Two Incredibly Bright X-Ray Flares Might Be Proof of Exotic Black Holes

It's often hard to intuitively reconcile cosmic time scales with every other cosmic scale. We say that astronomical objects mere single-digit light years from Earth are "close," while astronomical events that happen on single-minute time scales are "short." But put a minute next to a light year and the whole picture kind of falls apart, doesn't it?

Two x-ray flare sources detected recently by researchers at the University of Alabama compress those time scales even further—across six total flare events, Jimmy Irwin and colleagues observed eruptions bloom to their maximum intensity in less than a minute. In less than the time it would take you to skim this article, the sources flared to luminosities of nearly 100 million times that of our own Sun.

Irwin's observations, which are described in the current issue of Nature, don't really correspond to any known astronomical phenomenon. For one thing, other astronomical objects that produce extreme flares self-destruct in the process. "There's very few types of objects in our universe that we know that do this without blowing themselves up, like a supernova or gamma ray burst," Irwin said in a Nature podcast.

Meanwhile, x-ray flares from sources that don't blow themselves up in the process—magnetars and certain types of neutron star—blink away at time scales that are too short to allow for the hour-long fade-out seen in Irwin's flares.

So, we're left with a solid mystery. It's a mystery that happens to have been sitting in plain sight for several years. In the aforementioned podcast, Irwin said that he'd been looking for a project to occupy some undergraduate students and decided to task them with analyzing archived x-ray data collected by the Chandra and XMM-Newton space observatories. Within this dusty data, the group found the two x-ray sources, one near the Virgo galaxy, NGC 4636, and another near the Centaurus A galaxy, NGC 5128. Both are found near the periphery of the Milky Way—neighbors, practically.

NGC5128 with circled flare source. Image: ASA/CXC/U Birmingham/M Burke et al

Neutron star and supernovae explanations are further eliminated by the flare's origins within relatively old star populations. Both require young stars.

The explanation we're left with is black holes. It works like this: As a black hole chews up its surroundings, the infalling gas increasingly becomes compressed, resulting in energetic flashes as everything starts crashing together. This is the strange paradox of black holes—they're not all that black.

Two black hole scenarios might result in the observed x-ray flashes. The first is just a normal pair of black holes in a binary system beaming x-rays directly at Earth. The periodic flashes would occur as a result of increasing and decreasing levels of accretion (the process by which black holes gravitationally attract surrounding material) caused by the eccentric orbits found in binary systems.

The other possibility is even more interesting: an intermediate-mass black hole. This is a black hole with a mass of between 100 and 1,000 times that of our Sun: too large to be the result of a single collapsing star (stellar black hole), but not big enough to be the result of a whole collapsing regions of space (supermassive black hole). The thing is that we don't really know if intermediate-mass black holes exist or not. "We don't really know how to make black holes in that mass range," Irwin said.

The half-dozen x-ray flares discovered by Irwin's team then represent not just a good mystery, but a mystery that may solve the problem of whether intermediate black holes even exist, answering a decades-old question in astronomy.

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