Locked in an epic cosmic waltz 9 billion light years away, two supermassive
black holes appear to be orbiting around each other every two years. The two
giant bodies each have masses that are hundreds of millions of times larger
than that of our sun, and the objects are separated by a distance roughly 50
times that which separates our sun and Pluto. When the pair merge in roughly
10,000 years, the titanic collision is expected to shake space and time
itself, sending gravitational waves across the universe.
A Caltech-led team of astronomers has discovered evidence for this scenario
taking place within a fiercely energetic object known as a quasar. Quasars
are active cores of galaxies in which a supermassive black hole is siphoning
material from a disk encircling it. In some quasars, the supermassive black
hole creates a jet that shoots out at near the speed of light. The quasar
observed in the new study, PKS 2131-021, belongs to a subclass of quasars
called blazars in which the jet is pointing toward the Earth. Astronomers
already knew quasars could possess two orbiting supermassive black holes,
but finding direct evidence for this has proved difficult.
Reporting in The Astrophysical Journal Letters, the researchers argue that
PKS 2131-021 is now the second known candidate for a pair of supermassive
black holes caught in the act of merging. The first candidate pair, within a
quasar called OJ 287, orbit each other at greater distances, circling every
nine years versus the two years it takes for the PKS 2131-021 pair to
complete an orbit.
The telltale evidence came from radio observations of PKS 2131-021 that span
45 years. According to the study, a powerful jet emanating from one of the
two black holes within PKS 2131-021 is shifting back and forth due to the
pair's orbital motion. This causes periodic changes in the quasar's
radio-light brightness. Five different observatories registered these
oscillations, including Caltech's Owens Valley Radio Observatory (OVRO), the
University of Michigan Radio Astronomy Observatory (UMRAO), MIT's Haystack
Observatory, the National Radio Astronomy Observatory (NRAO), Metsähovi
Radio Observatory in Finland, and NASA's Wide-field Infrared Survey Explorer
(WISE) space satellite.
The combination of the radio data yields a nearly perfect sinusoidal light
curve unlike anything observed from quasars before.
"When we realized that the peaks and troughs of the light curve detected
from recent times matched the peaks and troughs observed between 1975 and
1983, we knew something very special was going on," says Sandra O'Neill,
lead author of the new study and an undergraduate student at Caltech who is
mentored by Tony Readhead, Robinson Professor of Astronomy, Emeritus.
Ripples in Space and Time
Most, if not all, galaxies possess monstrous black holes at their cores,
including our own Milky Way galaxy. When galaxies merge, their black holes
"sink" to the middle of the newly formed galaxy and eventually join together
to form an even more massive black hole. As the black holes spiral toward
each other, they increasingly disturb the fabric of space and time, sending
out gravitational waves, which were first predicted by Albert Einstein more
than 100 years ago.
The National Science Foundation's LIGO (Laser Interferometer
Gravitational-Wave Observatory), which is managed jointly by Caltech and
MIT, detects gravitational waves from pairs of black holes up to dozens of
times the mass of our sun. However, the supermassive black holes at the
centers of galaxies have millions to billions of times as much mass as our
sun, and give off lower frequencies of gravitational waves than those
detected by LIGO.
In the future, pulsar timing arrays—which consist of an array of pulsing
dead stars precisely monitored by radio telescopes—should be able to detect
the gravitational waves from supermassive black holes of this heft. (The
upcoming Laser Interferometer Space Antenna, or LISA, mission would detect
merging black holes whose masses are 1,000 to 10 million times greater than
the mass of our sun.) So far, no gravitational waves have been registered
from any of these heavier sources, but PKS 2131-021 provides the most
promising target yet.
In the meantime, light waves are the best option to detect coalescing
supermassive black holes.
The first such candidate, OJ 287, also exhibits periodic radio-light
variations. These fluctuations are more irregular, and not sinusoidal, but
they suggest the black holes orbit each other every nine years. The black
holes within the new quasar, PKS 2131-021, orbit each other every two years
and are 2,000 astronomical units apart, about 50 times the distance between
our sun and Pluto, or 10 to 100 times closer than the pair in OJ 287. (An
astronomical unit is the distance between Earth and the sun.)
Revealing the 45-Year Light Curve
Readhead says the discoveries unfolded like a "good detective novel,"
beginning in 2008 when he and colleagues began using the 40-meter telescope
at OVRO to study how black holes convert material they "feed" on into
relativistic jets, or jets traveling at speeds up to 99.98 percent that of
light. They had been monitoring the brightness of more than 1,000 blazars
for this purpose when, in 2020, they noticed a unique case.
"PKS 2131 was varying not just periodically, but sinusoidally," Readhead
says. "That means that there is a pattern we can trace continuously over
time." The question, he says, then became how long has this sine wave
pattern been going on?
The research team then went through archival radio data to look for past
peaks in the light curves that matched predictions based on the more recent
OVRO observations. First, data from NRAO's Very Long Baseline Array and
UMRAO revealed a peak from 2005 that matched predictions. The UMRAO data
further showed there was no sinusoidal signal at all for 20 years before
that time—until as far back as 1981 when another predicted peak was
observed.
"The story would have stopped there, as we didn't realize there were data on
this object before 1980," Readhead says. "But then Sandra picked up this
project in June of 2021. If it weren't for her, this beautiful finding would
be sitting on the shelf."
O'Neill began working with Readhead and the study's second author Sebastian
Kiehlmann, a postdoc at the University of Crete and former staff scientist
at Caltech, as part of Caltech's Summer Undergraduate Research Fellowship
(SURF) program. O'Neill began college as a chemistry major but picked up the
astronomy project because she wanted to stay active during the pandemic. "I
came to realize I was much more excited about this than anything else I had
worked on," she says.
With the project back on the table, Readhead searched through the literature
and found that the Haystack Observatory had made radio observations of PKS
2131-021 between 1975 and 1983. These data revealed another peak matching
their predictions, this time occurring in 1976.
"This work shows the value of doing accurate monitoring of these sources
over many years for performing discovery science," says co-author Roger
Blandford, Moore Distinguished Scholar in Theoretical Astrophysics at
Caltech who is currently on sabbatical from Stanford University.
Like Clockwork
Readhead compares the system of the jet moving back and forth to a ticking
clock, where each cycle, or period, of the sine wave corresponds to the
two-year orbit of the black holes (though the observed cycle is actually
five years due to light being stretched by the expansion of the universe).
This ticking was first seen in 1976 and it continued for eight years before
disappearing for 20 years, likely due to changes in the fueling of the black
hole. The ticking has now been back for 17 years.
"The clock kept ticking," he says, "The stability of the period over this
20-year gap strongly suggests that this blazar harbors not one supermassive
black hole, but two supermassive black holes orbiting each other."
The physics underlying the sinusoidal variations were at first a mystery,
but Blandford came up with a simple and elegant model to explain the
sinusoidal shape of the variations.
"We knew this beautiful sine wave had to be telling us something important
about the system," Readhead says. "Roger's model shows us that it is simply
the orbital motion that does this. Before Roger worked it out, nobody had
figured out that a binary with a relativistic jet would have a light curve
that looked like this."
Kiehlmann says their "study provides a blueprint for how to search for such
blazar binaries in the future."
Reference:
S. O'Neill et al, The Unanticipated Phenomenology of the Blazar PKS
2131–021: A Unique Supermassive Black Hole Binary Candidate, The
Astrophysical Journal Letters (2022).
DOI: 10.3847/2041-8213/ac504b
Tags:
Space & Astrophysics