In early November, 2011, a months-long blast
of energy launched by an enormous black hole almost 11 billion years ago
swept past Earth. 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. 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: a galaxy known as 4C +71.07. 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.
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 case.The
2011 flares from 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 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.
"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.
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.
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 emission.
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 inverse-Compton scattering.
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.
Image at the top of the page is a NASA's Chandra X-ray Observatory
discovery of an extraordinary outburst by a black hole in the spiral
galaxy M83, located about 15 million light years from Earth. Using
Chandra, astronomers found a new ultraluminous X-ray source, or ULX.
These objects give off more X-rays than most normal binary systems in
which a companion star is in orbit around a neutron star or black hole.
Source: The Daily Galaxy via NASA/Goddard Space Flight Center
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