Harvard ‘Black Hole’ Is a ‘Discovery of a Lifetime’

On October 12 2022, astronomer and Redditor u/Andromeda321 shared post to r/science, about novel behavior by a black hole:

That post contained a link to an October 12 2022 press release from the Center for Astrophysics | Harvard & Smithsonian (CfA). It had a headline identical to the Reddit post, and it began with an explanation of what astronomers had observed — the unexpected activity of a “mysteriously reanimated” black hole:

In October 2018, a small star was ripped to shreds when it wandered too close to a black hole in a galaxy located 665 million light years away from Earth. Though it may sound thrilling, the event did not come as a surprise to astronomers who occasionally witness these violent incidents while scanning the night sky.

But nearly three years after the massacre, the same black hole is lighting up the skies again — and it hasn’t swallowed anything new, scientists say.

“This caught us completely by surprise — no one has ever seen anything like this before,” says Yvette Cendes, a research associate at the Center for Astrophysics | Harvard & Smithsonian (CfA) and lead author of a new study analyzing the phenomenon.

The team concludes that the black hole is now ejecting material traveling at half of the speed of light, but are unsure why the outflow was delayed by several years. The results, described this week in the Astrophysical Journal, may help scientists better understand black holes’ feeding behavior, which Cendes likens to “burping” after a meal.

The team spotted the unusual outburst while revisiting tidal disruption events (TDEs) — when encroaching stars are spaghettified by black holes — that occurred over the last several years.

Radio data from the Very Large Array (VLA) in New Mexico showed that the black hole had mysteriously reanimated in June 2021. Cendes and the team rushed to examine the event more closely.

“Tidal disruption events” or “TDEs” were referenced in the press release. In 2015, NASA published a page about TDEs and explained:

Astronomers have observed material being blown away from a black hole after it tore a star apart. This event, known as a “tidal disruption,” is depicted in the artist’s illustration [on the site] … Gas often falls toward black holes by spiraling inward in a disk. But how this process starts has remained a mystery. In ASASSN-14li, astronomers were able to witness the formation of such a disk by looking at the X-ray light at different wavelengths (known as the “X-ray spectrum”) and tracking how that changed over time. The researchers determined that the observed X-rays come from material that is either very close to or is actually in the smallest possible stable orbit around the black hole.

Typically, popular Reddit news-related posts are submitted by readers, not individuals connected to the news. However, the first comment on the r/science thread was by the u/Andromeda321 (whose subreddit flair was “PhD | Radio Astronomy.”) That account, we were delighted to discover, belonged to Yvette Cendes, the lead author on a study of the observation, which Centes described as the “discovery of a lifetime”:

Astronomer here! I am the lead author on this paper, which is definitely the discovery of a lifetime! The TL;DR is we discovered a bunch of material spewing out of a black hole’s surroundings two years after it shredded a star, going as fast as half the speed of light! While we have seen two black holes that “turned on” in radio 100+ days after shredding a star, this is the first time we have the details, and no one expected this!

I wrote a more detailed summary here when the preprint first came out a few months ago, but feel free to AMA [ask me anything]. 🙂

Centes’ comment linked to her June 30 2022 post, where she described the findings and linked to the study’s preprint. She also answered questions in the comments, which included the following exchange:

[User one]: hi forgive my ignorance but does this mean that “even light can’t escape” isn’t true anymore?

[Centes]: No, that still stands. What we think happened is this material was in an accretion disc surrounding the black hole after it was unbound. In 20% of cases you then see a radio outflow at the part where it’s torn apart, but in this case we have really good radio limits that this didn’t happen then (ie, didn’t see anything). Then after ~750 days for whatever reason this outflow began …

[User two]: So like in a food proccessor there’s a ring of material always just outside the reach of the blades. That’s the material that slid free?

[Centes]: Not bad! Basically yea, this black hole had a tidal radius outside the event horizon and the star got shredded when it crossed that line. Took about a few hours.

Fun fact though, “always” is not accurate bc if a black hole exceeds ~100 million times the mass of the sun, the tidal radius is inside the event horizon. So the star just gets swallowed whole and you never see it.

Another commenter asked about the “the implications of the time delay [and whether it] correlated to anything like black hole mass or the disc.” Centes responded, indicating that theorists did not anticipate the observed black hole behavior:

We actually don’t know [because] theorists didn’t predict this- which is actually super exciting [because] it gives us a brand new laboratory to test extreme physics we didn’t have before! (Which made for a heck of a discussion section to write- we had to call in a theorist famous for the “we didn’t expect this” kind of discoveries.)

Right now though the strange thing is this was NOT an unusual TDE [tidal disruption events] in any way when it first was detected- average size, average brightness, everything. We really need to get my full sample out of these to try and find patterns on what’s going on, but that takes time …

Centes crossposted the press release to r/space, where another commenter asked Centes if the “ejected material” could have originated with another celestial body; Centes described how researchers determined that was not the case:

[User]: Is it definitively known that the ejected material is that of the shredded star? Could it be something else?

[Centes]:Good question! We know there wasn’t say a second star that got shredded or other large influx of material because the all sky survey would have spotted this. And while we can’t say for sure it’s from this it’s an astronomically super time scale to have no connection …

[User]: If the thought is that the material was in a super tight accretion disk right near the event horizon, wouldn’t we expect to see strong emissions from all that high energy material during those two years between consumption and ejection?

[Centes]: Short answer is there still is some optical, UV, and X-ray emission from that, but nowhere near as luminous as the original shredding of the star, or what we are seeing now in radio! You’d miss this totally if you had no radio observations. And we were lucky- we have a very good non detection of this source just nine months before its detection (plus several when the event happened), so we know it’s not like the radio was constantly there.

Finally, the press release submitted linked to the study, which was published by the Astrophysics Journal on October 11 2022. It was titled “A Mildly Relativistic Outflow Launched Two Years after Disruption in Tidal Disruption Event AT2018hyz,” and its abstract read in part:

We present late-time radio/millimeter (as well as optical/UV and X-ray) detections of tidal disruption event (TDE) AT2018hyz … This is the first definitive evidence for the production of a delayed mildly relativistic outflow in a TDE; a comparison to the recently published radio light curve of ASASSN-15oi suggests that the final rebrightening observed in that event (at a single frequency and time) may be due to a similar outflow with a comparable velocity and energy. Finally, we note that the energy and velocity of the delayed outflow in AT2018hyz are intermediate between those of past nonrelativistic TDEs (e.g., ASASSN-14li, AT2019dsg) and the relativistic TDE Sw J1644+57. We suggest that such delayed outflows may be common in TDEs.

A popular r/science (and r/space) submission on October 12 2022, “‘We’ve Never Seen Anything Like This Before:’ Black Hole Spews Out Material Years After Shredding Star,” linked to a press release about astronomers observing unexpected behavior in a previously observed black hole. The links were submitted by u/Andromeda321, the account of lead author Yvette Centes. Centes invited questions and provided information about the newly published study in the comments.