Scientists Uncover Important Clue in Mysterious Radio Signals From Space

Scientists have used the biggest single-dish radio telescope on Earth to probe the unknown origins of radio signals from space that repeat, sometimes in clockwork patterns, reports a new study. 

The observations could help distinguish between the hypothetical sources of these exotic signals, known as fast radio bursts (FRBs), based on the magnetic patterns imprinted in their light.

First discovered in 2007, FRBs are unidentified radio pulses that flash brightly for mere milliseconds, hinting that they are generated by energetic cosmic events. The bursts typically originate from beyond the Milky Way, though one has been observed within our galaxy. 

Some bursts are one-offs that are never seen again, while others repeat and sometimes follow weird periodic cycles that can last weeks or months. While there has been speculation that FRBs could be messages from intelligent aliens, the overwhelming consensus is that they are naturally-occurring phenomena that are probably fueled by the intense environments around dead or dying stars.

A few dozen FRBs are currently known, each with idiosyncratic properties, suggesting that there is a range of different origin stories for these flashes. Now, astronomers led by Li Di, a professor at the National Astronomical Observatories of the Chinese Academy of Sciences, have identified a key signature of FRB polarization, which is a magnetic pattern that gets embedded in the radio signals by their environment, that can help narrow down the possible sources of these bursts.

The team suggests that light that is heavily twisted by the magnetic fields likely indicates sources with messy magnetic environments associated with the deaths of younger stars, such as the fallout of recent supernovae or the windy nebulas that surround dead stars called pulsars. On the other hand, more orderly polarization implies that an FRB source “is consistent with the environment being less turbulent, dense, and/or magnetized than that of other FRBs, as might be expected for an old stellar population,” according to a paper published on Thursday in the journal Science.

“Information about FRB hosts and environments could be obtained from their polarization,” Di and his colleagues said in the new research. “In this study, we analyzed the polarization properties of 21 repeating FRB sources.”

The team directly observed five repeaters with two telescopes and used existing studies to probe the remaining 16. Observations were captured by the Robert C. Byrd Green Bank Telescope in West Virginia and the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in Pingtang County, China. Measuring 1,640 feet in diameter, FAST is the largest single-dish radio telescope on Earth.

Di and his colleagues initially found no traces of polarization in the FAST data, which was captured at lower frequencies, meaning that the radio signals had longer wavelengths. When they followed up at higher frequencies with data from other observatories, the team saw that the shorter wavelengths were more likely to show the polarized effects of magnetic fields. In other words, repeating FRBs become “depolarized” at lower frequencies. 

The researchers suggest this can be explained by a model centered on a property called the rotation measure scatter, which describes the unpredictable paths that light can take in an intense magnetic environment. Longer radio wavelengths are more likely to be scattered in different directions, which explains why the team observed depolarization at lower-frequency bands of the radio spectrum in the burst data. 

The effect was consistent across repeaters, but different bursts showed the effect at different bands of the spectrum: For instance, FRB 20180916B, which emits bursts on a 16-day cycle, depolarized below frequencies of 200 MHz, while FRB 20121102A, which repeats on a 157-day cycle, depolarized below 3.5 GHz. 

FRBs that depolarize at higher frequencies are more likely to have more dynamic magnetic environments, suggesting that they emanate from the tempestuous regions surrounding freshly dead stars located in a younger stellar population, according to the study. Bursts that depolarize at lower frequencies may originate from the remains of older and more subdued populations of exotic dead stars. 

“The repeaters with large observed [rotation measure scatter] could be more affected by turbulence, resulting in large fluctuations of electron density and magnetic field, which may explain the diversity among repeaters,” the team said. 

The new study adds one more piece to the puzzle of these enigmatic bursts, which have perplexed scientists for more than a decade. However, many mysteries remain, including the question of whether one-off and repeater FRBs have different origins or stem from similar emission mechanisms.

“It remains unclear whether repeating FRBs are ubiquitous or uncommon sources,” the researchers said. “Whether all FRBs repeat on some time scale, or whether repeaters form a separate population from single-burst sources, has implications” for understanding their sources and environments, they concluded.