In a pair of merging supermassive black holes, a new method for
measuring the void

Date:
May 9, 2022
Source:
Columbia University
Summary:
Researchers have devised a potentially easier way of gazing into
the abyss. Their imaging technique could allow astronomers to
study black holes smaller than M87's, a monster with a mass of
6.5 billion suns, harbored in galaxies more distant than M87,
which at 55 million light- years away, is still relatively close
to our own Milky Way.



FULL STORY ========================================================================== Three years ago, the first ever image of a black hole stunned the world. A black pit of nothingness enclosed by a fiery ring of light. That iconic
image of the black hole at the center of galaxy Messier 87 came into focus thanks to the Event Horizon Telescope, a global network of synchronized
radio dishes acting as one giant telescope.


==========================================================================
Now, a pair of Columbia researchers have devised a potentially easier way
of gazing into the abyss. Outlined in complementary studies in Physical
Review Lettersand Physical Review D, their imaging technique could allow astronomers to study black holes smaller than M87's, a monster with a
mass of 6.5 billion suns, harbored in galaxies more distant than M87,
which at 55 million light- years away, is still relatively close to our
own Milky Way.

The technique has just two requirements. First, you need a pair of
supermassive black holes in the throes of merging. Second, you need to
be looking at the pair at a nearly side-on angle. From this sideways
vantage point, as one black hole passes in front of the other, you
should be able to see a bright flash of light as the glowing ring of the
black hole farther away is magnified by the black hole closest to you,
a phenomenon known as gravitational lensing.

The lensing effect is well known, but what the researchers discovered
here was a hidden signal: a distinctive dip in brightness corresponding
to the "shadow" of the black hole in back. This subtle dimming can last
from a few hours to a few days, depending on how massive the black holes,
and how closely entwined their orbits. If you measure how long the dip
lasts, the researchers say, you can estimate the size and shape of the
shadow cast by the black hole's event horizon, the point of no exit,
where nothing escapes, not even light.

"It took years and a massive effort by dozens of scientists to make
that high- resolution image of the M87 black holes," said the study's
first author, Jordy Davelaar, a postdoc at Columbia and the Flatiron Institute's Center for Computational Astrophysics. "That approach only
works for the biggest and closest black holes -- the pair at the heart of
M87 and potentially our own Milky Way." He added, "with our technique,
you measure the brightness of the black holes over time, you don't need
to resolve each object spatially. It should be possible to find this
signal in many galaxies." The shadow of a black hole is both its most mysterious and informative feature.

"That dark spot tells us about the size of the black hole, the shape
of the space-time around it, and how matter falls into the black hole
near its horizon," said co-author Zoltan Haiman, a physics professor
at Columbia.



========================================================================== Black hole shadows may also hold the secret to the true nature of
gravity, one of the fundamental forces of our universe. Einstein's theory
of gravity, known as general relativity, predicts the size of black
holes. Physicists, therefore, have sought them out to test alternative
theories of gravity in an effort to reconcile two competing ideas of
how nature works: Einstein's general relativity, which explains large
scale phenomena like orbiting planets and the expanding universe, and
quantum physics, which explains how tiny particles like electrons and
photons can occupy multiple states at once.

The researchers became interested in flaring supermassive black holes
after spotting a suspected pair of supermassive black holes at the
center of a far- off galaxy in the early universe. NASA's planet-hunting
Kepler space telescope was scanning for the tiny dips in brightness corresponding to a planet passing in front of its host star. Instead,
Kepler ended up detecting the flares of what Haiman and his colleagues
claim are a pair of merging black holes.

They named the distant galaxy "Spikey" for the spikes in brightness
triggered by its suspected black holes magnifying each other on each
full rotation via the lensing effect. To learn more about the flare,
Haiman built a model with his postdoc, Davelaar.

They were confused, however, when their simulated pair of black holes
produced an unexpected, but periodic, dip in brightness each time one
orbited in front of the other. At first, they thought it was a coding
mistake. But further checking led them to trust the signal.

As they looked for a physical mechanism to explain it, they realized
that each dip in brightness closely matched the time it took for the
black hole closest to the viewer to pass in front of the shadow of the
black hole in back.

The researchers are currently looking for other telescope data to try
and confirm the dip they saw in the Kepler data to verify that Spikey is,
in fact, harboring a pair of merging black holes. If it all checks out,
the technique could be applied to a handful of other suspected pairs
of merging supermassive black holes among the 150 or so that have been
spotted so far and are awaiting confirmation.

As more powerful telescopes come online in the coming years, other opportunities may arise. The Vera Rubin Observatory, set to open this
year, has its sights on more than 100 million supermassive black
holes. Further black hole scouting will be possible when NASA's
gravitational wave detector, LISA, is launched into space in 2030.

"Even if only a tiny fraction of these black hole binaries has the right conditions to measure our proposed effect, we could find many of these
black hole dips," Davelaar said.


========================================================================== Story Source: Materials provided by Columbia_University. Original written
by Kim Martineau.

Note: Content may be edited for style and length.


========================================================================== Related Multimedia:
* Shadow_of_a_black_hole ========================================================================== Journal References:
1. Jordy Davelaar, Zolta'n Haiman. Self-Lensing Flares from Black Hole
Binaries: Observing Black Hole Shadows via Light Curve Tomography.

Physical Review Letters, 2022; 128 (19) DOI: 10.1103/
PhysRevLett.128.191101
2. Jordy Davelaar, Zolta'n Haiman. Self-lensing flares from black hole
binaries: General-relativistic ray tracing of black hole binaries.

Physical Review D, 2022; 105 (10) DOI: 10.1103/PhysRevD.105.103010 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/05/220509132625.htm

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