China's Einstein Probe detected a mysterious cosmic explosion — and scientists have no idea what caused it

China's Einstein Probe has spotted a cosmic explosion from a mysterious source that's unlike anything seen before.

The burst was made up of two X-ray flares, about 200 seconds apart, that likely came from the same object. Their behavior is most consistent with powerful cosmic explosions known as gamma ray bursts — except no gamma rays were detected, scientists reported June 13 in the Monthly Notices of the Royal Astronomical Society.

Launched into low Earth orbit in 2024 by the Chinese Academy of Sciences in collaboration with the European Space Agency, the Einstein Probe is designed to scan the sky for high-energy X-ray emission events. Those events are usually short-lived, but scientists can use the data to perform detailed follow-up studies with other instruments. The Einstein Probe orbits Earth every 96 minutes and can scan almost the entire night sky about every five hours.

On March 5, 2024, the probe captured one such event, called an X-ray transient, from a celestial object scientists named EP240305a. The first flare lasted about two minutes. Roughly 200 seconds later, the probe detected a second flare lasting just over 4 minutes.

In the new study, researchers directed several ground- and space-based telescopes toward the area to collect data at X-ray, infrared, optical and radio wavelengths over the weeks that followed. They found that, following the initial burst, the X-rays faded after a few days, while radio emissions slowly petered out over several weeks.

To predict what kind of object could emit these bursts, the team compared their data to the expected emissions from several types of X-ray transients. None matched the emission patterns from EP240305a.

For example, tidal disruption events, which occur when a supermassive black hole rips apart a passing star, emit light for months or years, while radio emissions from stellar flares fade after a few hours. And other types of X-ray bursts that occur on similar timescales to EP240305a emit no radio signals at all.

The most similar type of event that matches EP240305a’s behavior is a gamma ray burst (GRB), the team found. Gamma ray bursts can occur when massive stars die or collide. But without detecting any gamma rays directly, the team can’t yet say that this was the signal's origin.

"In the case of EP240305a, the current data do not allow us to firmly establish a GRB origin, and we therefore conservatively classify it as a gamma-ray-dark GRB-like transient or more broadly an extragalactic fast X-ray transient," the researchers wrote in the study.

If the signal was due to a gamma ray burst, the jet of gamma rays may have been pointed away from Earth, or it may have been surrounded by material that hid or reduced the gamma radiation emitted.

Collecting data on this and other unusual X-ray transients could help scientists figure out what causes them, the team wrote in the study.