Tick-Tock: The Imminent Merger of a Supermassive Black Hole Binary

Jan 27, 2020
by Ning Jiang, Huan Yang, Tinggui Wang, Jiazheng Zhu, Zhenwei Lyu, Liming Dou, Yibo Wang, Jianguo Wang, Zhen Pan, Hui Liu, Xinwen Shu, Zhenya Zheng

Supermassive black hole binaries (SMBHs) are a fascinating byproduct of galaxy mergers in the hierarchical universe. In the last stage of their orbital evolution, gravitational wave radiation drives the binary inspiral and produces the loudest siren awaiting to be detected by gravitational wave observatories. Periodically varying emission from active galactic nuclei has been proposed as a powerful approach to probe such systems, although none of the identified candidates are close to their final coalescence such that the observed periods stay constant in time. In this work, we report on the first system with rapid decaying periods revealed by its optical and X-ray light curves, which has decreased from about one year to one month in three years. Together with its optical hydrogen line spectroscopy, we propose that the system is an uneven mass-ratio, highly eccentric SMBH binary which will merge within three years, as predicted by the trajectory evolution model. If the interpretation is true, coordinated, multi-band electromagnetic campaign should be planned for this first binary SMBH merger event observed in human history, together with possible neutrino measurements. Gravitational wave memory from this event may also be detectable by Pulsar Timing Array with additional five-to-ten year observation.

SDSSJ143016.05+230344.4 (hereafter SDSSJ1430+2303) is known as a Seyfert 1 galaxy at redshift 0.08105[21], which shows typical AGN-like narrow emission line ratios yet somewhat unusual blueshifted broad Hα emission (see Extended Data Fig. 3) in its optical spectrum from Sloan Digital Sky Survey (SDSS). In particular, in the past three years the optical luminosity of SDSSJ1430+2303 has shown an unprecedented time-dependent variation. The g and r band light curves from Zwicky Transient Facitity (ZTF)[22] display an oscillation pattern since early 2019, which has completed at least 3 cycles up to August 2021, with a decreasing oscillation amplitude and period (see Figure 1). To our best knowledge, such a chirping AGN with simultaneously rapid damping amplitude and period has never been reported in the past. Unfortunately, the target is invisible in the subsequent months because it is too close to the solar direction. We have triggered the Neil Gehrels Swift X-ray telescope (XRT)[23] monitoring on this intriguing target since Nov. 24, 2021 (see details of Swift observations in Methods) and discovered a further shortened periodic variation in X-ray bands, whose period is approximately one month at the end of 2021.

The chirping flares are not compatible with known disk oscillation/instabilities, which have been tentatively used to explain other recurring AGN variabilities such as quasi-periodic eruptions ∼ 1 year to ∼ 1 month within only three years, also disfavour dissipation mechanisms such as dy- namical friction at accretion disk crossings and/or tidal gravitational heating of stars near pericenter passages as the main drivers for orbital evolution (see details in alternative model part in Methods). It appears the only plausible scenario is a secondary black hole orbits around the primary SMBH in an inclined, highly eccentric trajectory. The secondary black hole crosses the accretion disk shortly before and after the pericenter passages, where significant energy and angular momentum are radiated away through gravitational waves (see trajectory model part in Methods), and the in- duced shock waves at disk-crossings eject plasma balls to produce observed flares in the optical band. Note that the flare luminosity is on the same order of magnitude as the background disk luminosity, indicating that the mass ratio between these two black holes cannot be too extreme. The X-ray emission from hot corona around the SMBH(s) are likely affected by the pericenter passages, where the direct accretion onto black holes are mostly perturbed, but they may be also subject to variations in other circum-single disk conditions.

Figure 3: The evolution of the binary separation from trajectory model and predicted merger time. The separations (in unit of 100Rg of the primary SMBH) inferred from different scenarios (with or without X-ray peaks, 3.5PN or 4.5PN, see details in Methods) are shown in red dotted, blue dashed, magenta dot-dashed and black lines, respectively. The observed peak times suggested by light curves are denoted with grey shadow regions. The possibility distribution function (PDF) of merger time (4.5PN) predicted from optical plus X-ray peaks is shown with orange histogram while that without X-ray peaks is shown in green histogram.

