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2022. augusztus. 13. szombat

A ‘strange signal’ is coming from the Milky Way. What’s causing it?

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Ez a hír már több, mint egy éves, így elképzelhető, hogy a tartalma már nem releváns, esetleg a képek már törlésre kerültek!

A fast radio burst was detected from within our galaxy for the first time. We may be closer to uncovering its origin.

On April 28, 2020, two ground-based radio telescopes detected an intense pulse of radio waves. It only lasted a mere millisecond but, for astonished astronomers, it was a major discovery, representing the first time a fast radio burst (FRB) had ever been detected so close to Earth.

Located just 30,000 light-years from our planet, the event was firmly within the Milky Way, and it was, to all intents and purposes, almost impossible to miss. The Canadian Hydrogen Intensity Mapping Experiment (CHIME) and the Survey for Transient Astronomical Radio Emission 2 (STARE2) certainly had no problems picking it up. “CHIME wasn’t even looking in the right direction and we still saw it loud and clear in our peripheral vision,” said Kiyoshi Masui, assistant professor of physics at the Massachusetts Institute of Technology. “STARE2 also saw it, and it’s only a set of a few radio antennae literally made out of cake pans.”

Until that point, all FRBs had been observed outside our galaxy. “They’ve been billions of light years away, making them a lot harder to study,” said doctoral candidate in physics Pragya Chawla from McGill University in Canada. April 2020’s discovery was also notable for being the most energetic radio blast that astronomers have ever recorded in the Milky Way, but what made it most exciting is that scientists are now closer to determining the origin of FRBs than at any point since they were first discovered.

That happened in 2007, when Duncan Lorimer and David Narkevic were studying data taken by the Parkes radio dish in Australia. Discovering an FRB so close to home has been the breakthrough astronomers have wished for ever since. “We can learn more from a source that’s 30,000 light-years away than one that’s a billion or more light-years’ distance,” Masui affirms. “We finally have a nearby source to study.”

One of the major problems with detecting FRBs, aside from most of them having been so far away, is that they are so fleeting. They’ve been and gone in the blink of an eye despite being 100 million times more powerful than the sun — they can release as much energy in a few thousandths of a second as the sun in 100 years. Ideally, astronomers would discover an object and focus one or more different telescopes at it, but the ephemeral nature of these bursts removes any such opportunity.

But despite these challenges, astronomers have succeeded in building up a bank of knowledge about FRBs, most of which has been based on the dozens of recorded events from beyond our own galaxy. For starters, we know they are bright flashes of radio light lasting for microseconds to milliseconds. “All-sky searches for them also suggest that thousands of these bursts occur in the sky every day,” Chawla added.

An artist’s impression of the SGR 1935+2154 magnetar during an outburst, highlighting its complex magnetic field structure and beamed emissions. (Image credit: © McGill University Graphic Design Team)

We also know that most of them come from billions of light-years away. But while dozens of models have been proposed to explain the origins of FRBs — with progenitors ranging from neutron stars to white dwarfs to cosmic strings — have any theories really prevailed? “Well, we know that they come from very small sources — no more than a few hundred kilometres in size,” Masui said. “And the most likely sources are neutron stars since they are both very small and very energetic.”

The FRB discovered in the Milky Way is now helping astronomers to firm up such theories, and it’s become something of a breakthrough for scientists trying to get to the bottom of what is causing them.

Thanks to some nifty cosmic detective work involving the data of other telescopes monitoring the same patch of sky, observational evidence is now suggesting that the origin of FRBs is very likely a magnetar, a type of young neutron star born from the embers of supernovas with a magnetic field 5,000 trillion times more powerful than Earth’s, thereby making them the universe’s most powerful magnets.

An artist’s impression of a fast radio burst with its different radio wavelengths — red being long and blue short — as they reach Earth.  (Image credit: © Jingchuan Yu, Beijing Planetarium)

But how has this conclusion been drawn? To explain, we must consider the work that has gone into studying FRBs in relation to magnetars, which are known to emit high-energy electromagnetic radiation, notably gamma rays and X-rays. Both of these erupt in short-lived flares, and there has been speculation that radio waves could be emitted in such a process that would pinpoint magnetars as the source for FRBs.

When this latest FRB was discovered in our galaxy — known by astronomers as FRB 200428 — it was found to have originated in the constellation of Vulpecula, which just so happens to be where the galactic magnetar SGR 1935+2154 is located. It was also accompanied by a burst of X-rays that further excited astronomers.

The first detection of X-rays from that sky region came the day before CHIME and STARE2 discovered FRB 200428. The Neil Gehrels Swift Observatory and the Fermi Gamma-ray Space Telescope detected multiple X-ray and gamma-ray bursts coming from SGR 1935+2154, which was known to exhibit transient radio pulsations.

Other telescopes were also found to have observed an X-ray burst from SGR 1935+2154 — crucially, at the same time as the fast radio burst. These included the Konus-Wind detector on board NASA’s GGS-Wind spacecraft and the European Space Agency’s INTEGRAL space telescope, both picking up an X-ray burst at the moment CHIME and STARE2 recorded the FRB.

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