gamma rays from eta carinae ranging in energy from 20 billion to 50
billion electron volts, roland Walter
of the university of Geneva reported
december 9. the most plausible
explanation for the high energy is
that protons bouncing back and
forth between the two stellar winds
attain energies of at least 10 trillion electron volts. the protons then
collide with protons in the winds
to produce pions, which decay into
the energetic gamma rays seen by
fermi. — Ron Cowen
www.sciencenews.org
january 1, 2011 | science news | 11