The expected time till merger is approximately 100-300 days considering both optical and X-ray light curves, and within three years considering only the optical data (see Figure 3). Upon merger, the gravitational wave frequency is around the low-end of the frequency band of Laser Interferometer Space Antenna (LISA)[29], which is launching in 2030s, and well above the detec- tion band of Pulsar Timing Array (PTA). The gravitational wave memory effect, however, may be detectable by PTA with additional five-ten years observation post merger. Although the black hole spins are unknown, because of the uneven mass ratio, the expected kick velocity is likely below 103 km/s.

(Highlights from the original paper)

See: https://arxiv.org/pdf/2201.11633.pdf

Crash of the titans: imminent merger of giant black holes predicted
Never-before-seen event could spark cosmic fireworks, but many fear signal will evaporate

In the center of a galaxy 1.2 billion light-years from Earth, astronomers say they have seen signs that two giant black holes, with a combined mass of hundreds of millions of Suns, are gearing up for a cataclysmic merger as soon as 100 days from now. The event, if it happens, would be momentous for astronomy, offering a glimpse of a long-predicted, but never witnessed mechanism for black hole growth. It might also unleash an explosion of light across the electromagnetic spectrum, as well as a surge of gravitational waves and ghostly particles called neutrinos that could reveal intimate details of the collision.

As soon as the paper appeared last week on the preprint server arXiv, other astronomers, eager to confirm the tantalizing signals, rushed to secure telescope observing time, says team member Huan Yang of the Perimeter Institute in Waterloo, Canada. “We’ve seen people acting pretty fast,” he says. Emma Kun of Konkoly Observatory in Budapest, Hungary, began to scour archives of radio observations for confirmation of the signal. “If the boom happens, it will confirm many things,” she says.

But the prediction may be a mirage. It’s not clear that the observed galaxy holds a pair of black holes, let alone a pair that’s about to merge, says Scott Ransom of the National Radio Astronomy Observatory, who finds the presented evidence “pretty circumstantial.”

Supermassive black holes are thought to lurk at the heart of most, if not all, galaxies, but theorists don’t know how they grow so big. Some sporadically suck in surrounding material, fiercely heating it and causing the galaxy to shine brightly as a so-called active galactic nucleus (AGN). But the trickle of material may not be enough to account for the black holes’ bulk. They could gain weight more quickly through mergers: After galaxies collide, their central black holes become gravitationally bound and they gradually spiral together.

Such black hole pairs are not easy to detect. X-ray telescopes have discovered a handful of AGNs with two bright, separated central sources, but the putative black holes are hundreds of light-years apart and wouldn’t collide for billions of years. Once they get closer, it’s almost impossible to separate their light with a telescope. But some AGNs regularly dim and brighten, which astronomers have recently argued is a sign they harbor pairs of black holes orbiting each other that regularly churn and heat the surrounding material. Some of these periodic oscillations have faded, however, calling into question the binary interpretation. “AGNs do all sorts of crazy things we don’t understand,” Ransom says.

In data from a survey telescope in California called the Zwicky Transient Facility (ZTF), a team led by Ning Jiang of the University of Science and Technology of China stumbled on a periodic AGN called SDSSJ1430+2303. “My first instinct was it must be related to a pair of supermassive black holes,” Jiang says.

Then, the researchers found something more: a trend they interpret as a binary pair closing in on a merger. The cycles were getting shorter, going from 1 year to 1 month in the space of 3 years. It is “the first official report of decaying periods which reduced over time,” says Youjun Lu, a theoretical astrophysicist at the National Astronomical Observatories of China, who was not part of the team.

The researchers confirmed the monthlong oscillation in x-ray observations from NASA’s orbiting Neil Gehrels Swift Observatory. If this decreasing trend continues, the black holes, which Jiang says come as close to each other as the Sun is to Pluto, will merge in the next 100 to 300 days, they report in the paper, which has not been peer reviewed.

If the merger comes to pass, observers could have a field day. “There should be a huge burst across the electromagnetic spectrum, from gamma rays to radio,” Kun says. Some also expect a flood of neutrinos, which the IceCube detector at the South Pole—1 cubic kilometer of polar ice outfitted with light sensors to detect neutrino impacts—could pick up. Neither, however, is certain. Some predict a whimper rather than a bang. “We really don’t know what to expect,” Ransom says.

