Puzzles about cosmic X-ray flashes

Suddenly flaring sources of X-rays do not resemble any known phenomena in space

View of the galaxy Centaurus A (NGC 5128) with the newly discovered X-ray flash (circle). © NASA / CXC / UA / J. Irwin et al.
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Mysterious Lightning: Astronomers have discovered extremely unusual X-ray flares in two galaxies near the Milky Way galaxy. The two X-ray sources became abruptly several times abruptly at least 100 times brighter to fade slowly after one minute. The enigmatic fact: The behavior of these X-ray flashes is similar to none of the known phenomena, neither gamma-ray bursts nor magnetars, as the researchers report in the journal "Nature".

It flashes at all wavelengths in the universe: exploding stars produce gamma-ray bursts, black holes cause light and X-ray pulses from ingested gases, and only recently did astronomers discover ultrashort radio flashes whose origins are still unexplained.

Sudden lightning

Now the spectrum of enigmatic flashes has expanded: from two different sources, Jimmy Irwin from the University of Alabama and his colleagues have captured X-ray flashes that do not fit into any of the previous explanatory patterns. They discovered the flares as the 70 nearby galaxies scanned the X-ray telescopes Chandra and XMM-Newton.

"These flares are extraordinary, " says co-author Peter Maksym of the Harvard-Smithsonian Center for Astrophysics. "For a short time, one of the X-ray sources became the brightest X-ray object you've ever seen in an elliptical galaxy."

Two galaxies, similar flares

The first lightning source is located outside of Centaurus A, an elliptical galaxy about twelve million light-years away. The Chandra telescope registered a sudden X-ray explosion there: within seconds an otherwise weak radiation source became 200 to 300 times brighter. After about a minute of lighting, the radiation faded in the course of an hour again. display

This sequence of images from the Chandra X-ray telescope shows the flare glowing and fading near Centaurus A. NASA / CXC / UA / J. Irwin et al.

The second flash source is 47 million light-years away near the galaxy NGC 4636 in the constellation Virgo. Even this object increased its X-rays within seconds by more than a hundredfold, and then gradually fade again. For Centaurus A, the astronomers registered a total of five flares, one for NGC 4636. However, they suspect that both objects give off such X-ray flashes at a fast rate - with a repetition rate of only a few days.

"We have never seen anything like it"

But what is it? Exactly this question prepares the astronomers so far headache. Because the characteristics of the two flashing objects do not match any of the previously known X-ray sources. "We've never seen anything like it before, " says Irwin. "Astronomers know many different objects that can light up, but these flares could be a whole new phenomenon."

Although gamma-ray bursts may appear abruptly, they do not repeat at the same point several times. Because their source, a dying star, is destroyed in the explosion. Similar abrupt, repeated X-ray flashes are known from magnetar-young, fast-rotating neutron stars. However, these typically do not occur in regions with rather dense, old star environments, as is the case with these two sources of X-rays.

An intermediary matter smoother?

Another possibility would be a star orbiting a black hole or a neutron star and periodically losing gas. If this matter is sucked in by the partner, radiation pulses can be released. However: Normally these X-ray pulses are a thousand times weaker than the newly discovered flares, as the astronomers explain.

An intermediary black hole might explain the unusually high intensity of the X-ray flashes. These objects of a hundred to a thousand solar masses are so rare and difficult to observe that it was long disputed whether they exist at all.

Which object actually exhibits the unusual X-ray flashes remains unclear for the time being further research must follow. "Now that we have discovered these flashing objects, applied astronomers and theoreticians will equally feverishly seek an explanation for this phenomenon, " says Gregory Sivakoff of the University of Alberta. (Nature, 2016; doi: 10.1038 / nature19822)

(Chandra / Nature, 24.10.2016 - NPO)