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.
It’s a trip that took 50 years.
The VERITAS and MAGIC teams began monitoring the night sky and
tracking the pulsar’s orbit in September 2016. At first, they weren’t
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 earth’s 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 he’d 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.”
What’s more, according to Holder, while astrophysicists expected the National Aeronautics and Space Administration’s (NASA) Fermi gamma-ray space telescope
to record these gamma rays, it didn’t. 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 don’t accelerate a large
portion of the particles in our galaxy, they allow scientists to study
the type of acceleration mechanisms which could produce them.
Charting a promising future
Astronomers won’t 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
telescope’s 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, he’ll 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 I’ve ever
met because this event that happens only once every 50 years — one of
the most exciting things we’ve 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