Black Holes Tango in Distant Galaxy

A bizarre X-ray flare first spotted in 2010 could be a signal from two black holes that will ultimately unite into a single beast.

Supermassive black hole binary

An artist's impression shows the second, smaller black hole interrupting the flow of plasma feeding its larger companion.
ESA / C. Carreau

In a galaxy 2 billion light-years away, two black holes are locked in a whirling embrace. They are spiraling toward each other in a dance that will end only when they unite into a single supermassive black hole 2 million years from now.

At least that’s one possible explanation for a curious flare seen in a galaxy with the telephone number SDSS J120136.02+ 300305.5. And if that explanation is correct, then this system becomes one of the closest pairs of black holes known, separated by an average of only 120 times the distance between Earth and the Sun.

A Mysterious X-ray Flare

On June 10, 2010, the XMM-Newton space telescope spotted an X-ray outburst from an otherwise quiet galaxy. The X-rays, initially quite bright, immediately started fading away exactly as expected for a supermassive black hole dining on a sudden feast. From the light curve, astronomers gathered that a star had passed too close to the dark beast lurking in this galaxy’s core, suffering a tidal rip that sent its plasma flowing into the black hole’s maw.

But as the X-rays were dimming in that characteristic way, they all of a sudden turned off altogether — only to reappear 21 days later. The dimming resumed as if nothing had happened. Then a year after the initial outburst, XMM-Newton slewed to the source again for follow-up observations, and once again found nothing there.

In a paper published in the Astrophysical Journal, Fukun Liu (Peking University, China) and colleagues argue that the best explanation for this flare is not one black hole feeding on an unexpected treat, but two black holes engaged in a deadly dance. Back in 2009 Liu had proposed that the presence of a second black hole would pull on and interrupt the plasma stream flowing into the first black hole. Now he and his colleagues suggest that his model is the best way to explain the flare’s odd behavior.

Even with only 11 X-ray observations conducted over the period of a year, the team is able to calculate what the system looks like. Most likely, this duo is locked in an elliptical orbit with an average separation of 120 astronomical units. The primary, that is, the black hole feeding on the star, has the mass of 10 million Suns. Its dancing partner is about 10 times lighter. Assuming the partners lose energy by radiating gravitational waves, they will unite as one in 2 million years.

Mystery Still Unsolved

XMM-Newton Slew Survey

The mysterious flare was captured in XMM-Newton's Slew Survey. This image shows the slew tracks between 2001 and 2012.
ESA / A. Read

“This is truly a fascinating object, and the explanation suggested by Liu and colleagues is intriguing,” says Laura Blecha (University of Maryland). “That said, I do think it’s too early to call this a firm result.”

There is only one strong dip in the X-rays, Blecha points out, possibly two if you count the non-detection a year after the initial outburst. These dips could be explained by the passage of a cloud of gas, star cluster, or some other obscuring material. Though the authors calculate that such a transit would be exceedingly rare, Blecha says the uncertainties in those calculations are large.

Observing additional X-ray dips would solidify the case for a binary black hole. The source might still be detectable today, though barely so given the continued decline in X-rays. Indeed the authors are requesting follow-up observations, though predicting exactly when the X-rays should disappear again is difficult. “Meanwhile, we continue our search for new events, using the same technique with XMM-Newton,” says co-author Stefanie Komossa (Max Planck Institute for Radio Astronomy, Germany).

Another sticking point is how quickly the source faded in X-rays: by a factor of 50 over just a week. If that’s due to a second black hole interrupting the plasma flow, Blecha adds, it means that the black hole is eating so quickly, it’s swallowing almost before it can chew. That’s not impossible, but it’s certainly rare.

“In any case, it’s a very strange object,” Blecha says.

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