The only certain signal is gravitational waves, but the ponderous colliding masses would emit them at too low a frequency to be picked up by detectors such as the Laser Interferometer Gravitational-Wave Observatory, which is tuned to smaller mergers. They should, however, leave an imprint on spacetime itself, a sort of relaxation of distance and time dubbed gravitational wave memory, which could be detected over many years by monitoring the metronomic pulses of spinning stellar remnants known as pulsars. “It’s a very tricky signal to measure,” Ransom says, “but that would be definitive, a total smoking gun” of merging supermassive black holes.

But Ransom is prepared for disappointment. He and others point out the team is basing its prediction on just a handful of observed cycles. Theorist Daniel D’Orazio of the Niels Bohr Institute in Copenhagen, Denmark, says some aspects of the AGN’s light curve also raise doubts. For example, he says, the ZTF archives show SDSSJ1430+2303 lacked a periodic oscillation in the years before Jiang’s team discovered it; its dim, steady emission then looked more like a standard AGN with a single supermassive black hole. “Why has [the oscillation] just turned on now?” D’Orazio asks. “I’m not sure how that steady emission fits with binary emission models.”

Observations in the coming months should show whether the oscillation continues to shorten. The team had to halt its observing in August 2021 when Earth’s orbit put the distant galaxy too close to the Sun for telescopes to observe it safely. Observations restarted in November, but since then technical glitches have idled both ZTF and Swift.

Andrew Fabian of the University of Cambridge is among the astronomers who will be chasing the will o’ the wisp, having applied for time on NASA’s Neutron star Interior Composition Explorer, an x-ray telescope attached to the International Space Station. “If this is true, then it’s important to get as many observations as possible now to see what it’s doing,” he says. Fabian says the chance of such a merger taking place so close to Earth in any given year is one in 10,000. He’s skeptical that one is imminent, but says it’s worth monitoring for a few months to see whether the claim holds up. “Rare events do happen,” he says.

With additional reporting by Ling Xin.

Daniel Clery is Science’s senior correspondent in the United Kingdom, covering astronomy, physics, and energy stories as well as European policy.

See: https://www.science.org/content/article/crash-titans-imminent-merger-giant-black-holes-predicted?utm_source=sfmc&utm_medium=email&utm_campaign=DailyLatestNews&utm_content=alert&et_rid=255259432&et_cid=4095802

To detect this increasingly fast black hole cha-cha, physicists measure the tiny shockwaves this highly dynamic merger produces. As massive objects like black holes merge, they warp space and time, which is predicted by Albert Einstein’s general theory of relativity, creating ripples in the fabric of the Universe that shoot outward at the speed of light from the event. These gravitational waves are huge when they’re first produced, but by the time they reach our modest backwater planet in our unassuming galaxy, the Milky Way, they are incredibly faint and extremely difficult to detect and measure.

In a gravitational wave observatory, the distance between two suspended mirrors is measured with a laser. The measurement technique relies on the overlap of reflected laser light within the experiment. Two light waves are arranged so that the signals cancel each other out exactly. Changing the distance between the mirrors by even a tiny fraction of a wavelength produces a measurable light signal.

The basic idea behind the theory of relativity is that space itself possesses a kind of elastic structure, even in the absence of any matter. Similar to an inflated balloon, you can squeeze it one way and it expands in the perpendicular direction.

Relativity predicts that matter warps space (and time) and a collision between two compact objects like two black holes rapidly changes the compression and relaxation of the space in the vicinity of the objects. Waves of periodic compression and expansion are emitted. The way to measure these waves is to monitor the distance between two otherwise fixed objects, because the gravitational wave will periodically change the extent of the space between these objects, as it passes.

Scientists have become pretty adept at detecting these tiny gravitational waves thanks to observatories in the US and Italy. Known as LIGO and Virgo, the observatories are specifically designed to detect these infinitesimal waves from cataclysmic mergers — by measuring how the ripples affect suspended mirrors here on Earth. Ever since LIGO made the first detection of gravitational waves in 2015, the observatories have made impressive gains, finding almost 70 mergers of black holes, neutron stars, and black holes merging with neutron stars.