NASA's Fermi Gamma-ray Space Telescope orbits our planet every 95
minutes, building up increasingly deeper views of the universe with
every circuit. Its wide-eyed Large Area Telescope (LAT) sweeps across
the entire sky every three hours, capturing the highest-energy form of
light -- gamma rays -- from sources across the universe. These range
from supermassive black holes billions of light-years away to intriguing
objects in our own galaxy, such as X-ray binaries, supernova remnants
and pulsars.
Now a Fermi scientist has transformed LAT data of a famous pulsar into a mesmerizing movie that visually encapsulates the spacecraft's complex motion.
Now a Fermi scientist has transformed LAT data of a famous pulsar into a mesmerizing movie that visually encapsulates the spacecraft's complex motion.
The Vela pulsar outlines a fascinating pattern in this movie showing 51
months of position and exposure data from Fermi's Large Area Telescope
(LAT). The pattern reflects numerous motions of the spacecraft,
including its orbit around Earth, the precession of its orbital plane,
the manner in which the LAT nods north and south on alternate orbits,
and more. The movie renders Vela's position in a fisheye perspective,
where the middle of the pattern corresponds to the central and most
sensitive portion of the LAT's field of view. The edge of the pattern is
90 degrees away from the center and well beyond what scientists regard
as the effective limit of the LAT's vision. Better knowledge of how the
LAT's sensitivity changes across its field of view helps Fermi
scientists better understand both the instrument and the data it
returns. Credit: NASA/DOE/Fermi LAT Collaboration . › Download video in HD formats - › Watch this video on YouTube
Pulsars are neutron stars, the crushed cores of massive suns that
destroyed themselves when they ran out of fuel, collapsed and exploded.
The blast simultaneously shattered the star and compressed its core into
a body as small as a city yet more massive than the sun. The result is
an object of incredible density, where a spoonful of matter weighs as
much as a mountain on Earth. Equally incredible is a pulsar's rapid
spin, with typical rotation periods ranging from once every few seconds
up to hundreds of times a second. Fermi sees gamma rays from more than a
hundred pulsars scattered across the sky.
One pulsar shines especially bright for Fermi. Called Vela, it spins 11
times a second and is the brightest persistent source of gamma rays the
LAT sees. Although gamma-ray bursts and flares from distant black holes
occasionally outshine the pulsar, they don't have Vela's staying power.
Because pulsars emit beams of energy, scientists often compare them to
lighthouses, a connection that in a broader sense works especially well
for Vela, which is both a brilliant beacon and a familiar landmark in
the gamma-ray sky.
Fermi's Large Area Telescope is the spacecraft's main scientific
instrument. This animation shows how a gamma ray (purple) entering the
LAT is converted into an electron (red) and a positron (blue).
High-precision detectors track the motion of the particles, whose paths
point back to the gamma ray's source. Credit: NASA's Goddard Space Flight Center Conceptual Image Lab . › Download video in HD formats
This image compresses the Vela movie sequence into a single snapshot by merging pie-slice sections from eight individual frames. Credit: NASA/DOE/Fermi LAT Collaboration . › Larger image
Most telescopes focus on a very small region of the sky, but the LAT is a
wide-field instrument that can detect gamma rays across a large portion
of the sky at once. The LAT is, however, much more sensitive to gamma
rays near the center of its field of view than at the edges. Scientists
can use observations of a bright source like Vela to track how this
sensitivity varies across the instrument's field of view.
With this in mind, LAT team member Eric Charles, a physicist at the
Kavli Institute for Particle Astrophysics and Cosmology and the SLAC
National Accelerator Laboratory at Stanford University in California,
used the famous pulsar to produce a novel movie. He tracked both Vela's
position relative to the center of the LAT’s field of view and the
instrument's exposure of the pulsar during the first 51 months of
Fermi’s mission, from Aug. 4, 2008, to Nov. 15, 2012.
The movie renders Vela’s position in a fisheye perspective, where the
middle of the pattern corresponds to the central and most sensitive
portion of the LAT’s field of view. The edge of the pattern is 90
degrees away from the center and well beyond what scientists regard as
the effective limit of the LAT's vision.
The pulsar traces out a loopy, hypnotic pattern reminiscent of art
produced by the colored pens and spinning gears of a Spirograph, a
children's toy that produces geometric patterns.
The pattern created in the Vela movie reflects numerous motions of the
spacecraft. The first is Fermi's 95-minute orbit around Earth, but
there's another, subtler motion related to it. The orbit itself also
rotates, a phenomenon called precession. Similar to the wobble of an
unsteady top, Fermi's orbital plane makes a slow circuit around Earth
every 54 days.
In order to capture the entire sky every two orbits, scientists
deliberately nod the LAT in a repeating pattern from one orbit to the
next. It first looks north on one orbit, south on the next, and then
north again. Every few weeks, the LAT deviates from this pattern to
concentrate on particularly interesting targets, such as eruptions on
the sun, brief but brilliant gamma-ray bursts associated with the birth
of stellar-mass black holes, and outbursts from supermassive black holes
in distant galaxies.
The Vela movie captures one other Fermi motion. The spacecraft rolls to
keep the sun from shining on and warming up the LAT's radiators, which
regulate its temperature by bleeding excess heat into space.
The braided loops and convoluted curves drawn by Vela hint at the
complexity of removing these effects from the torrent of data Fermi
returns, but that’s a challenge LAT scientists long ago proved they
could meet. Still going strong after more than four years on the job,
Fermi continues its mission to map the high-energy sky, which is now
something everyone can envision as a celestial Spriograph traced by a
pulsar pen.
