tidal disruption events
(NASA’s Goddard Space Flight Center/Chris Smith (USRA/GESTAR))

Tidal Disruption Events: When Suns Moving at Light Speed Around a Black Hole Collide

Imagine the sun moving close to the speed of light and colliding with another sun. This must be a spectacular sight to watch. But it could get even better, as these fireworks get amplified when the collision occurs near a supermassive black hole. As it turns out, nature orchestrates such events. What should we expect when we watch them with our best telescopes?

The center of most galaxies hosts a supermassive black hole surrounded by stars. For black holes more massive than a hundred million suns, like the 6.5-billion solar mass black hole in the giant elliptical galaxy M87,  the stars can get arbitrarily close to the event horizon without getting disrupted by the gravitational tide induced by the black hole.  In that neighborhood, they move near the speed of light.

On occasion, two suns could collide there while moving close to the speed of light. The collision itself would produce a weak signal that would be followed by a much brighter flare when the debris from the collision accretes onto the nearby black hole.

My student from the Harvard Physics Department, Betty Hu, was set to calculate the radiation emitted from a supermassive black hole at the center of a galaxy when two stars collide in its vicinity. She was getting a puzzling result, namely a bright emission peak followed by a dramatic dimming for about a year and a sudden rise in emission afterward. Our immediate reaction was that there must be a bug in her computer code. She checked it over and over again, and this feature persisted. At that point, I realized that there might be a simple explanation.

When two stars collide, the explosion ejects debris that expands in all directions. The material that ends up near a black hole and produces the radiation originates from two opposite sides of the star. One side releases material that goes straight in the direction of the black hole, and the other sends material that first goes out and then comes back straight into the black hole’s mouth. This results in the gap between the two emission episodes.

Betty went on to trace the material that feeds the black hole over time and confirmed the results. At that point, we realized that we discovered an interesting temporal feature that could distinguish the flares of star-star collision from the flares associated with the disruption of a single star as a result of the gravitational tide from the black hole, the so-called tidal disruption event. The Sun could be tidally disrupted by the black hole at the center of the Milky Way, SgrA* – which has a mass of 4 million suns, at a distance that is roughly the Earth-Sun separation. But if the Sun were to collide with another star at a larger distance, the flare that would result from the black hole would have the characteristic dip in the middle.

It was obvious to us that this dip is a “fingerprint” of a new class of events that should be common near supermassive black holes. A decade earlier, I wrote a paper with my Physics graduate student Doug Rubin which predicted that such collisions should occur every hundred thousand years in the Galactic center.

This is all theory, but do we see such events?

Every Thursday, I chair a luncheon hosted by Harvard’s Institute for Theory and Computation in my capacity as its director. When Betty confirmed our theoretical result, I was reminded that a couple of weeks earlier, a postdoctoral fellow, Yvette Cendes, described in a luncheon talk exciting observational data on a flaring black hole that showed a paradoxical rapid rise in emission after an early peak followed by a dimming episode. The plot that Yvette showed (the blue and green data point in Figure 2 of her paper) showed two events, AT2018hyz and ASASSN-15oi, where this feature is observed. In her luncheon presentation, Yvette noted that the events are associated with outflows at a fifth of the speed of light and that the dimming and sudden rise feature does not have a simple explanation in the context of tidal disruption events.  “Well, it looks like we just explained it without expecting it,” I told Betty. She smiled in agreement.

Over the past two years, we fitted data on these and other events and wrote an extended paper to be released soon. Remarkably, by fixing the collision distance based on the outflow speed, we obtained a dip in luminosity at the correct times, a sudden rise in emission after the dip, and the correct overall luminosity.

These circumstances illustrate why science is so exciting. First, Betty and I obtained a surprising theoretical result that we did not expect when we were set to do the calculation. Then, we realized in retrospect that the result made sense and should have been expected. Then came the data regarding real events that do not have a simple explanation but feature the same unique behavior. Finally, we were able to fit different aspects of the data with our model.

What Betty and I uncovered is a new class of transients in galactic nuclei, characterized by a dimming and rebrightening fingerprint that separates them from tidal disruption flares. There is a simple way to distinguish between the two populations of transients. Black holes more massive than a hundred million suns swallow a star whole because their event horizon is bigger than the distance at which tidal disruption occurs. However, these supermassive black holes would still exhibit collisions of stars near the speed of light in the vicinity of their event horizons.

Within a year, the Rubin Observatory in Chile will employ a 3.2-billion-pixel camera that will survey the southern sky every four days and document numerous astrophysical flares. Some of those transients may flag the collisions of stars in the vicinity of a supermassive black hole. Now that we know what the light curve of such events looks like, we can search for them.

The beauty of scientific discovery is that after resolving a puzzle, everything falls into place. Nature is simple. We just need to imagine it in order to make sense of what we observe.

Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s – Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011-2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021.