A recent study has revealed that nearly half of black holes that consume stars during tidal disruption events (TDEs) later emit remnants of those stars, sometimes years after the initial event. TDEs occur when a star ventures too close to a black hole, where the black hole’s gravitational pull exerts intense tidal forces. This results in the star being stretched and compressed, a process known as spaghettification, which tears the star apart within hours. This destruction is marked by a burst of electromagnetic radiation visible as a bright flash.
As the star is consumed, part of its material is expelled, while the remaining material forms an accretion disk—a thin, rotating structure around the black hole. The accretion disk initially releases material in chaotic bursts, detectable through radio waves, but these emissions typically fade within a few months. Traditionally, astronomers only observed these radio emissions for a short period after the star’s destruction, missing any longer-term activity.
Surprising Delays in Emission from Black Holes
The new study, led by Yvette Cendes, a research associate at the Harvard and Smithsonian Center for Astrophysics, involved monitoring black holes for several years after TDEs. Published on Aug. 25 in the preprint database arXiv, the findings showed that in up to 50% of the cases, black holes expelled material years after consuming a star. In 10 of the 24 studied black holes, this delayed emission occurred between two and six years after the initial star-destroying event. These unexpected “burps” were observed as sudden bursts of radio waves, indicating that the black holes “turned on” again long after the initial event.
Cendes described this phenomenon as a delayed reaction, with material being expelled much later than previously thought. It remains unclear why this delay happens, but it appears that the ejection is influenced by conditions around the black hole rather than events occurring within the black hole itself. This observation challenges current models of TDEs, which typically assume that material outflows stabilize within weeks.
The study also recorded cases where radio waves from black holes peaked, faded, and then peaked again. This re-brightening phenomenon was unexpected, suggesting that black holes may have multiple phases of activity following a TDE. Cendes and her team plan to continue observing these black holes, particularly as some still show signs of increasing radio wave activity.
This research sheds new light on the behavior of black holes and suggests that existing models need to be revised to account for the prolonged activity seen in TDEs. It highlights the complexity of black holes, showing that even years after devouring a star, they can exhibit unpredictable behavior, reinforcing the idea that “black holes are messy eaters.”
these radio emissions come from outflows of material. Because this data set is such a beast, we can get information about the physical parameters of the system- these outflows are similar to a supernova shockwave in terms of speed and energy (except for the part where they start years later), and the densities are all similar to our own Milky Way’s so it’s not that the environment is particularly unusual.
So to continue, a TDE where a star is ripped apart is a hugely energetic event, where an entire star is torn apart in just a few hours, and a ton of mass suddenly falls onto the black hole. We only see outflows in ~20-30% of these cases within the first few months of the TDE. On the other hand, we see up to half of them turning “on” in radio waves years later- this doesn’t make sense! Imagine going years later to the site of an explosion and things are happening then that weren’t when it happened! This tells us we are misunderstanding something basic about how this process works, and maybe something about black holes more generally.
Is this something that can now be analyzed in past data to gain a larger data set? Going forward, what predictions does your findings make and what will you be looking for in the future?
Yep that’s exactly what we did! This is a relatively new phenomenon and this is incidentally also just the first systematic study of radio emission from these events (before they were just individual event papers). Which might not sound like a big deal to someone not in the field, but I am proud of!
Going forward is tough to predict, but my big curiosity lies in the ones we haven’t seen turn on yet. People always ask “how do you know they never do?” and I say “you don’t- it’s called job security!” For example, our oldest one happened in 2014, and turned on six years later. Most in our sample aren’t that old yet…
Is there any correlation with mass of the star or the black hole and the delay?
oh, okay, for a second there I was thinking it was implying that somehow matter was escaping from inside the event horizon, which I thought was impossible (it is), but what they’re saying is that after an extended period of time, some stellar remnants are being flung out of the accretion disc. So, physics is still physics
I’m super not a physicist or astronomer but I always wondered how the theory every action of force there has to be an opposite equal reaction.
To me that would include the energy of a star. If it all gets ate up by a black hole, then it’s gotta go somewhere, maybe out the holes butt?
Anyways I’m an idiot.
Too much gas
There’s a new field of study now available at colleges and universities across the country: astronomy gastronomy.
Burping up? Like, they come out the way they went in?
So a black hole is more like a wood chipper rather than a hydrogen bomb