The first observation of a brand-new kind of supernova had been predicted by theorists but never before confirmed
Dillon Dong, with a 40-meter radio dish at Caltech's Owens Valley Radio Observatory in the background.
In 2017, a
particularly luminous and unusual source of radio waves was discovered in data
taken by the Very Large Array (VLA) Sky Survey, a project that scans the night
sky in radio wavelengths. Now, led by Caltech graduate student Dillon Dong (MS
'18), a team of astronomers has established that the bright radio flare was
caused by a black hole or neutron star crashing into its companion star in a
never-before-seen process.
"Massive stars
usually explode as supernovae when they run out of nuclear fuel,"
says Gregg Hallinan, professor of
astronomy at Caltech. "But in this case, an invading black hole or neutron
star has prematurely triggered its companion star to explode." This is the
first time a merger-triggered supernova has ever been confirmed.
A paper about the
findings appears in the journal Science on September 3.
Bright Flares in the
Night Sky
Hallinan and his team
look for so-called radio transients—short-lived sources of radio waves that
flare brightly and burn out quickly like a match lit in a dark room. Radio
transients are an excellent way to identify unusual astronomical events, such
as massive stars that explode and blast out energetic jets or the mergers of
neutron stars.
As Dong sifted through
the VLA's massive dataset, he singled out an extremely luminous source of radio
waves from the VLA survey called VT 1210+4956. This source is tied for the
brightest radio transient ever associated with a supernova.
Dong determined that
the bright radio energy was originally a star surrounded by a thick and
dense shell of gas. This gas shell had been cast off the star a few hundred
years before the present day. VT 1210+4956, the radio transient, occurred when
the star finally exploded in a supernova and the material ejected from the
explosion interacted with the gas shell. Yet, the gas shell itself, and the
timescale on which it was cast off from the star, were unusual, so Dong
suspected that there might be more to the story of this explosion.
Two Unusual Events
Following Dong's
discovery, Caltech graduate student Anna Ho (PhD '20) suggested that this radio
transient be compared with a different catalog of brief bright events in the
X-ray spectrum. Some of these X-ray events were so short-lived that they were
only present in the sky for a few seconds of Earth time. By examining this
other catalog, Dong discovered a source of X-rays that originated from the same
spot in the sky as VT 1210+4956. Through careful analysis, Dong established
that the X-rays and the radio waves were likely coming from the same event.
"The X-ray
transient was an unusual event—it signaled that a relativistic jet was launched
at the time of the explosion," says Dong. "And the luminous radio
glow indicated that the material from that explosion later crashed into a
massive torus of dense gas that had been ejected from the star centuries
earlier. These two events have never been associated with each other, and on
their own they're very rare."
A Mystery Solved
So, what happened?
After careful modeling, the team determined the most likely explanation—an
event that involved some of the same cosmic players that are known to generate
gravitational waves.
They speculated that a
leftover compact remnant of a star that had previously exploded—that is, a
black hole or a neutron star—had been closely orbiting around a star. Over
time, the black hole had begun siphoning away the atmosphere of its companion
star and ejecting it into space, forming the torus of gas. This process dragged
the two objects ever closer until the black hole plunged into the star, causing
the star to collapse and explode as a supernova.
The X-rays were produced
by a jet launched from the core of the star at the moment of its collapse. The
radio waves, by contrast, were produced years later as the exploding star
reached the torus of gas that had been ejected by the inspiraling compact
object.
Astronomers know that
a massive star and a companion compact object can form what is called a stable
orbit, in which the two bodies gradually spiral closer and closer over an
extremely long period of time. This process forms a binary system that is
stable for millions to billions of years but that will eventually collide and
emit the kind of gravitational waves that were discovered by LIGO in 2015 and 2017.
However, in the case
of VT 1210+4956, the two objects instead collided immediately and
catastrophically, producing the blasts of X-rays and radio waves observed.
Although collisions such as this have been predicted theoretically, VT
1210+4956 provides the first concrete evidence that it happens.
Serendipitous
Surveying
The VLA Sky Survey
produces enormous amounts of data about radio signals from the night sky, but
sifting through that data to discover a bright and interesting event such as
VT 1210+4956 is like finding a needle in a haystack. Finding this
particular needle, Dong says, was, in a way, serendipitous.
"We had ideas of
what we might find in the VLA survey, but we were open to the possibility of
finding things we didn't expect," explains Dong. "We created the
conditions to discover something interesting by conducting loosely constrained,
open-minded searches of large data sets and then taking into account all of the
contextual clues we could assemble about the objects that we found. During this
process you find yourself pulled in different directions by different
explanations, and you simply let nature tell you what's out there."
The paper is titled
"A transient radio source consistent with a merger-triggered core collapse
supernova." Dillon Dong is the first author. In addition to Hallinan and
Ho, additional co-authors are Ehud Nakar, Andrew Hughes, Kenta Hotokezaka,
Steve Myers (PhD '90), Kishalay De (MS '18, PHD '21), Kunal Mooley (PhD '15),
Vikram Ravi, Assaf Horesh, Mansi Kasliwal (MS '07, PhD '11), and Shri Kulkarni.
Funding was provided by the National Science Foundation, the United
States–Israel Binational Science Foundation, the I-Core Program of the Planning
and Budgeting Committee and the Israel Science Foundation, Canada's Natural
Sciences and Engineering Research Council, the Miller Institute for Basic
Research in Science at the UC Berkeley, the Japan Society for the Promotion of
Science Early-Career Scientists Program, the National Radio Astronomy Observatory,
and the Heising-Simons Foundation.
This
illustration shows a massive star that is about to explode. The explosion was
triggered after its dead-star companion (a black hole or neutron star) plunged
into the star's core. Scientists say that the black hole or neutron star rammed
into the massive star, and then, as it traveled inward over the course of
centuries, ejected a spiral of material from the star's atmosphere (pictured
surrounding the star). When it reached the star's core, material from the core
rapidly fell onto the stellar corpse and this led to the launching of a pair of
jets at nearly the speed of light. In this artist's depiction, the jets are
shown tunneling through the star, and will soon set off the supernova
explosion. After a few years, the supernova will crash through the bulk of the
ejected spiral, which extends to about 10,000 times the size of the star. This
will create the luminous transient radio source observed by the Very Large
Array.Credit: Chuck Carter
September 02, 2021
WRITTEN BY
Lori Dajose
Source: A
Black Hole Triggers a Premature Supernova | www.caltech.edu
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