GALAXY'S GAMMA-RAY FLARES ERUPTED FAR FROM ITS BLACK HOLE
WASHINGTON -- In 2011, a months-long blast of energy launched by an
enormous black hole almost 11 billion years ago swept past Earth.
Using a combination of data from NASA's Fermi Gamma-ray Space
Telescope and the National Science Foundation's Very Long Baseline
Array (VLBA), the world's largest radio telescope, astronomers have
zeroed in on the source of this ancient outburst.
Theorists expect gamma-ray outbursts occur only in close proximity to
a galaxy's central black hole, the powerhouse ultimately responsible
for the activity. A few rare observations suggested this is not the
The 2011 flares from a galaxy known as 4C +71.07 now give astronomers
the clearest and most distant evidence that the theory still needs
some work. The gamma-ray emission originated about 70 light-years
away from the galaxy's central black hole.
The 4C +71.07 galaxy was discovered as a source of strong radio
emission in the 1960s. NASA's Compton Gamma-Ray Observatory, which
operated in the 1990s, detected high-energy flares, but the galaxy
was quiet during Fermi's first two and a half years in orbit.
In early November 2011, at the height of the outburst, the galaxy was
more than 10,000 times brighter than the combined luminosity of all
of the stars in our Milky Way galaxy.
"This renewed activity came after a long slumber, and that's important
because it allows us to explicitly link the gamma-ray flares to the
rising emission observed by radio telescopes," said David Thompson, a
Fermi deputy project scientist at NASA's Goddard Space Flight Center
in Greenbelt, Md.
Located in the constellation Ursa Major, 4C +71.07 is so far away that
its light takes 10.6 billion years to reach Earth. Astronomers are
seeing this galaxy as it existed when the universe was less than
one-fourth of its present age.
At the galaxy's core lies a supersized black hole weighing 2.6 billion
times the sun's mass. Some of the matter falling toward the black
hole becomes accelerated outward at almost the speed of light,
creating dual particle jets blasting in opposite directions. One jet
happens to point almost directly toward Earth. This characteristic
makes 4C +71.07 a blazar, a classification that includes some of the
brightest gamma-ray sources in the sky.
Boston University astronomers Alan Marscher and Svetlana Jorstad
routinely monitor 4C +71.07 along with dozens of other blazars using
several facilities, including the VLBA.
The instrument's 10 radio telescopes span North America, from Hawaii
to St. Croix in the U.S. Virgin Islands, and possess the resolving
power of a single radio dish more than 5,300 miles across when their
signals are combined. As a result, The VLBA resolves detail about a
million times smaller than Fermi's Large Area Telescope (LAT) and
1,000 times smaller than NASA's Hubble Space Telescope.
In autumn 2011, the VLBA images revealed a bright knot that appeared
to move outward at a speed 20 times faster than light.
"Although this apparent speed was an illusion caused by actual motion
almost directly toward us at 99.87 percent the speed of light, this
knot was the key to determining the location where the gamma-rays
were produced in the black hole's jet," said Marscher, who presented
the findings Monday at the American Astronomical Society meeting in
Long Beach, Calif.
The knot passed through a bright stationary feature of the jet, which
the astronomers refer to as its radio "core," on April 9, 2011. This
occurred within days of Fermi's detection of renewed gamma-ray
flaring in the blazar. Marscher and Jorstad noted that the blazar
brightened at visible wavelengths in step with the higher-energy
During the most intense period of flaring, from October 2011 to
January 2012, the scientists found the polarization direction of the
blazar's visible light rotated in the same manner as radio emissions
from the knot. They concluded the knot was responsible for the
visible and the gamma-ray light, which varied in sync.
This association allowed the researchers to pinpoint the location of
the gamma-ray outburst to about 70 light-years from the black hole.
The astronomers think that the gamma rays were produced when electrons
moving near the speed of light within the jet collided with visible
and infrared light originating outside of the jet. Such a collision
can kick the light up to much higher energies, a process known as
The source of the lower-energy light is unclear at the moment. The
researchers speculate the source may be an outer, slow-moving sheath
that surrounds the jet. Nicholas MacDonald, a graduate student at
Boston University, is investigating how the gamma-ray brightness
should change in this scenario to compare with observations.
"The VLBA is the only instrument that can bring us images from so near
the edge of a young supermassive black hole, and Fermi's LAT is the
only instrument that can see the highest-energy light from the
galaxy's jet," said Jorstad.
NASA's Fermi Gamma-ray Space Telescope is an astrophysics and particle
physics partnership. Fermi is managed by NASA's Goddard Space Flight
Center. 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.