Using X-ray data from NASA’s Chandra X-ray Observatory and optical data from ground-based telescopes, astronomers have spotted what could be the most tightly coupled pair of supermassive black holes ever seen.
X-ray source LGGS J004527.30+413254.3. Image credit: NASA / CXC / University of Washington / T. Dorn-Wallenstein et al / ESA / J. Dalcanton et al / R. Gendler.
Until recently, astronomers thought this source, known as LGGS J004527.30+413254.3 (J0045+41 for short), was an object within the Andromeda Galaxy, which is approximately 2.5 million light-years from Earth.
The new data, however, revealed that J0045+41 was actually at a much greater distance, around 2.6 billion light-years from Earth.
“We were looking for a special type of star in the Andromeda Galaxy and thought we had found one. We were surprised and excited to find something far stranger,” said Dr. Trevor Dorn-Wallenstein, from the University of Washington.
The estimated total mass for two giant black holes in J0045+41 is about 200 million solar masses.
They orbit each other with a separation of only a few hundred times the distance between the Earth and the Sun — this corresponds to less than 1/100 of a light-year.
Such a system could be formed as a consequence of the merger, billions of years earlier, of two galaxies that each contained a supermassive black hole.
At their current close separation, the two black holes are inevitably being drawn closer together as they emit gravitational waves.
“We’re unable to pinpoint exactly how much mass each of these black holes contains. Depending on that, we think this pair will collide and merge into one black hole in as little as 350 years or as much as 360,000 years,” said Dr. John Ruan, also of the University of Washington.
Previously, a different research team had seen periodic variations in the optical light from this source, and, believing it to be a member of the Andromeda Galaxy, classified it as a pair of stars that orbited around each other about once every 80 days.
The intensity of the X-ray source observed by Chandra revealed this original classification was incorrect.
Rather, J0045+41 had to be either a binary system in the Andromeda Galaxy containing a neutron star or black hole that is pulling material from a companion or a much more massive and distant system that contains at least one rapidly growing supermassive black hole.
However, a spectrum from the Gemini-North telescope in Hawaii taken by Dr. Dorn-Wallenstein and colleagues showed that J0045+41 must host at least one supermassive black hole and allowed the team to estimate the distance.
The spectrum also provided possible evidence that a second black hole was present in J0045+41 and moving at a different velocity from the first, as expected if the two black holes are orbiting each other.
The astronomers then used optical data from Caltech’s Palomar Transient Factory to search for periodic variations in the light from J0045+41.
They found several periods in J0045+41, including ones at about 80 and 320 days. The ratio between these periods matches that predicted by theoretical work on the dynamics of two giant black holes orbiting each other.
“This is the first time such strong evidence has been found for a pair of orbiting giant black holes,” said Dr. Emily Levesque, also from the University of Washington.
A paper describing the discovery will appear in the Astrophysical Journal and is now available online at arXiv.org.
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Trevor Dorn-Wallenstein et al. 2017. A Mote in Andromeda’s Disk: a Misidentified Periodic AGN behind M31. ApJ, in press; arXiv: 1704.08694
