Finding binaries with a compact object orbiting a supermassive black hole is essential since they are considered antecedents of gravitational wave occurrences. However, this has proven to be an incredibly challenging process.
Researchers at MIT, Italy, the Czech Republic, and other universities have seen a silent black hole until recently erupting. It is situated at the heart of a galaxy around 800 million light-years away. Every 8.5 days, the black hole emits gas plumes before quieting down again.
These recurring explosions show a novel behavior in black holes that has never been observed before. Scientists believe that the existence of a second, smaller black hole orbiting the original supermassive black hole is the most plausible explanation for these explosions. Every 8.5 days, material is thought to be thrown out of the gas ring encircling the larger black hole by this smaller one, which is believed to be orbiting the larger one at a high speed.
The research casts doubt on the belief that black hole accretion disks are essentially uniform gas disks revolving around a central black hole. The new findings imply that the contents of accretion disks might be more diversified, perhaps including whole stars or other black holes.
The All-Sky Automatic Survey for SuperNovae (ASAS-SN), a network of 20 robotic telescopes dispersed throughout the Northern and Southern Hemispheres, made the automatic discovery that led to the team’s breakthrough. Once a day, these telescopes explore the sky for indications of supernovae and other transitory phenomena.
ASAS-SN discovered a light burst in December 2020 in a galaxy roughly 800 million light-years away. Before the telescopes saw the galaxy’s abrupt brightening, which grew by a factor of 1,000, this area of the sky had been comparatively calm.
Scientists decided to use NASA’s NICER (the Neutron Star Interior Composition Explorer), an X-ray telescope installed on the International Space Station, to concentrate on the flare after learning about its identification in a community alert. NICER continuously scans the sky for X-ray bursts to detect activity from neutron stars, black holes, and other extreme gravitational events. Luckily, the timing was perfect since the team was allowed to “trigger” the target by pointing the telescope toward it before the observation period ended.
When the flare outburst occurred, which lasted for around four months before ending, scientists trained NICER to monitor the far-off galaxy continuously. NICER obtained high-cadence daily measurements of the galaxy’s X-ray radiation during this time.
After closely examining the data, scientists discovered an odd pattern in the four-month flare: faint decreases in a narrow band of X-rays that occurred every 8.5 days. Every 8.5 days, the galaxy’s energetic outburst underwent periodic dimming. This signal was similar to what astronomers see when an orbiting planet briefly reduces the brightness of its home star by passing in front of it. But it didn’t seem possible for a star to stop a flare coming from a whole galaxy.
While looking for an explanation for the periodic dips in the galaxy’s energy, scientists came upon a recent paper by Czech theoretical physicists. These theorists had independently postulated the possibility of a second, much smaller black hole hidden inside the supermassive black hole at the center of a galaxy. This smaller black hole may orbit at an angle concerning the accretion disk of the larger one.
Theorists proposed that during its orbit, the secondary black hole would periodically pass across the disk of the parent black hole. During this procedure, it would emit a plume of gas resembling a bee soaring through a pollen cloud. The plume might then be propelled upward and out of the disk by magnetic solid fields above and below the black hole.
The smaller black hole ejected another plume regularly and periodically each time it moved around the disk. These plumes may be seen as periodic dips in the galaxy’s total energy that would momentarily block disk light if they coincided with an observing telescope.
Study author Dheeraj “DJ” Pasham, a research scientist at MIT’s Kavli Institute for Astrophysics and Space Research, said, “I was super excited by this theory, and I immediately emailed them to say, ‘I think we’re observing exactly what your theory predicted.”
Scientists tested this idea with simulations incorporating NICER’s observations of the original outburst and the regular 8.5-day dips.
They found that the observed outburst was likely a signal of a second, smaller black hole orbiting a central supermassive black hole and periodically puncturing its disk.
The team found that the galaxy was surprisingly quiet before 2020 detection. According to the study, the galaxy’s center supermassive black hole could weigh up to 50 million suns. Prior to the outburst, the black hole was probably surrounded by a faint, diffuse accretion disk. In the meantime, a second, smaller black hole, with a mass between 100 and 10,000 times that of the sun, was circling it in relative darkness.
The enormous gravitational power of the supermassive black hole is thought to have torn apart a third item, maybe a nearby star, when it approached the system too closely in December 2020. This event is referred to as a “tidal disruption event.”
As the star’s debris spiraled into the black hole, a sudden surge of stellar material caused the black hole’s accretion disk to brighten momentarily. The second black hole continued its orbit as the black hole swallowed the star debris over four months. The smaller black hole produced a larger-than-usual plume each time it traversed the disk, which pointed directly toward NICER’s scope.
The researchers ran several simulations to investigate the reported periodic drops. They concluded that a novel kind of astronomical system—a little, intermediate-mass black hole around a supermassive black hole—is the most likely explanation.
Pasham said, “This is a different beast. It doesn’t fit anything that we know about these systems. We’re seeing evidence of objects going in and through the disk at different angles, which challenges the traditional picture of a simple gaseous disk around black holes. We think there is a huge population of these systems out there.”
Richard Saxton, an X-ray astronomer from the European Space Astronomy Centre (ESAC) in Madrid, who was not involved in the study, said, “This is a brilliant example of how to use the debris from a disrupted star to illuminate the interior of a galactic nucleus which would otherwise remain dark. It is akin to using fluorescent dye to find a leak in a pipe. This result shows that very close supermassive black hole binaries could be common in galactic nuclei, which is an exciting development for future gravitational wave detectors.”
Journal Reference:
- Dheeraj Pasham, Francesco Tombesi, et al. A case for a binary black hole system revealed via quasi-periodic outflows. Science Advances. DOI: 10.1126/sciadv.adj8898