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A strange cosmic explosion has astronomers baffled. It's either a very rare case of stars aligning just right (literally), or something powerful that's never been seen before. The event is designated EP240408a because it was first detected by the Einstein Probe, an X-ray space telescope, on April 8, 2024. At first glance, it seemed like a regular gamma-ray burst, which also typically emits bright X-rays.
But when a stellar array of telescopes observed it across a range of wavelengths, including ultraviolet, optical, near-infrared, radio, X-ray and gamma rays, they found that it didn't quite fit 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, fortunately, is headed straight for Earth.
"EP240408a fits some of the requirements for several different types of phenomena, but it doesn't fit all of them," says Brendan O'Connor, an astronomer at Carnegie Mellon University and lead author of the study. "In particular, the short duration and high brightness are difficult to explain by other scenarios. The alternative is that we are seeing something completely new!"
The universe is ablaze with fleeting events—flashes of energy from the explosions of stars and black holes, the explosions of stars as supernovae, the ingestion of stars by black holes, and other kinds of cosmic drama. Astronomers can determine what each event is by its duration, frequency, source, and the specific combination of wavelengths it emits.
Since its discovery by the Einstein probe, EP240408a has been observed by a group of other ground-based and space-based telescopes, including the Nuclear Spectroscopic Telescope Array (NuSTAR), Swift, Gemini, Keck, the Dark Energy Camera (DECam), Very Large Array (VLA), Australia Telescope Compact Array (ATCA), and the Neutron star Interior Composition Explorer (NICER).
Armed with this data, astronomers pieced together the properties of the event, but it only deepened the mystery. EP240408a flared in soft X-rays for the first 10 seconds, plateaued at a steady glow for about four days, and then quickly faded away over the next day. That's much longer than most gamma-ray bursts, which last up to a few hours, but not long enough to fit into other known categories.
Its X-ray brightness was in a similar range to the reverse Goldilocks: too bright for some events and not bright enough for others. Most surprisingly, the VLA detected no signs of radio emission from the source when it was checked 11, 158, and 258 days after the initial flare.
"When we see something that bright in X-rays for that long, it usually has an extremely bright radio equivalent," O'Connor says. "And here we don't see anything, which is very strange."
After ruling out several possible explanations, such as quasars or mysterious fast blue optical transients, astronomers have come up with the most likely cause: a tidal disruption event (TDE). These are flashes of light thrown off when black holes randomly devour stars. On rare occasions, TDEs produce huge jets of material that shoot out from the poles of the black hole. They can accidentally point directly at Earth, creating a visible signature. The characteristics of the signal suggest that it was an intermediate-mass black hole eating a white dwarf star.
The thing is, there should still be radio emission from a TDE jet. The team's hypothesis for why nothing has been found so far is that the event was recorded too early—previous studies suggest it can take hundreds or even thousands of days for the jet material to slow down enough to start emitting radio signals.
If future observations do indeed detect radio emissions, that could close the case for EP240408a. But if it remains silent, it could mean it's a particularly strange gamma-ray burst, or perhaps a completely new type of transient. The study is published in The Astrophysical Journal Letters.
