• Scientists have discovered a mysterious pulsating light—and they don’t know what it could be.
• It pulses at a rate of about once every 21 minutes, and has been doing so since at least 1988.
• It doesn’t nicely fit the description of any of the pulsating space objects we know of, so researchers are still trying to figure out what this object might be.

Some things in the universe shine with a constant, life-sustaining light. Some beam with the light of other sources. Some flash in a large explosion, never to be seen again. And some pulse. Like a ticking—or, rather, flashing—clock, some energy sources in the universe hit us with beam after beam of photons, lighting up and darkening on a very set schedule.

There’s actually a fair few types of objects that do this. But scientists recently discovered one particular source of flashing light has already proven especially puzzling. It blinks about once every 21 minutes, and according to archival data, it’s been doing that at least since 1988.

It’s called GPM J1839–10, and we don’t know what it is.

Usually, if you spot a pulsing object, your first instinct would be to identify it as a pulsar. Pulsars are rapidly rotating neutron stars with strong magnetic fields that generate radio jets at the magnetic poles. If their poles aren’t pointed directly at Earth, we will only see those radio jets when they spin around to hit us, causing an incredibly consistent pulse.

But in addition to the, well, pulsing, the key to identifying a pulsar is the timing. Pulsars spin incredibly fast—some hit us with their radio beams once a millisecond, and the longest pulse out way about once a minute. And they kind of can’t go slower than that. The rapid spin that makes them “blink” also powers the radio jets of the star. If they slow down, the jets die off completely (researchers actually call it the “pulsar death line”). So, the chances of us spotting a pulsar with a 21-minute pulse interval is incredibly small.

There are other options, but those don’t fit great either. It could be a magnetar (another kind of neutron star that is prone to bursts of activity), but they usually produce X-rays along with their radio bursts, and the last one we found with a pulse interval like this stopped emitting after about three years. GPM J1839–10 doesn’t seem to be producing X-rays, and it’s been emitting for three and a half decades.

Or, it could be what’s called a magnetized white dwarf. It’s also a long shot, as we’ve never seen a white dwarf give off super-bright radio emission (let alone bursts) before. But because white dwarfs are so much bigger in mass than neutron stars, they would take longer to spin around, and could achieve a rotation rate like the one observed from GPM J1839–10.

Researchers aren’t fully ready to give up on any of these ideas yet, no matter how long-shot they may be, largely because they don’t have a nicely fitting alternative. Further investigations will likely be needed to determine what exactly this thing is, and the observations needed to examine this things aren’t exactly easy to get.

It may be a while until we solve the mystery of GPM J1839–10. Until we do, it’ll just keep pulsing away, taunting scientists with the promise of answers to big questions.