Black Hole Binary En Route to Merger?

Astronomers poring through two decades of archival and survey observations have discovered what looks like a pair of supermassive black holes closing in for a merger.

When galaxies merge, astronomers expect the supermassive black holes lurking in the galaxies’ cores to form their own dancing duos. Theorists predict that these black holes inspiral and coalesce, and such mergers undoubtedly play a role in building up the beefiest black holes (we’re talking billions of solar masses).

black hole binary artist's impression

An artist's conception of a black hole binary in a heart of a quasar, with hot accreting gas shown in orange and the data that reveal the periodic variability superposed (sinusoidal curve betwixt the black holes).
Santiago Lombeyda / Caltech Center for Data-Driven Discovery

Yet despite their theoretical abundance, black hole binaries have proven difficult to find. Astronomers have found plenty of circumstantial evidence for black hole pairs in galactic centers, but often the evidence can be interpreted in many ways, or the pair is too far apart to conclusively qualify as a merger-bound binary.

The best candidate so far has been OJ 287, which puts on a pair of optical outbursts every 12 years. These flares stand out even against the violent variability typical of blazars, quasars that point their jets right at us as OJ 287 does. OJ 287 shows up in photographic plates as far back as 1887, as seen in the Harvard College Observatory’s archive, Jonathan Grindlay (Harvard-Smithsonian Center for Astrophysics) reported on January 7th at the winter American Astronomical Society meeting. Astronomers have speculated that the double-peaked outbursts might come from a supermassive black hole punching through the accretion disk around a second black hole of comparable size.

Now an even better candidate might join the very short list of known black hole binaries. Matthew Graham (Caltech) and colleagues announced their discovery of the closest-hugging black hole binary candidate yet on January 7th in Nature and at the AAS meeting.

Black Hole Binary in a Haystack

The source, PG 1302–102, is a “vanilla” quasar, lacking the ferocious Earth-pointing jet that can block the view of black hole dynamics. Oddly enough, it pulsates with a period of about 5 years. The team found the source within the data pouring out of the Catalina Real-time Transient Survey (CRTS), which uses three ground-based telescopes to monitor about 80% of the night sky.

PG 1302–102 is actually one of 20 periodic sources that the team found essentially on a whim. They were aiming to study quasar variability: quasars are notoriously variable at all wavelengths, but randomly so — usually, there’s no regularity to the peaks and dips in brightness. Yet when the team ran algorithms to check a sample of 247,000 quasars for regular pulsations in brightness, these 20 popped up.

Statistically, the team would only have seen 1 periodic source if it were a fluke, so seeing 20 gives them confidence that the periodic phenomenon is real. The discovery rate (20 out of 247,000) is also close to that expected by theorists for binary black holes separated by less than a tenth of a light-year, as this one seems to be.

PG 1302–102 is the “best-looking” of the bunch, which is why the astronomers are presenting it first. But study coauthor S. George Djorgovski (Caltech) says they’re working on the rest. “Studying the full sample would give us more confidence than any individual case.”

Astronomers still don’t have a good handle on what happens in the final few light-years of a black hole merger (a conundrum called the “final parsec problem”), but PG 1302–102’s (purported) black holes will likely merge in a few hundred thousand to a couple million years. The elliptical galaxy they sit in has also long shown signs of being the product of a merger; it’s hard to say exactly how long ago it formed, but probably a few hundred million years ago.

The team thinks most of the 20 sources it found are black hole duos en route to merging. Given the span of time CRTS has observed, the other 19 should also have periods on the order of a few years and will likely have similar separations. The exact distances between the leviathans depend on what the black hole masses are. The team can’t separate those out, but they can estimate a combined mass for PG 1302–102’s pair of a few hundred million solar masses.

Black hole binaries are a potential source of gravitational waves, ripples in spacetime creating by accelerating masses. Astronomers have not yet directly detected gravitational waves, although they’ve seen indirect evidence of them. But PG 1302–102’s gravitational waves would have too low a frequency for ground- or space-based instruments to detect them. And even studies using pulsar timing arrays — which look for spacetime ripples by watching for hiccups in the neutron stars’ lighthouse-like pulses — aren’t currently sensitive enough. Although the gravitational wave frequency would be on the order of 10 nanohertz, which is where PTAs are most sensitive, PG 1302–102 is too far away and its binary isn’t massive enough for detection as an individual source, explains researcher Alberto Sesana (Max Planck Institute for Gravitational Physics, Germany). The system would need to be at least 10 times more massive for next-gen PTA studies — possible, given the error bars, but a stretch. Instead, this source is likely one of several thousand that contribute to the expected “gravitational wave background” (like the cosmic microwave background, but in gravitational radiation).

Even watching for the period to shorten with time as the black holes inspiral is beyond human timescales, says Scott Hughes (MIT). Assuming for simplicity’s sake that both black holes are the same mass, it would take 18,500 years for the period to shrink by 100 days. “A bit long for a PhD thesis, I’m afraid!” he says.

You can read more about the science behind the discovery in Caltech’s press release.

Web editor Monica Young contributed to the reporting of this news blog.

 

Reference: M. J. Graham et al. “A possible close supermassive black-hole binary in a quasar with optical periodicity.” Nature. Published online January 7, 2015.

