Radio emissions from the depths of the cosmos tailed off, leaving researchers scrambling for answers
Scientists have observed two blasts in space unlike anything ever seen before.
Two bursts were spotted by China's Einstein Probe, each made of two X-ray flares, each about 200 seconds apart, which likely came from the same object.
Researchers then quickly pointed several telescopes to gather more data - and analysis of the multiwavelength data is presented.
This behaviour is consistent with gamma rays - except no gamma radiation was detected.
The Einstein Probe, launched in 2024, scans space for high-energy X-ray blasts.
While usually short-lived, scientists can examine these events with more-detailed follow-up studies.
The probe completes an orbit every 96 minutes and scans the night sky about every five hours.
Now a study, published in the Monthly Notices of the Royal Astronomical Society, has found the X-rays to fade after a few days, while radio emissions tailed off after a number of weeks.
What remains a mystery is the identity of the celestial object -named EP240305a - which caused the X-rays.
Possible sources included tidal disruption events, which occur when a supermassive black hole rips apart a passing star - but these events emit light for months or years.
Radio emissions from stellar flares, meanwhile, have radio emissions fade after a few hours.
Also considered was a thermonuclear blast on a neutron's star surface.
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And other types of X-ray bursts which occur on similar timescales to EP240305a emit no radio signal at all.
The most similar celestial event is a gamma ray burst (GRB) - but without the detection of gamma rays, the event is a mystery.
Scientists have described the event as a "gamma-ray-dark" GRB-like event - which means it has GRB-like behaviour but failed to show any gamma ray flash.
The researchers wrote in the study: "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."
If the signal was caused by a gamma ray burst, the radiation jet may have been propelled away from Earth.
Gamma radiation may also be obscured with a "dirty fireball", loaded with extra material which dampens its gamma-ray output.
Other possibilities for the lack of gamma rays include the potential it was surrounded by material which hid or reduced the gamma radiation emitted.
Further study on other unusual X-ray transients could be needed to determine what causes them, scientists said.