A twinkling pulsar reveals invisible structures in space

The twinkling stars in the night sky are not just beautiful to look at. Their flickering reveals something about the varying temperatures and densities in the layers of Earth’s atmosphere, which refract the light as it travels toward us. Certain stellar remnants that emit radio waves can exhibit a very similar effect.

Although their radio waves—which have longer wavelengths than visible light—can penetrate Earth’s atmosphere almost undisturbed, they are scattered by the thin gas between the stars. Their twinkling—known as scintillation—thus provides unique insights into interstellar space.

An international team led by Tim Sprenger from the Max Planck Institute for Radio Astronomy (MPIfR) has measured the flickering radio radiation from an object using an innovative observation technique. The results are published in Astronomy & Astrophysics.

Flickering stellar remnants

Scintillation occurs only in point sources, which is why distant stars twinkle but planets do not. In the radio spectrum, flickering can be observed in pulsars—the remnants of massive stars. They are among the most compact objects in the universe: the mass of an entire star is compressed into a sphere with the diameter of a major city.

The radio signals emitted by pulsars fluctuate in brightness due to scintillation, and their position in the sky appears smeared. The pulsar observed in the current study is only the second one in which the distortion caused by scintillation could be directly imaged.

Unexpectedly straight

The research team led by Tim Sprenger observed the pulsar designated PSR B1508+55, located about 7,000 light-years away in the constellation Draco. In the long-exposure image, the pulsar appears distorted into a line.

“Usually, one imagines that the pulsar is distorted into a blurred disk by random density fluctuations. Instead, the interstellar medium here seems to form ordered structures with a preferred orientation,” explains lead author Tim Sprenger. These could be, for example, parallel filaments or thin, folded layers.

Exactly what the structures look like in this case is not yet clear. This is partly because the observed scattering they cause is very small on an astronomical scale and difficult to observe. Of particular interest are small irregularities in the otherwise straight scattered line.

“Observing the contrast between the primary linear image and its complex deviations is fascinating. It makes us wonder: What are the microscopic structures that created them—structures that elude our current picture of the interstellar medium?” adds co-author Xun Shi from Yunnan University in China. Using model calculations, it is at least possible to determine that the interstellar cloud is located about 430 light-years from Earth.

Novel observation technique

The scintillation of a pulsar causes such small positional changes that they cannot be spatially resolved with individual telescopes. The researchers therefore used a sophisticated observational technique and two of the world’s most powerful radio telescopes, the 100-meter Effelsberg radio telescope in Germany and the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in China. Due to Earth’s motion, when both telescopes are pointed simultaneously at PSR B1508+55, their positions change significantly over time.

This means that over the course of a day, sometimes one telescope and sometimes the other sees the same flickering first, depending on whether Germany or China is currently pointing more in the direction of the Earth’s motion. From this, an image can be calculated.

Co-author Olaf Wucknitz notes, “Taking advantage of the large distance between the two radio telescopes and of the Earth’s motion relative to the observed structures, we were able to achieve a resolution that is not possible with any other technique in the observed frequency range.”

At higher frequencies, comparable resolutions can be achieved by combining many telescopes around the world into a virtual telescope. This is technically complex, and the resulting data must be correlated in a time-consuming process.

“The observation technique we used does not place high demands on the infrastructure. It works with locally processed data sets that we were able to merge using our standard laptops,” reports Sprenger. Following this success, observations of additional pulsars are planned. These should then reveal more about the invisible structures of the interstellar medium.

Michael Kramer, Executive Director of the MPIfR, points out that FAST is currently the most sensitive telescope in the world, emphasizing, “This beautiful work demonstrates what’s possible when two of the most powerful instruments in the world are working together. Both telescopes are great, but their rare combination is far better.”

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Stephanie Baum

Master’s in TESOL from The New School. Passionate about language learning and editing science news on biology and space exploration.

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Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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