flashing lights

Astronomers see the light from a black hole collision for the first time

How do these dark objects erupt with light?

NASA’s Goddard Space Flight Center

As two black holes orbit close to one another, they will begin to spiral around each other and eventually merge into one larger cosmic beast. Black hole mergers are typically detected by signals of gravitational waves that act like ripples of space and time.

However, a recent merger between two black holes produced something rather unfamiliar to these massive, dark objectslight.

The merger event is detailed in a study published Thursday in the journal Physical Review Letters.

When two black holes merge together, they release an immense amount of energy that is given off as gravitational waves, and these mergers are the strongest source of gravitational waves in the whole universe.

Black holes orbit around one another, swirling closer together till they merge into one.


On May 21, 2019, the National Science Foundation's Laser Interferometer Gravitational-wave Observatory (LIGO) and the European Virgo detector captured the gravitational wave signal of the merger of two black holes.

At the same time, scientists at Caltech’s Zwicky Transient Facility (ZTF) spotted what appeared to be a flare of light sparking from a pair of merging black holes at the same region where the gravitational waves were detected.

The event was dubbed S190521g, and marks the first time light was observed from a black hole merger.

“This supermassive black hole was burbling along for years before this more abrupt flare,” Matthew Graham, a research professor of astronomy at Caltech and the project scientist for ZTF, said in a statement. “The flare occurred on the right timescale, and in the right location, to be coincident with the gravitational-wave event."

But how did the merger of these two light-deprived objects produce a flare? It seems unlikely since black holes are naturally dark beings, a space vacuum where not even light can escape from the pull of their gravity.

However, some astronomers have theorized about the possibility of a light-emitting black hole merger.

Black holes are surrounded by a disk of material, gas and stars. After a pair of black holes merge together into one larger black hole, that new big boy experiences a little kick that sends it off into a random direction and it plows through the gas in the surrounding disk, according to the researchers.

The reaction of the gas to that sudden, speeding black hole ramming through it is what creates a bright flare that is visible with telescopes.

The light flare is visible days after the actual merger event, and slowly faded over a period of a month after. By the time the researchers went back to observe the flare of light further, it was already gone.

However, they are hoping to catch another flare of light from this supermassive black hole in the next few years as the kick should cause it to smash into the disk of gas once again and possibly produce another light flare.

“Supermassive black holes like this one have flares all the time. They are not quiet objects, but the timing, size, and location of this flare was spectacular,” Mansi Kasliwal, an assistant professor of astronomy at Caltech, and co-author of the study, said in a statement. “The reason looking for flares like this is so important is that it helps enormously with astrophysics and cosmology questions. If we can do this again and detect light from the mergers of other black holes, then we can nail down the homes of these black holes and learn more about their origins.”

Abstract: We report the first plausible optical electromagnetic counterpart to a (candidate) binary black hole merger. Detected by the Zwicky Transient Facility, the electromagnetic flare is consistent with expectations for a kicked binary black hole merger in the accretion disk of an active galactic nucleus [B. McKernan, K. E. S. Ford, I. Bartos et al., Astrophys. J. Lett. 884, L50 (2019)] and is unlikely [<O(0.01%))] due to intrinsic variability of this source. The lack of color evolution implies that it is not a supernova and instead is strongly suggestive of a constant temperature shock. Other false-positive events, such as microlensing or a tidal disruption event, are ruled out or constrained to be <O(0.1%). If the flare is associated with S190521g, we find plausible values of total mass MBBH∼100  M⊙, kick velocity vk∼200  km s−1 at θ∼60° in a disk with aspect ratio H/a∼0.01 (i.e., disk height H at radius a) and gas density ρ∼10−10  g cm−3. The merger could have occurred at a disk migration trap (a∼700rg; rg≡GMSMBH/c2, where MSMBH is the mass of the active galactic nucleus supermassive black hole). The combination of parameters implies a significant spin for at least one of the black holes in S190521g. The timing of our spectroscopy prevents useful constraints on broad-line asymmetry due to an off-center flare. We predict a repeat flare in this source due to a reencountering with the disk in ∼1.6  yr(MSMBH/108  M⊙)(a/103rg)3/2.
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