NSF VLA Contributes Crucial Puzzle Piece to ‘Peculiar’ High Energy Transient

Non-detection at radio wavelengths may prove to be the critical clue toward categorizing EP240408a as an entirely new phenomenon

High-energy transient signals are most often determined to be gamma-ray burst events, but the recently-launched Einstein Probe has expanded astronomers’ ability to quickly respond to similar signals occurring at X-ray wavelengths. Now, a multi-wavelength study of EP240408a concludes that while many of the signal’s characteristics might lead to the conclusion that it is a gamma-ray burst, the non-detection at radio wavelengths precludes that possibility. Instead, the international team of astronomers suggest that EP240408a is either a rare jetted tidal disruption event or, perhaps, an entirely new type of astronomical phenomenon. This was discovered in only the first two months of the commissioning phase. 

Tidal disruption events (TDEs) occur when a star is shredded by a nearby black hole; these events are themselves rare, with fewer than 100 discovered so far. In even more rare cases, the black hole’s powerful tidal forces propel some of the shredded stellar material outward in high-velocity jets, which then interact with nearby clouds of dust and gas and shine brightly in X-ray and radio. Thus far, only four TDEs are known to have relativistic-velocity jets associated with them.

An international team of astronomers led by Brendan O’Connor, an astronomer at Carnegie Mellon University, analyzed the signal from EP240408a across the span of wavelengths from radio to X-ray and concluded that this X-ray transient is—thus far—unique. “It ticks the boxes for a bunch of different kinds of phenomena, but it doesn’t tick all of the boxes for anything,” O’Connor summarizes. “And I think the radio non-detection is a massive box that we don’t know how to not tick.”

The team’s expansive follow-up campaign further characterized the X-ray emissions from EP240408a and identified a potential host galaxy in optical wavelengths. Crucially, however, O’Connor notes the non-detection in radio wavelengths as potentially the deciding factor in fully categorizing the source. Observations from the U.S. National Science Foundation Very Large Array (NSF VLA), operated by the U.S. National Science Foundation National Radio Astronomy Observatory (NSF NRAO), at 11 days,  158 days, and 258 days after EP240408a’s initial discovery indicated no radio emission from the source.  

“I think where radio really fits in is that when we see something this bright, for this long, in X-rays, it usually has an extremely luminous radio counterpart. And here we see nothing, which is extremely peculiar,” O’Connor says.

After methodically eliminating a number of potential explanations including active galactic nuclei, fast blue optical transients, fast X-ray transients, and other variations of previously-characterized phenomena, O’Connor and his co-authors conclude that EP240408a is extragalactic in origin and is most likely a relativistically-jetted Tidal Disruption Event.

“Because of this new wide field view of the X-ray universe, there’s a diverse range of phenomena we can see that weren’t possible before. And it looks like this transient, EP 240408a, is new. It’s something that we don’t think we’ve seen before,” O’Connor says. “It’s falling in a range of energy, of wavelengths, that it can be detected at, and the time scales are so short, that it’s probably something that we’ve just missed before now.”

O’Connor emphasizes that the current lack of radio emissions is pivotal, but that follow-up observations in radio wavelengths will hopefully yield future detections as the material within the jets slows down to energies corresponding to radio—a process expected to occur on timescales of roughly 1000 days. Thus, follow-up radio observations with the NRAO VLA will be imperative.

Thus, EP240408a appears to be giving astronomers an in-between glimpse of a high-energy transient’s signal after its initial X-ray outburst but before its relativistic-speed jet flares in radio.

“It seems to me that this is the most likely explanation for why we aren’t seeing radio emission. Hopefully, eventually, we will see a jet at radio wavelengths, either with the current setup of the VLA or the Next Generation VLA, and we can monitor it for years to come in order to learn even more about this explosion,” O’Connor muses

“These results highlight the importance of multiwavelength observations in fully understanding the astronomical object,” says Joe Pesce, NSF Program Director for the NRAO.  “The complete picture of what’s really happening requires a holistic study.”

 An international team of astronomers were involved in the study, including Dheeraj Pasham at MIT, Igor Andreoni at the University of North Carolina Chapel Hill, Jeremy Hare at the Catholic University of America, Paz Beniamini at the Open University of Israel, and Eleonora Troja at the University of Rome Tor Vergata, among others.Read the full scientific paper.

About the NRAO

The National Radio Astronomy Observatory (NRAO) is a facility of the U.S. National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

This news article was originally published on the NRAO website on January 27, 2025.