Colossal black holes locked in dance at heart of galaxy
Astronomers find evidence for the tightest-knit supermassive black hole
duo observed to date

Date:
February 23, 2022
Source:
California Institute of Technology
Summary:
A sinusoidal light curve spanning 45 years points to the presence of
orbiting supermassive black holes at the core of a distant galaxy.



FULL STORY ========================================================================== 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), Metsa"hovi 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." Says Kiehlmann: "Our study provides
a blueprint for how to search for such blazar binaries in the future."
Video: https://youtu.be/B_q9tYjvgiY The Astrophysical Journal Letters
study titled "The Unanticipated Phenomenology of the Blazar PKS 2131-021:
A Unique Super-Massive Black hole Binary Candidate" was funded by Caltech,
the Max Planck Institute for Radio Astronomy, NASA, National Science
Foundation (NSF), the Academy of Finland, the European Research Council, ANID-FONDECYT (Agencia Nacional de Investigacio'n y Desarrollo-Fondo
Nacional de Desarrollo Cienti'fico y Tecnolo'gico in Chile), the Natural Science and Engineering Council of Canada, the Foundation for Research
and Technology -- Hellas in Greece, the Hellenic Foundation for Research
and Innovation in Greece, and the University of Michigan. Other Caltech
authors include Tim Pearson, Vikram Ravi, Kieran Cleary, Matthew Graham,
and Tom Prince. Other authors from the Jet Propulsion Laboratory, which is managed by Caltech for NASA, include Michele Vallisneri and Joseph Lazio.

========================================================================== Story Source: Materials provided by
California_Institute_of_Technology. Original written by Whitney
Clavin. Note: Content may be edited for style and length.


========================================================================== Related Multimedia:
* Tightest-knit_supermassive_black_hole_duo ========================================================================== Journal Reference:
1. S. O'Neill, S. Kiehlmann, A. C. S. Readhead, M. F. Aller, R. D.

Blandford, I. Liodakis, M. L. Lister, P. Mro'z, C. P. O'Dea, T. J.

Pearson, V. Ravi, M. Vallisneri, K. A. Cleary, M. J. Graham,
K. J. B.

Grainge, M. W. Hodges, T. Hovatta, A. La"hteenma"ki, J. W. Lamb,
T. J. W.

Lazio, W. Max-Moerbeck, V. Pavlidou, T. A. Prince, R. A. Reeves, M.

Tornikoski, P. Vergara de la Parra, J. A. Zensus. The Unanticipated
Phenomenology of the Blazar PKS 2131-021: A Unique Supermassive
Black Hole Binary Candidate. The Astrophysical Journal Letters,
2022; 926 (2): L35 DOI: 10.3847/2041-8213/ac504b ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220223133506.htm

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