A Dutch team of astronomers has found that the repeating pattern in the cosmic radio flash FRB20180916B is not caused by the strong stellar wind from a companion star, as previously suspected. Instead, the flashes may come from a highly magnetized but solitary neutron star called a magnetar. The astronomers made this discovery within a unique combination of observations with two of the largest radio telescopes in the world: LOFAR and Westerbork.

The Westerbork telescope (left) first observed a rhythmic fast radio burst in the blue short-wave radio sky. Much later the source emitted red, long-wave flashes. The LOFAR telescope (right) has now observed such flashes for the first time. The color-dependent flashing behavior means that the source is not periodically obscured by the wind from a companion star. (Image Credit: Joeri van Leeuwen)

Fast Radio Bursts (FRBs) are among the most violent bursts in the universe, but until now, astronomers thought they were obscured by an electron fog. From the new observations, however, they are clearly visible.

The use of 'radio colors' led to the breakthrough. In visible light, we see the different wavelengths as different colors. For example, the rainbow runs from blue light (shorter wavelength) to red light (longer wavelength). Electromagnetic radiation whose wavelength is too short or too long for the human eye can also be described as light. Astronomers call this ultraviolet light, for example, or radio light. Radio light is found beyond the red side of the regular rainbow.

Bluer light (shorter wavelength) can again be distinguished from redder light (longer wavelength) within the radio light band. Radio waves are a million times longer than those in visible blue and red light, but essentially different radio wavelengths are just colors.

The researchers studied the fast radio flash FRB20180916B on two wavelengths simultaneously: one bluer and one much redder. The radio flashes are highly energetic phenomena and last only one-thousandth of a second.

The energy source behind the flash must be immense, but astronomers don't quite understand its origin yet. Some FRBs emit multiple flashes; FRB20180916 B does this at regular intervals. Astronomers, therefore, suspected that the flashes originate from binary stars.

Binary stars orbit each other very regularly and can obscure each other with their stellar wind. "The companion star wind should transmit most of the blue, short-wave radio light, but not the red, long-wave radio light," commented first author Inés Pastor-Marazuela (University of Amsterdam and ASTRON).

To test that idea, the astronomers combined the LOFAR telescope with the upgraded Westerbork telescope. This allowed them to observe FRB20180916B in two radio colors at the same time.

Westerbork investigated the bluer wavelength of 21 centimeters; LOFAR looked at the much redder wavelength of 3 meters. The telescopes each made a high-speed movie of the source at thousands of frames per second.

The huge Westerbork Synthesis Radio Telescope in the Dutch province of Drenthe - Image Credit: Universal-Sci (CC BY 4.0)

A self-learning supercomputer searched the images directly and continuously. "When we compared the two colors, we had a big surprise," said Pastor-Marazuela. "From the usual stellar wind models for FRBs, you would expect to see only, or at least mainly, bluer flashes. But what we found was two days of bluer radio bursts, followed by three days of redder ones. So the earlier models can't be right; something else is going on."

This was the first time astronomers observed a fast radio burst with LOFAR. They had never been observed before at wavelengths longer than 1 meter. "We have been trying to discover FRBs with LOFAR for over ten years. We had already searched an unimaginable amount of data. So far, without result. I had almost given up," said co-author Sander ter Veen (ASTRON).

The detection is important because it means that the redder, long-wave radio light can still escape from the immediate vicinity of the bright source. "Some FRBs appear to be crystal clear, unimpeded by any electron fog in their galaxy. That is very interesting," said co-author Liam Connor (UvA/ASTRON), "because it allows us to use FRBs as beacons to map the atoms in the universe. A large part of that matter seems to have been lost."

The two Dutch radio telescopes played a key role in the discoveries. LOFAR is a long-wave radio telescope spread across Europe, with the Dutch province of Drenthe as the center. The dishes at Westerbork have recently been renewed with the (shorter wave) Apertif receivers, high-speed radio wave cameras. The breakthrough came when the team directly connected the two, making them work together as one.

Research leader Dr. Joeri van Leeuwen (ASTRON/UvA): “We first built a self-learning supercomputer for Westerbork. It acts as the visual brain and can recognize the flashes at lightning speed. Westerbork sent in LOFAR fully automatically with every short-wave FRB, but LOFAR saw nothing. At first, we suspected the predicted fog around the FRB source was blocking those redder, long-wave flashes—but to our surprise, the redder flashes appeared after the bluer ones had already stopped. The fast radio bursts escape unimpeded, and are likely emitted by magnetars."

Such magnetars are neutron stars with a density many times higher than lead and are also exceptionally strongly magnetized. "A lone, slow-spinning magnetar best explains the newly discovered behavior," Pastor-Marazuela says. "It feels like you are a detective close to the denouement – our observations leave few models for FRBs left."

If you are interested in the details of the team's research, be sure to check out the result, which appeared in the journal Nature, listed below.

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