A bizarre cosmic explosion has puzzled astronomers. It’s either a very rare case of the stars aligning just right (literally) – or something powerful never seen before.
The event is designated EP240408a, as it was first detected by the Einstein Probe, an X-ray space telescope, on 8 April 2024. At a glance, it appeared to be a run-of-the-mill gamma ray burst, which typically emits bright X-rays too.
But when an all-star cast of telescopes observed it in a range of wavelengths, including ultraviolet, optical, near-infrared, radio, X-rays, and gamma rays, they found that it didn’t quite match any particular known type of event.
The current leading explanation, according to a new study, is that it’s the death throes of a white dwarf being torn apart by a medium-sized black hole. This created a high-speed jet of material that, as luck would have it, is pointing directly at Earth.
“EP240408a ticks some of the boxes for several different kinds of phenomena, but it doesn’t tick all the boxes for anything,” says Brendan O’Connor, astronomer at Carnegie Mellon University and lead author of the study.
“In particular, the short duration and high luminosity are hard to explain in other scenarios. The alternative is that we are seeing something entirely new!”
The Universe is ablaze with transient events – energetic flashes caused by outbursts from stars and black holes, stars exploding as supernovae, stars being devoured by black holes, and all kinds of other cosmic drama. Astronomers can figure out what each event is by its duration, frequency, source, and the specific combination of wavelengths it emits.
After its discovery by the Einstein Probe, EP240408a was observed by a squad of other ground- and space-based telescopes, including the Nuclear Spectroscopic Telescope Array (NuSTAR), Swift, Gemini, Keck, the Dark Energy Camera (DECam), the Very Large Array (VLA), the Australia Telescope Compact Array (ATCA), and the Neutron star Interior Composition Explorer (NICER).
Armed with this data, astronomers pieced together the event’s properties – but that only deepened the mystery. EP240408a flared up in soft X-rays for the first 10 seconds, plateaued at a steady glow for about four days, then faded quickly within another day. That’s much longer than most gamma-ray bursts, which last up to several hours, but not long enough to fit into other known categories.
Its brightness in X-rays was in a similar reverse-Goldilocks zone: too bright for some phenomena and not bright enough for others. Weirdest of all, the VLA saw no sign of radio emission from the source when it checked 11 days, 158 days, and 258 days after the initial flare-up.
“When we see something this bright for this long in X-rays, it usually has an extremely luminous radio counterpart,” says O’Connor. “And here we see nothing, which is very peculiar.”
After ruling out several possible explanations, such as quasars or the mysterious fast blue optical transients, the astronomers put forward the most likely culprit: a tidal disruption event (TDE). These are flashes of light thrown off when black holes messily gobble up stars.
In rare cases, TDEs produce huge jets of material that blast off from the black hole’s poles. These can, by chance, point straight towards Earth, which produces the signature seen. The characteristics of the signal suggest that specifically, it was an intermediate-mass black hole chowing down on a white dwarf star.
The thing is, there should still be some radio emissions from a jetted TDE. The team’s hypothesis for why none have been found so far is that the event was caught too early – previous research suggests that it can take hundreds or even thousands of days for jet material to slow down enough to begin beaming radio signals.
If future observations do detect radio emissions, this could close the case on EP240408a. But if it stays silent, it could mean it’s a particularly weird gamma-ray burst – or perhaps a brand new type of transient.
The research was published in The Astrophysical Journal Letters.
