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UD Professor Jamie Holder (left) and doctoral student Tyler
Williamson have been studying gamma rays with the help of the VERITAS
telescopes at the Fred Lawrence Whipple Observatory in Arizona.
Scientists have discovered something amazing.
In a cluster of some of the most massive and luminous stars in our
galaxy, about 5,000 light years from Earth, astronomers detected
particles being accelerated by a rapidly rotating neutron star as it
passed by the massive star it orbits only once every 50 years.
The discovery is extremely rare, according to University of Delaware
astrophysicist Jamie Holder and doctoral student Tyler Williamson, who
were part of the international team that documented the occurrence.
Holder called this eccentric pair
of gravitationally linked stars a gamma-ray binary system and likened
the once-in-a-lifetime event to the arrival of Halleys comet or last
years U.S. solar eclipse.
Massive stars are among the brightest stars in our galaxy. Neutron
stars are extremely dense and energetic stars that result when a massive
This binary system is a massive star with a neutron star orbiting
around it. Of the 100 billion stars in our galaxy, less than 10 are
known to be this type of system.
Even fewer only two systems, including this one are known to have
an identified neutron star, or pulsar, that emits pulses of radio waves
that scientists can measure. This is important because it tells
astronomers very accurately how much energy is available to accelerate
particles, something scientists know little about.
Move this whole section up, swapping places with the section above it.
UD Professor Jamie Holder is dwarfed by one of the VERITAS cameras.
You couldn't ask for a better natural laboratory to study particle
acceleration in a continually changing environment - at energies far
beyond anything we can produce on the Earth, said Holder, a professor
in UDs Department of Physics and Astronomy.
The project was led by a team of scientists, including Holder and
Williamson, using the VERITAS telescope array at the Fred Lawrence
Whipple Observatory in Arizona, in collaboration with scientists using
the MAGIC telescopes at the Roque de los Muchachos Observatory located
in La Palma, an island of the Canary Islands, Spain. (VERITAS stands for
Very Energetic Radiation Imaging Telescope Array System and MAGIC
stands for Major Atmospheric Gamma Imaging Cherenkov telescopes.)
The researchers recently reported their findings in the Astrophysical Journal Letters.
The natural question, to many minds, is why do scientists care about accelerated particles?
Because our galaxy is full of them. We call them cosmic rays and
they carry as much energy as the light from all the stars, said Holder.
Astronomers discovered more than 100 years ago that accelerated
particles exist, yet how or where these particles speed up remains a
mystery. Pulsars are among the most extreme objects in the universe and
they have magnetic fields around them that are millions of times
stronger than anything scientists could hope to build on earth. When a
pulsar encounters dust or gas close to a massive star, the particles
nearby accelerate to near speed of light velocities and collide with
whats around them. The result is a beam of high-energy light called
gamma-radiation or gamma rays.
Sophisticated telescopes, like those operated by VERITAS and MAGIC,
can detect these gamma rays because they emit a blue flash of light when
they reach the Earths atmosphere. While our eyes cant see these
flashes of light because they are too quick, only nanoseconds long,
these telescopes can.
Astronomers first discovered gamma rays coming from the pulsar in
this unusual pair of stars in 2008. About the size of Newark, Delaware,
the pulsar is spinning like the attachment on a kitchen blender,
emitting little pulses of gamma rays and radio waves with every
One of the VERITAS telescopes at sunset.
By measuring these radio pulse frequencies, astronomers were able to
tell how fast the pulsar was moving and calculate exactly when it would
be closest to the massive star that it was orbiting Nov. 13, 2017.
Its a trip that took 50 years.
The VERITAS and MAGIC teams began monitoring the night sky and
tracking the pulsars orbit in September 2016. At first, they werent
even sure if they would see anything. But in September 2017 the
astronomers began to detect a rapid increase in the number of gamma rays
hitting the top of the earths atmosphere.
As they monitored the data coming
from the VERITAS telescopes, Holder and Williamson realized that the
pulsar was doing something different each day.
I would wake up every morning and check and see if we had new data,
then analyze it as fast as I could, because there were times where the
number of gamma rays we were seeing was changing rapidly over a day or
two, said Williamson, a fourth-year doctoral student.
During the closest approach between the star and the pulsar in
November 2017, Williamson noticed that the VERITAS telescopes had
overnight recorded ten times the number of gamma rays detected only a
few days before.
I double checked everything before sending the data to our
collaborators, Williamson said. Then one of our partners, Ralph Bird
at UCLA, confirmed hed gotten the same results; that was exciting.
Even more interesting this observational data did not match what predictive models had predicted.
Generally speaking, Holder said, existing models predicted that as
the pulsar approached the massive star it was orbiting, the number of
gamma rays produced would slowly accelerate, experience some volatility
and then slowly decay over time.
But our recorded data showed a huge spike in the number of gamma
rays instead, Holder said. This tells us that we need to revise the
models of how this particle acceleration is happening.
Whats more, according to Holder, while astrophysicists expected the National Aeronautics and Space Administrations (NASA) Fermi gamma-ray space telescope
to record these gamma rays, it didnt. Holder said the reason for this
is unclear, but that is part of what makes the VERITAS results so
Astrophysicists want to learn just which particles are being
accelerated, and what processes are pushing them up to these extreme
speeds, in order to understand more about the Universe. Holder said that
although gamma-ray binary systems probably dont accelerate a large
portion of the particles in our galaxy, they allow scientists to study
the type of acceleration mechanisms which could produce them.
Astronomers wont be able to see this binary system at work again
until 2067 when the two stars are once again close together. By then,
Williamson joked that he just might be an emeritus professor with time
on his hands.
At the moment, Williamson is not worried about running out of things
to do. He spent three months at the Arizona-based observatory earlier
this year, taking measurements, performing hardware maintenance and
devising a remote control to allow the researchers to turn on the
telescopes cameras from a computer inside a control room.
It was a great chance to spend hands-on time with the telescopes and get to know the instrument, said Williamson.
Going forward, hell spend the remainder of his doctoral studies
combing through and analyzing in greater detail the nearly 175 hours of
data the VERITAS telescopes collected in 2016 and 2017.
Tyler is, without a doubt, the luckiest graduate student Ive ever
met because this event that happens only once every 50 years one of
the most exciting things weve seen with our telescopes in a decade
occurred right in the middle of his doctoral work, said Holder.
Funding for this work was provided by the National Science Foundation and NASA.
Article by Karen B. Roberts; photos by Evan Krape, John Millis, John Quinn and courtesy of Jamie Holder