Astronomers discovered a suspected tight binary system composed of two supermassive black holes at the center of the active galaxy Markarian 501 (Mrk 501 for short). The relevant research was led by the Max Planck Institute for Radio Astronomy in Germany and has been accepted by the Monthly Notices of the Royal Astronomical Society.
Long-term high-resolution radio observations show that there is not only a long-known powerful particle jet in the core of the galaxy (a jet refers to a stream of high-energy particles ejected at nearly the speed of light), but also a second jet hidden, providing direct evidence for a pair of supermassive black holes orbiting extremely close to each other.
Current research shows that almost every large galaxy resides in the center of a supermassive black hole with a mass of millions to billions of times that of the sun. However, it is difficult to grow to such a "size" within the age of the universe simply by accreting surrounding gas. Therefore, mergers between black holes are considered to be one of the important ways to "fatten". It is not uncommon to observe collisions between galaxies, and it is also speculated that the black holes in the centers of these galaxies should gradually move closer under the influence of gravity and eventually merge. However, theoretical models are still imperfect in describing this "final stage", and a "close binary black hole" system has never been reliably confirmed by imaging before. This observation of Mrk 501 provides a key puzzle piece for this long-standing astrophysical picture.
The research team conducted a systematic analysis of observation data in the core region of Mrk 501 at different radio frequencies spanning about 23 years. The data came from a network of radio telescopes distributed around the world, forming extremely high angular resolution. The results showed that in addition to the known jet that pointed toward the Earth and was particularly bright, there was also a second jet hidden in the data. Its direction was obviously different from the first jet, and it showed significant position changes in just a few weeks. With the comparison of multi-epoch data, astronomers not only "saw" the second jet, but also tracked its orbit, which was interpreted as the projection effect of its orbit around another black hole.
Observation records show that the second jet seems to be emitted from behind the known black hole with a greater mass and orbits in a counterclockwise direction, showing periodic displacement changes in continuous observations, as if the entire jet system is "swaying." The research team explained this phenomenon as the swing of the orbital plane of the binary black hole system: the two black holes orbit each other, causing the angle between the direction of the jet and our line of sight to constantly change. In an observation in June 2022, the radiation from the jet system happened to arrive at the Earth via an extremely "skewed" path. Under the bending effect of the strong gravity of the known foreground black hole, the light from the rear jet was "pulled" into an approximately ring-shaped structure, the so-called "Einstein ring." This provides strong support for the explanation that "the foreground black hole acts as a gravitational lens, and the background jet comes from the second black hole."
By analyzing the periodicity of the jet's brightness changes and position evolution, the research team calculated that it takes about 121 days for the two black holes to orbit each other. The distance between them is estimated to be about 250 to 540 times the distance between the Earth and the Sun - this is still a huge scale for ordinary stars, but for supermassive black holes with masses between 100 million and 1 billion solar masses, this distance is already quite "tight". Based on the mass range and orbital parameters, this binary black hole system may eventually merge due to the loss of orbital energy due to gravitational radiation in as little as about 100 years. This time scale is considered "imminent" in the evolution of the universe.
It is worth mentioning that although the two black holes themselves are extremely huge, because Mrk 501 is extremely far away from the earth, even the Event Horizon Telescope (EHT), which has photographed the ring structure of the black hole's "event horizon", is currently unable to directly resolve them into two independent celestial bodies. As the orbital radius further shrinks, the "final rotation circle" of the binary black hole system will still be difficult to see directly in imaging, but scientists hope to capture its final steps through another "signal" - gravitational wave radiation in the extremely low frequency band. These signals are expected to be detected through the "Pulsar Timing Array" (PTA) observation method, which "hears" the large-scale gravitational wave background of the universe by precisely monitoring the periodic micro-perturbations of millisecond pulsars.
In fact, supermassive black hole binaries are already one of the main candidate sources to explain the "gravitational wave background" signal reported by teams such as the European Pulsar Timing Array in 2023. Mrk 501 has now become a very valuable "target laboratory", which is expected to directly correspond certain low-frequency gravitational wave signals measured by PTA with specific binary black hole systems, giving the previous statistical "background" a clear identity of celestial bodies. Research collaborators pointed out that if gravitational waves can be successfully captured in the direction of this source in the future, not only are they expected to see their frequency gradually increase over time, corresponding to the process of black holes spiraling closer, but they may also be able to obtain close to "real-time tracking" evolution records on the scale of "supermassive black hole mergers" for the first time.
In this study, Silke Britzen, Frédéric Jaron and Nicholas Roy McDonald from the Max Planck Institute for Radio Astronomy are signed as co-authors, and the relevant results will be published in the Monthly Notices of the Royal Astronomical Society. This pair of supermassive black hole binary stars at the center of Mrk 501 not only provides a key case for understanding how the black hole at the center of the Milky Way grows, but also opens up a rare "firing range" for future gravitational wave astronomy based on pulsar timing, allowing humans to witness a cosmic-scale "black hole merger" with our own eyes in the next few decades to hundreds of years.