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Thursday, August 25, 2016
NASA's WISE, Fermi Missions Reveal a Surprising Blazar Connection
An analysis of blazar properties observed by the
Wide-field Infrared Survey Explorer (WISE) and Fermi's Large Area
Telescope (LAT) reveal a correlation in emissions from the mid-infrared
to gamma rays, an energy range spanning a factor of 10 billion. When
plotted by gamma-ray and mid-infrared colors, confirmed Fermi blazars
(gold dots) form a unique band not shared by other sources beyond our
galaxy. A blue line marks the best fit of these values. The relationship
allows astronomers to identify potential new gamma-ray blazars by
studying WISE infrared data.
Credits: NASA's Goddard Space Flight Center/Francesco Massaro, University of Turin
Black-hole-powered galaxies called blazars are the
most common sources detected by NASA's Fermi Gamma-ray Space Telescope.
As matter falls toward the supermassive black hole at the galaxy's
center, some of it is accelerated outward at nearly the speed of light
along jets pointed in opposite directions. When one of the jets happens
to be aimed in the direction of Earth, as illustrated here, the galaxy
appears especially bright and is classified as a blazar. Credits: M. Weiss/CfA.Hi-res image
Astronomers studying distant galaxies powered by monster black holes
have uncovered an unexpected link between two very different wavelengths
of the light they emit, the mid-infrared and gamma rays. The discovery,
which was accomplished by comparing data from NASA’s Wide-field
Infrared Survey Explorer (WISE) and Fermi Gamma-ray Space Telescope, has
enabled the researchers to uncover dozens of new blazar candidates.
Francesco Massaro at the University of Turin in Italy and Raffaele
D’Abrusco at the Harvard-Smithsonian Center for Astrophysics in
Cambridge, Massachusetts, show for the first time that the mid-infrared
colors of blazars in WISE data correlate to an equivalent measurement of
their gamma-ray output.
"This connection links two vastly different forms of light over an
energy range spanning a factor of 10 billion," said Massaro.
"Ultimately, it will help us decipher how supermassive black holes in
these galaxies manage to convert the matter around them into vast
amounts of energy."
Blazars constitute more than half of the discrete gamma-ray sources
seen by Fermi's Large Area Telescope (LAT). At the heart of a blazar
lies a supersized black hole with millions of times the sun's mass
surrounded by a disk of hot gas and dust. As material in the disk falls
toward the black hole, some of it forms dual jets that blast subatomic
particles straight out of the disk in opposite directions at nearly the
speed of light. A blazar appears bright to Fermi for two reasons. Its
jets produce many gamma rays, the highest-energy form of light, and we
happen to be viewing the galaxy face on, which means one of its jets is
pointing in our direction.
From January to August 2010, NASA's WISE mapped the entire sky
in four infrared wavelengths, cataloging more than half a billion
sources. In 2011, Massaro, D’Abrusco and their colleagues began using
WISE data to investigate Fermi blazars.
"WISE made it possible to explore the mid-infrared colors of known
gamma-ray blazars," said D’Abrusco. "We found that when we plotted Fermi
blazars by their WISE colors in a particular way, they occupied a
distinctly different part of the plot than other extragalactic gamma-ray
The scientists detail new aspects of the infrared/gamma-ray connection in a paper
published in The Astrophysical Journal on Aug. 9. They say the
electrons, protons and other particles accelerated in blazar jets leave a
specific "fingerprint" in the infrared light they emit. This same
pattern is also clearly evident in their gamma rays. The relationship
effectively connects the dots for blazars across an enormous swath of
the electromagnetic spectrum.
About a thousand Fermi sources remain unassociated with known objects
at any other wavelength. Astronomers suspect many of these are blazars,
but there isn't enough information to classify them. The
infrared/gamma-ray connection led the authors to search for new blazar
candidates among WISE infrared sources located within the positional
uncertainties of Fermi's unidentified gamma-ray objects. When the
researchers applied this relationship to Fermi's unknown sources, they
quickly found 130 potential blazars. Efforts are now under way to
confirm the nature of these objects through follow-up studies and to
search for additional candidates using the WISE connection.
"About a third of the gamma-ray objects seen by Fermi remained
unknown in the most recent catalog, and this result represents an
important advance in understanding their natures," said David Thompson, a
Fermi deputy project scientist at NASA’s Goddard Space Flight Center in
NASA's Jet Propulsion Laboratory in Pasadena, California, manages and
operates WISE for NASA's Science Mission Directorate in Washington. The
spacecraft was put into hibernation mode in 2011 after twice scanning
the entire sky, thereby completing its main objectives. In September
2013, WISE was reactivated, renamed NEOWISE and assigned a new mission
to assist NASA's efforts to identify potentially hazardous near-Earth
NASA's Fermi Gamma-ray Space Telescope is an astrophysics and
particle physics partnership, developed in collaboration with the U.S.
Department of Energy and with important contributions from academic
institutions and partners in France, Germany, Italy, Japan, Sweden and
the United States.