In 2005, a very strange event was observed. An unknown object, undetectable by visible light, gave off an intense burst of X-rays. It took about a minute for the solar flare to reach its full brightness, about 90 times brighter than its resting output and about a million times brighter than the Sun. The flare lasted for about an hour before extinguishing. 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 and how often they occur to determine what processes are causing them. These flares don’t correspond to any known mechanism, which makes them mysterious indeed.
To find out more, a team of researchers decided to look at archival data from the Chandra and XMM-Newton space observatories. They wondered if similar phenomena are happening elsewhere in the universe. If so, it could provide clues to the nature of these strange flares. And the researchers were not disappointed. Their search, which included 70 nearby galaxies, turned up two more such flares.
Like the first flare, the other two reached their peak brightness in less than a minute and lasted about an hour. One of the flares recurred as many as five times during the observations, and the researchers estimated that the recurrences could be as many as every 1.8 days. When not flaring, the objects looked like normal black holes or neutron stars.
All of the flares almost certainly came from their apparent host galaxies – meaning they are not nearby stars within our own Milky Way galaxy that happened to be in the foreground.
Instead, the flares are in globular clusters (although one of them could be a dwarf galaxy) on the edge of their galaxies. Globular clusters are blobs of stars orbiting 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) and high-energy supernovae — require a population of young stars. And the stars in globular clusters are quite old, making those explanations unlikely.
The source of these flares is also puzzling, because even when they don’t flare, they are brighter than a neutron star can normally get. Neutron stars are the remnants of a dead star that has been crushed as far as possible 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 inward, even triggering a runaway thermonuclear burn in some cases. This process emits a lot of light. It also causes repeated flares, but they only last a few seconds and are about a hundred times less luminous, so they can’t be the same.
So what the hell are they?
This question currently has no answer other than “we will find out more with more research.” But the viable explanations put forward by the researchers all involve black holes. one hundred thousand solar masses. Based on their brightness, these two new flare sources would have 800 and 80 solar masses, respectively. Black holes of these masses, eating up matter as fast as possible, could produce the observed flares.
Another possibility is that the black holes are smaller, but their poles point directly at us. They can produce a dense, cone-shaped beam of X-rays from their magnetic poles, which are focused on us every few days as they orbit. If the poles are pointed our way, we would perceive it as a flare. However, this does not explain why the flares slowly fade after flare-up.
It’s also possible that a black hole, or even a neutron star, is sometimes consuming 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 through the black hole or neutron star and cause matter to fall into it faster.
Future research could examine the frequency of these outbursts more closely to provide clues to the source of these strange new phenomena.
Nature2015. DOI: doi:10.1038/nature19822 (About DOIs)