Credit: Columbia University
This simulation helps explain an odd light signal thought to be
coming from a close-knit pair of merging black holes, PG 1302-102,
located 3.5 billion light-years away. The close-up view at right shows
that the smaller of the two black holes gives off more light (left side
of picture). While the black holes themselves don't emit light, they
accumulate and heat up surrounding gas, which then radiates light. The
reason the smaller black hole gives off more light is that it is
orbiting farther from the center of mass and closer to the surrounding
gas disk, allowing it to gather up most of the gas as it orbits. The
result is that the more massive central black hole is starved of gas and
doesn't glow as brightly.
As these black holes orbit around each other, they are thought to
send out a varying light signal. The signal was detected by astronomers
using telescopes on the ground and in space.
The simulation comes from Brian Farris of Columbia University and New York University, both in New York City. Credit: Columbia University
Entangled by gravity and destined to merge, two candidate black holes
in a distant galaxy appear to be locked in an intricate dance.
Researchers using data from NASA's Galaxy Evolution Explorer (GALEX) and
NASA's Hubble Space Telescope have come up with the most compelling
confirmation yet for the existence of these merging black holes and have
found new details about their odd, cyclical light signal.
The candidate black hole duo, called PG 1302-102, was first
identified earlier this year using ground-based telescopes. The black
holes are the tightest orbiting pair detected so far, with a separation
not much bigger than the diameter of our solar system. They are expected
to collide and merge in less than a million years, triggering a titanic
blast with the power of 100 million supernovae.
Researchers are studying this pair to better understand how galaxies
and the monstrous black holes at their cores merge -- a common
occurrence in the early universe. But as common as these events were,
they are hard to spot and confirm.
PG 1302-102 is one of only a handful of good binary black hole
candidates. It was discovered and reported earlier this year by
researchers at the California Institute of Technology in Pasadena, after
they scrutinized an unusual light signal coming from the center of a
galaxy. The researchers, who used telescopes in the Catalina Real-Time
Transient Survey, demonstrated that the varying signal is likely
generated by the motion of two black holes, which swing around each
other every five years.
While the black holes themselves don't give off
light, the material surrounding them does.
In the new study, published in the Sept. 17 issue of Nature,
researchers found more evidence to support and confirm the close-knit
dance of these black holes. Using ultraviolet data from GALEX and
Hubble, they were able to track the system's changing light patterns
over the past 20 years.
"We were lucky to have GALEX data to look through," said co-author
David Schiminovich of Columbia University in New York. "We went back
into the GALEX archives and found that the object just happened to have
been observed six times."
Hubble, which sees ultraviolet light in addition to visible and other
wavelengths of light, had likewise observed the object in the past.
The ultraviolet light was important to test a prediction of how the
black holes generate a cyclical light pattern. The idea is that one of
the black holes in the pair is giving off more light -- it is gobbling
up more matter than the other one, and this process heats up matter that
emits energetic light. As this black hole orbits around its partner
every five years, its light changes and appears to brighten as it heads
toward us.
"It's as if a 60-Watt light bulb suddenly appears to be 100 Watts,"
explained Daniel D'Orazio, lead author of the study from Columbia
University. "As the black hole light speeds away from us, it appears as a
dimmer 20-Watt bulb."
What's causing the changes in light? One set of changes has to do
with the "blue shifting" effect, in which light is squeezed to shorter
wavelengths as it travels toward us in the same way that a police car's
siren squeals at higher frequencies as it heads toward you. Another
reason has to do with the enormous speed of the black hole.
The brighter black hole is, in fact, traveling at nearly seven
percent the speed of light -- in other words, really fast. Though it
takes the black hole five years to orbit its companion, it is traveling
vast distances. It would be as if a black hole lapped our entire solar
system from the outer fringes, where the Oort cloud of comets lies, in
just five years. At speeds as high as this, which are known as
relativistic, the light becomes boosted and brighter.
D'Orazio and colleagues modeled this effect based on a previous
Caltech paper and predicted how it should look in ultraviolet light.
They determined that, if the periodic brightening and dimming previously
seen in the visible light is indeed due to the relativistic boosting
effect, then the same periodic behavior should be present in ultraviolet
wavelengths, but amplified 2.5 times. Sure enough, the ultraviolet
light from GALEX and Hubble matched their predictions.
"We are strengthening our ideas of what's going on in this system and
starting to understand it better," said Zoltan Haiman, a co-author from
Columbia University who conceived the project.
The results will also help researchers understand how to find even
closer-knit merging black holes in the future, what some consider the
holy grail of physics and the search for gravitational waves. In the
final moments before the ultimate union of two black holes, when they
are tightly spinning around each other like ice skaters in a "death
spiral," they are predicted to send out ripples in space and time.
These
so-called gravitational waves, whose existence follows from Albert
Einstein's gravity theory published 100 years ago, hold clues about the
fabric of our universe.
The findings are also a doorway to understanding other merging black
holes across the universe, a widespread population that is only now
beginning to yield its secrets.
The California Institute of Technology in Pasadena led the Galaxy
Evolution Explorer mission, which ended in 2013 after more than a decade
of scanning the skies in ultraviolet light. NASA's Jet Propulsion
Laboratory, also in Pasadena, managed the mission and built the science
instrument. JPL is managed by Caltech for NASA.
The Hubble Space Telescope is a project of international cooperation
between NASA and ESA (European Space Agency). NASA's Goddard Space
Flight Center in Greenbelt, Maryland, manages the telescope. The Space
Telescope Science Institute (STScI) in Baltimore, Maryland, conducts
Hubble science operations. STScI is operated for NASA by the Association
of Universities for Research in Astronomy in Washington.
