An international team of astronomers, using NASA's Fermi observatory, has
made the first-ever gamma-ray measurements of a gravitational lens, a kind of
natural telescope formed when a rare cosmic alignment allows the gravity of a
massive object to bend and amplify light from a more distant source.
This accomplishment opens new avenues for research, including a novel way to
probe emission regions near supermassive black holes. It may even be possible to
find other gravitational lenses with data from the Fermi Gamma-ray Space
Telescope.
This movie illustrates the components of a
gravitational lens system known as B0218+357. Different sight lines to a
background blazar result in two images that show outbursts at slightly different
times. NASA's Fermi made the first gamma-ray measurements of this delay in a
lens system.
Image Credit: NASA's Goddard Space Flight
Center
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"We began thinking about the possibility of making this observation a couple
of years after Fermi launched, and all of the pieces finally came together in
late 2012," said Teddy Cheung, lead scientist for the finding and an
astrophysicist at the Naval Research Laboratory in Washington.
In September 2012, Fermi's Large Area Telescope (LAT) detected a series of
bright gamma-ray flares from a source known as B0218+357, located 4.35 billion
light-years from Earth in the direction of a constellation called Triangulum.
These powerful flares, in a known gravitational lens system, provided the key to
making the lens measurement.
Astronomers classify B0218+357 as a blazar -- a type of active galaxy noted
for its intense emissions and unpredictable behavior. At the blazar's heart is a
supersized black hole with a mass millions to billions of times that of the sun.
As matter spirals toward the black hole, some of it blasts outward as jets of
particles traveling near the speed of light in opposite directions.
The extreme brightness and variability of blazars result from a chance
orientation that brings one jet almost directly in line with Earth. Astronomers
effectively look down the barrel of the jet, which greatly enhances its apparent
emission.
This Hubble image of gravitational lens B0218+357
reveals two bright sources separated by about a third of an arcsecond, each an
image of the background blazar. Spiral arms belonging to the lensing galaxy also
can be seen. B0218+357 boasts the smallest separation of lensed images currently
known.
Image Credit: NASA/ESA and the Hubble Legacy
Archive
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In the heart of an active galaxy, matter falling
toward a supermassive black hole creates jets of particles traveling near the
speed of light. For active galaxies classified as blazars, one of these jets
beams almost directly toward Earth.
Image Credit: NASA/Goddard Space Flight Center Conceptual Image
Lab
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This interactive illustrates how gravitational
lensing can be observed by NASA's Fermi Gamma-ray Space
Telescope.
Image Credit: NASA's Goddard Space Flight
Center
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Long before light from B0218+357 reaches us, it passes directly through a
face-on spiral galaxy -- one very much like our own -- about 4 billion
light-years away.
The galaxy's gravity bends the light into different paths, so astronomers see
the background blazar as dual images. With just a third of an arcsecond (less
than 0.0001 degree) between them, the B0218+357 images hold the record for the
smallest separation of any lensed system known.
While radio and optical telescopes can resolve and monitor the individual
blazar images, Fermi's LAT cannot. Instead, the Fermi team exploited a "delayed
playback" effect.
"One light path is slightly longer than the other, so when we detect flares
in one image we can try to catch them days later when they replay in the other
image," said team member Jeff Scargle, an astrophysicist at NASA's Ames Research
Center in Moffett Field, Calif.
In September 2012, when the blazar's flaring activity made it the brightest
gamma-ray source outside of our own galaxy, Cheung realized it was a golden
opportunity. He was granted a week of LAT target-of-opportunity observing time,
from Sept. 24 to Oct. 1, to hunt for delayed flares.
At the American Astronomical Society meeting in National Harbor, Md., Cheung
said the team had identified three episodes of flares showing playback delays of
11.46 days, with the strongest evidence found in a sequence of flares captured
during the week-long LAT observations.
Intriguingly, the gamma-ray delay is about a day longer than radio
observations report for this system. And while the flares and their playback
show similar gamma-ray brightness, in radio wavelengths one blazar image is
about four times brighter than the other.
Astronomers don't think the gamma rays arise from the same regions as the
radio waves, so these emissions likely take slightly different paths, with
correspondingly different delays and amplifications, as they travel through the
lens.
"Over the course of a day, one of these flares can brighten the blazar by 10
times in gamma rays but only 10 percent in visible light and radio, which tells
us that the region emitting gamma rays is very small compared to those emitting
at lower energies," said team member Stefan Larsson, an astrophysicist at
Stockholm University in Sweden.
As a result, the gravity of small concentrations of matter in the lensing
galaxy may deflect and amplify gamma rays more significantly than lower-energy
light. Disentangling these so-called microlensing effects poses a challenge to
taking further advantage of high-energy lens observations.
The scientists say that comparing radio and gamma-ray observations of
additional lens systems could help provide new insights into the workings of
powerful black-hole jets and establish new constraints on important cosmological
quantities like the Hubble constant, which describes the universe's rate of
expansion.
The most exciting result, the team said, would be the LAT's detection of a
playback delay in a flaring gamma-ray source not yet identified as a
gravitational lens in other wavelengths.
A paper describing the research will appear in a future edition of The
Astrophysical Journal Letters.
NASA's Fermi Gamma-ray Space Telescope is an astrophysics and particle
physics partnership. Fermi is managed by NASA's Goddard Space Flight Center in
Greenbelt, Md. 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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