The flashing of a nearby star has drawn MIT astronomers to a new and
mysterious system 3,000 light years from Earth. The stellar oddity appears
to be a new "black widow binary"—a rapidly spinning neutron star, or pulsar,
that is circling and slowly consuming a smaller companion star, as its
arachnid namesake does to its mate.
Astronomers know of about two dozen black widow binaries in the Milky Way.
This newest candidate, named ZTF J1406+1222, has the shortest orbital period
yet identified, with the pulsar and companion star circling each other every
62 minutes. The system is unique in that it appears to host a third,
far-flung star that orbits around the two inner stars every 10,000 years.
This likely triple black widow is raising questions about how such a system
could have formed. Based on its observations, the MIT team proposes an
origin story: As with most black widow binaries, the triple system likely
arose from a dense constellation of old stars known as a globular cluster.
This particular cluster may have drifted into the Milky Way's center, where
the gravity of the central black hole was enough to pull the cluster apart
while leaving the triple black widow intact.
"It's a complicated birth scenario," says Kevin Burdge, a Pappalardo
Postdoctoral Fellow in MIT's Department of Physics. "This system has
probably been floating around in the Milky Way for longer than the sun has
been around."
Burdge is the author of a study appearing in Nature that details the team's
discovery. The researchers used a new approach to detect the triple system.
While most black widow binaries are found through the gamma and X-ray
radiation emitted by the central pulsar, the team used visible light, and
specifically the flashing from the binary's companion star, to detect ZTF
J1406+1222.
"This system is really unique as far as black widows go, because we found it
with visible light, and because of its wide companion, and the fact it came
from the galactic center," Burdge says. "There's still a lot we don't
understand about it. But we have a new way of looking for these systems in
the sky."
The study's co-authors are collaborators from multiple institutions,
including the University of Warwick, Caltech, the University of Washington,
McGill University, and the University of Maryland.
Day and night
Black widow binaries are powered by pulsars—rapidly spinning neutron stars
that are the collapsed cores of massive stars. Pulsars have a dizzying
rotational period, spinning around every few milliseconds, and emitting
flashes of high-energy gamma and X-rays in the process.
Normally, pulsars spin down and die quickly as they burn off a huge amount
of energy. But every so often, a passing star can give a pulsar new life. As
a star nears, the pulsar's gravity pulls material off the star, which
provides new energy to spin the pulsar back up. The "recycled" pulsar then
starts reradiating energy that further strips the star, and eventually
destroys it.
"These systems are called black widows because of how the pulsar sort of
consumes the thing that recycled it, just as the spider eats its mate,"
Burdge says.
Every black widow binary to date has been detected through gamma and X-ray
flashes from the pulsar. In a first, Burdge came upon ZTF J1406+1222 through
the optical flashing of the companion star.
It turns out that the companion star's day side—the side perpetually facing
the pulsar—can be many times hotter than its night side, due to the constant
high-energy radiation it receives from the pulsar.
"I thought, instead of looking directly for the pulsar, try looking for the
star that it's cooking," Burdge explains.
He reasoned that if astronomers observed a star whose brightness was
changing periodically by a huge amount, it would be a strong signal that it
was in a binary with a pulsar.
Star motion
To test this theory, Burdge and his colleagues looked through optical data
taken by the Zwicky Transient Facility, an observatory based in California
that takes wide-field images of the night sky. The team studied the
brightness of stars to see whether any were changing dramatically by a
factor of 10 or more, on a timescale of about an hour or less—signs that
indicate the presence of a companion star orbiting tightly around a pulsar.
The team was able to pick out the dozen known black widow binaries,
validating the new method's accuracy. They then spotted a star whose
brightness changed by a factor of 13, every 62 minutes, indicating that it
was likely part of a new black widow binary, which they labeled ZTF
J1406+1222.
They looked up the star in observations taken by Gaia, a space telescope
operated by the European Space Agency that keeps precise measurements of the
position and motion of stars in the sky. Looking back through decades old
measurements of the star from the Sloan Digital Sky Survey, the team found
that the binary was being trailed by another distant star. Judging from
their calculations, this third star appeared to be orbiting the inner binary
every 10,000 years.
Curiously, the astronomers have not directly detected gamma or X-ray
emissions from the pulsar in the binary, which is the typical way in which
black widows are confirmed. ZTF J1406+1222, therefore, is considered a
candidate black widow binary, which the team hopes to confirm with future
observations.
"The one thing we know for sure is that we see a star with a day side that's
much hotter than the night side, orbiting around something every 62
minutes," Burdge says. "Everything seems to point to it being a black widow
binary. But there are a few weird things about it, so it's possible it's
something entirely new."
The team plans to continue observing the new system, as well as apply the
optical technique to illuminate more neutron stars and black widows in the
sky.
Reference:
Kevin Burdge, A 62-minute orbital period black widow binary in a wide
hierarchical triple, Nature (2022).
DOI: 10.1038/s41586-022-04551-1
Tags:
Space & Astrophysics