Astronomers are gearing up for high-energy fireworks coming in early 2018, when a stellar remnant the size of a city meets one of the brightest stars in our galaxy. The cosmic light show will occur when a pulsar discovered by NASA's Fermi Gamma-ray Space Telescope swings by its companion star. Scientists plan a global campaign to watch the event from radio wavelengths to the highest-energy gamma rays detectable.
The pulsar, known as J2032+4127 (J2032 for short), is the crushed
core of a massive star that exploded as a supernova. It is a magnetized
ball about 12 miles across, or about the size of Washington, weighing
almost twice the sun's mass and spinning seven times a second. J2032's
rapid spin and strong magnetic field together produce a lighthouse-like
beam detectable when it sweeps our way. Astronomers find most pulsars
through radio emissions, but Fermi's Large Area Telescope (LAT) finds
them through pulses of gamma rays, the most energetic form of light.
J2032 was found in 2009 through a so-called blind search of LAT data.
Using this technique, astronomers can find pulsars whose radio beams
may not be pointed precisely in our direction and are therefore much
harder to detect.
"Two dozen pulsars were discovered this way in the first year of LAT
data alone, including J2032," said David Thompson, a Fermi deputy
project scientist at NASA's Goddard Space Flight Center in Greenbelt,
Maryland. "Nearly all of them would not have been found without Fermi."
Once they knew exactly where to look, radio astronomers also were
able to detect J2032. A team at the Jodrell Bank Centre for Astrophysics
at the University of Manchester in the U.K. kept tabs on the object
from 2010 through 2014. And they noticed something odd.
"We detected strange variations in the rotation and the rate at which
the rotation slows down, behavior we have not seen in any other
isolated pulsar," said Andrew Lyne, professor of physics at the
University of Manchester. "Ultimately, we realized these peculiarities
were caused by motion around another star, making this the
longest-period binary system containing a radio pulsar."
The massive star tugging on the pulsar is named MT91 213. Classified
as a Be star, the companion is 15 times the mass of the sun and shines
10,000 times brighter. Be stars drive strong outflows, called stellar
winds, and are embedded in large disks of gas and dust.
"When we discovered this pulsar in 2009, we noticed that it was in
the same direction as this massive star in the constellation Cygnus, but
our initial measurements did not give any evidence that either star was
a member of a binary system," explained Paul Ray, an astrophysicist at
the Naval Research Laboratory in Washington. "The only way to escape
that conclusion was if the binary system had a very long orbital period,
much longer than the longest known pulsar-massive star binary at the
time, which seemed unlikely."
Following an elongated orbit lasting about 25 years, the pulsar
passes closest to its partner once each circuit. Whipping around its
companion in early 2018, the pulsar will plunge through the surrounding
disk and trigger astrophysical fireworks. It will serve as a probe to
help astronomers measure the massive star's gravity, magnetic field,
stellar wind and disk properties.
Several features combine to make this an exceptional binary. Out of
six similar systems where the massive star uses hydrogen as its central
energy source, J2032's has the greatest combined mass, the longest
orbital period, and, at a distance of about 5,000 light-years, is
closest to Earth.
"This forewarning of the energetic fireworks expected at closest
approach in three years' time allows us to prepare to study the system
across the entire electromagnetic spectrum with the largest telescopes,"
added Ben Stappers, a professor of astrophysics at the University of
Manchester.
Astronomers think the supernova explosion that created the pulsar
also kicked it into its eccentric orbit, nearly tearing the binary apart
in the process. A study of the system led by Lyne and including Ray and
Stappers was published June 16 in the journal Monthly Notices of the
Royal Astronomical Society.
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