 

8 thoughts on “Black Hole Binary En Route to Merger?

  1. Jim-BaughmanJim-Baughman

    A problem presents itself with the merger of these two, or any two, black holes.

    According to the theory of relativity, the closer these holes approached each other the slower time in their vicinity would move from our perspective, so even if we found a pair calculated to merge within the next five years or so we would never see them merge. They would simply get closer and closer to each other; reducing their separation by half, then by half again, then again, ad infinitum.

    So the question arises–how is it that some GRBs have been postulated as coming from the merger of two back holes, since from our vantage this phenomenon cannot possibly ever be completed?

  2. Jim-BaughmanJim-Baughman

    A problem presents itself with the merger of these two, or any two, black holes.

    According to the theory of relativity, the closer these holes approached each other the slower time in their vicinity would move from our perspective, so even if we found a pair calculated to merge within the next five years or so we would never see them merge. They would simply get closer and closer to each other; reducing their separation by half, then by half again, then again, ad infinitum.

    So the question arises–how is it that some GRBs have been postulated as coming from the merger of two back holes, since from our vantage this phenomenon cannot possibly ever be completed?

    1. Peter WilsonPeter Wilson

      What happens is their separation keeps reducing until it equals the Schwarzchild radius of their combined mass. At that point, the event horizon has enveloped the pair, and no more information is available, except the total mass and angular momentum of the system. I.e, for all practical purposes, the two have merged.

      1. Jim-BaughmanJim-Baughman

        Thanks for the reply, but you are describing events from the point of view of the black holes. From our perspective (per relativity) it is impossible for the two event horizons to touch (let alone envelop the pair of black holes) because time is stopped dead at the event horizon of each hole. (That’s why the word “event” is in the term–no more “events” past that point because there is no time for them to unfold in.) Regardless of Schwarzchild radius or any other parameter, for anyone watching from earth the two event horizons can only approach each other ever more slowly but never meet, even after quadrillions of earth-years.

  3. Peter WilsonPeter Wilson

    Jim: The idea of BHs merging is correct; your idea of time “stopped dead” is incorrect.
    Your incorrect idea is probably based on the popular “simplification” of the situation, so grossly simplified it is wrong. Imagine dropping a clock into a BH, and watching it through a telescope from a safe distance. The popular accounting goes like this:
    As the clock nears the event horizon, you will see its second-hand slow down. Eventually, the clock will come to a complete stop, hovering just above the event horizon. It will hang there, frozen in time, never falling in, because at the event horizon…time stops!
    Reality: The clock will go into orbit around the BH. Tidal forces will rip it apart into subatomic particles, smearing it into a mini accretion disk. This will heat up, radiating first infrared, then visible light, then UV, x-rays and gamma rays. At that point, about 95% of the subatomic particles from our former clock will be shot out the BH’s twin jets, a la the iconic Cygnus A. Then the radiation will fade in reverse order: x-rays, UV, visible, IR, etc, to the point of invisibility.
    In other words, we will never see time “stop.” The last we will see of our clock is a fade-to-black as its subatomic remains race around the BH at nearly the speed of light.

  4. Jim-BaughmanJim-Baughman

    I just can’t get you to grasp what relativity is…don’t mean to be insulting, but time at an event horizon operates in different ways depending on one’s status as an observer. I’ll quote from Wikipedia, since the situation is laid out clearly and simply in short sentences, and happens to outline things correctly. Please note the highlighted parts:

    “An event horizon is most commonly associated with black holes. Light emitted from beyond the event horizon can never reach the outside observer. Likewise, any object approaching the horizon from the observer’s side appears to slow down and never quite pass through the horizon, with its image becoming more and more redshifted as time elapses.”

    Please let me repeat: [A]ny object approaching the horizon from the observer’s side appears to slow down and never quite pass through the horizon.

    This holds true whether it is a clock, a bobby pin, a kangaroo rat or another black hole that is approaching an event horizon. Whether you are prepared to grasp this or not, a conundrum exists. Surely you would not argue that a black hole merger could be prevented by people watching two approaching holes through a telescope, but that it would be completed as soon as they took their eyes off the encounter?

  5. Peter WilsonPeter Wilson

    Well Jim, you leave me no choice but to edit the Wikipedia page…

    Seriously, think about that quote. “…its image becoming more and more redshifted as time elapses.” Compare to mine: “Then the radiation will fade in reverse order: x-rays, UV, visible, IR, etc, to the point of invisibility.” Wiki and I agree on that much. It will not appear to slow down. It will appear to go faster and faster until it redshifts into blackness. And when the blackness of the object approaching the horizon becomes becomes indistinguishable from the blackness of the black hole…you could say the two have merged.

    On the other hand, you could argue that they really haven’t… 😉

    1. Jim-BaughmanJim-Baughman

      You are the kind of person whose life work seems to be muddling established science in favor of your own kooky misinformation. I will attempt to exert pressure to keep you from damaging Wikipedia entries on science in the future, since your tossing around of arcane (albeit irrelevant) terms in this discussion shows you obviously don’t know what you are talking about, and can only wreak havoc for the sake of seeming to win an argument you lost at the outset due to your ignorance of simple physics.

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