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. Download this video in HD formats from NASA Goddard's Scientific Visualization Studio
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
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. View interactive
"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.
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.
Related Links
J. D. Harrington
NASA Headquarters, Washington
202-358-5241
j.d.harrington@nasa.gov
Lynn Chandler
NASA's Goddard Space Flight Center, Greenbelt, Md.
301-286-2806
lynn.chandler-1@nasa.gov
NASA Headquarters, Washington
202-358-5241
j.d.harrington@nasa.gov
Lynn Chandler
NASA's Goddard Space Flight Center, Greenbelt, Md.
301-286-2806
lynn.chandler-1@nasa.gov
