In 2005, a very strange event was observed. An unknown object, not detectable through visible light, released an intense flare of X-rays. It took about a minute for the flare to reach its full brightness, about 90 times brighter than its resting output and about a million times as bright as the Sun. The flare lasted for about an hour before petering out. Four years later, it flared up again.

X-ray flares are not unheard of, but this event defied classification. Astronomers normally look at the length of the flares as well as how often they occur to determine what kinds of processes produce them. These flares don’t match any known mechanism, making them mysterious indeed.

To find out more, a team of researchers decided to look over archival data from the Chandra and XMM-Newton space observatories. They wondered if similar phenomena are taking place anywhere else in the Universe. If so, it might provide clues about the nature of these strange flares. And the researchers weren’t disappointed. Their search, which included 70 nearby galaxies, turned up two more such flares.

Anomalous characteristics

Like the first flare, the other two reached their peak luminosity in under a minute and lasted for about an hour. One of the flares recurred as many as five times during the observations, and the researchers estimate the recurrences could be as often as every 1.8 days. When not flaring, the objects looked like normal black holes or neutron stars.

All the flares were almost certainly coming from their apparent host galaxies—meaning they’re not closer stars within our own Milky Way galaxy that happened to be situated in the foreground.

Instead, the flares are in globular clusters (though one might be a dwarf galaxy) on the outskirts of their galaxies. Globular clusters are blobs of stars that orbit outside the galaxy proper. This is problematic because the easiest phenomena to compare them to—long-duration gamma ray bursts (incredibly bright bursts of gamma rays, lasting more than two seconds) as well as high-energy supernovae—requires a population of young stars. And the stars in globular clusters are pretty old, making those explanations unlikely.

The source of these flares is also enigmatic because even when not flaring, they’re brighter than a neutron star can normally get. Neutron stars are the remains of a dead star that has been crushed down as far as it can go without turning into a black hole. Imagine a body with more than twice the mass of the Sun in a sphere the size of Manhattan.

Because neutron stars are so compact, they have intense gravity, which allows them to shine as they gobble up matter. This matter can heat up as it spirals in, even triggering runaway thermonuclear combustion in some cases. This process emits a lot of light. It also triggers repeating flares, but they last for only a few seconds and are about a hundred times less luminous, so they couldn’t be the same thing.

So what the heck are they?

This question doesn’t currently have an answer beyond “we’ll find out more with more research." But the viable explanations put forward by the researchers all involve black holes. One possibility is that the black holes are of intermediate mass, between roughly a hundred and a thousand Solar masses. Based on their luminosity, these two new flare sources would have 800 and 80 Solar masses, respectively. Black holes of these masses, consuming matter as fast as they possibly can, might produce the observed flares.

Another possibility is that the black holes are smaller but that their poles are aiming right at us. They might produce a tight, cone-shaped beam of X-rays from their magnetic poles, which gets pointed at us every few days as they rotate. When the poles are aimed our way, we’d perceive it as a flare. This doesn’t explain why the flares slowly decrease after flaring up, however.

It’s also possible that a black hole, or even a neutron star, sometimes consumes matter faster than its normal physical limits. This could happen if it had a companion star with a very wide orbit. In the closest part of the star’s orbit, this star would pass the black hole or neutron star and cause matter to fall in more quickly.

Future research might investigate the frequency of these bursts more closely to gain clues about the source of these strange new phenomena.

Nature, 2015. DOI: doi:10.1038/nature19822 (About DOIs)