Astrophysicists turn fast radio bursts into cosmic probes

The deep synoptic array on a sunny day

The Deep Synoptic Array in California’s Owens Valley can pinpoint the point in the sky where a fast radio burst occurs. When completed, it will comprise 110 plates, each 4.65 meters wide.Credit: Gregg Hallinan

Powerful millisecond-long bursts of radiation known as fast radio bursts (FRBs) have baffled scientists. since they were discovered in 2007. Although their exact causes remain uncertain, astronomers are now beginning to use the bursts as tools to probe the cosmos, from unraveling the nature of the cosmic web to measuring the expansion of the Universe.

“I am quite optimistic that FRBs will become a basic astronomical tool in the near future,” says Di Li, an astronomer at the National Astronomical Observatories in Beijing.

FRBs are believed to come from sources like pulsating dead stars or cataclysmic events, mostly outside the Milky Way. These events are compact but incredibly powerful, releasing 500 million times more energy than the Sun during a given duration. The gas that each FRB passes through on its way to Earth, both as it leaves its home galaxy and as it traverses the space between galaxies, leaves telltale traces in the signal.

Until now, astronomers have largely used the available FRB data to locate and understand the source galaxies of the outbursts, but they are beginning to make broader inferences about the distribution and structure of matter that would otherwise be lost. hard to study.

Observatories are detecting FRBs at an unprecedented rate and are increasingly able to identify which galaxies they came from, says Kiyoshi Masui, a radio astronomer at the Massachusetts Institute of Technology in Cambridge. “We will have statistically powerful samples much sooner than we expected,” he says.

hidden treasures of information

FRBs make good cosmic probes because their radio waves interact with whatever medium they pass through. The total amount of matter, as well as the density fluctuations within it, leave a trace in the signal, just like magnetic fields. The ability of an FRB to reveal information about its journey has “great scientific power,” even if separating the footprint of the different stages remains a challenge, says Xavier Prochaska, an astrophysicist at the University of California, Santa Cruz.

A magnetic field, for example, flips the polarization of radio waves, the direction in which their electric fields oscillate. Astronomers want to know about magnetic fields because they affect how matter flows and how galaxies form. in a preprintone Published in arXiv last September, Prochaska and his colleagues showed that nine FRBs came from galaxies with magnetic fields similar in strength to the Milky Way. Once astronomers can identify the source galaxies of around 100 FRBs, they will be able to explore broader trends, such as whether galaxy mass or type correlates with magnetic field strength, Prochaska says. “At that stage, we can really inform galaxy formation models,” he says.

First, FRBs can reveal the total amount of matter they encounter while traveling. The low-frequency components of the waves slow down more than the high-frequency ones, leaving a smeared signal. The bigger the blob, known as scattering, the more matter the wave has passed through.

In 2020, astronomers used scattering measurements of five localized FRBs to shed light on a long-standing mystery: the location of more than half of the Universe’s ordinary matter that scatters as gas.2, instead of concentrating on galaxies. Cosmology predicts that this matter must exist, but because the density of the gas is low, it was maddeningly difficult to fully explain. The team, led by Jean-Pierre Macquart of Curtin University in Perth, Australia, showed that the correlation between the distance of the FRBs and their dispersal measurements roughly matched the expected amount of matter lost.

However, not all FRBs fit the correlation perfectly. This is because the missing matter is not evenly distributed through space, but swirls around and between galaxies in filaments known as the cosmic web. Astronomers working on the FLIMFLAM survey3 now I want to use a sample of 30 located FRBs to map the cosmic web more accurately. For each FRB, they plan to subtract estimates of the scattering caused by various stages of the journey (the host galaxy, other galaxies the signal skirts, and the Milky Way) so they can better constrain the amount of matter that must remain in the cosmic web. in between.

even more answers

The scattering of an FRB can also tell astronomers about the properties of the gas on a smaller scale. Vikram Ravi, an astronomer at the California Institute of Technology in Pasadena, and his colleagues have been using FRBs as spikes to observe the gas that lies around galaxies in halos, known as the circumgalactic medium. In an article published on arXiv this month4, the team used the scattering measurement of an FRB that brushed against the Milky Way’s own halo to put an upper limit on the amount of gas present there and showed that it is much smaller than expected. The researchers say the finding supports the idea that matter is regularly ejected from galaxies in a process known as feedback. Supernovae and stellar winds eject matter as gravity pulls it in, a process that is difficult to model in a computer simulation. “FRBs can help a lot” in understanding this process, which is critical to understanding how galaxies form, says Yin-Zhe Ma, an astronomer at the University of Kwazulu-Natal in Durban, South Africa.

Astronomers are even trying to use FRB scattering to measure how fast galaxies are receding from each other due to the expansion of the Universe, which is described by the Hubble constant. Theories of cosmology, along with the Hubble constant, predict a specific relationship between the distance of an FRB and how stretched it should appear. By inputting observed values ​​for the measure of dispersion and distance, astronomers can change the equation to give the best value for the Hubble constant.

The constant has been measured with high precision, but different methods have gave conflicting results. So far, the values ​​that the FRB researchers have calculated have such a large uncertainty, around 10%, that they cannot help resolve the question.5. But the figure will be more accurate with more localized FRBs, says Esanmouli Ghosh, a student at the Mohali Indian Institute of Science Education and Research in Manauli, who presented one such study.5 at the International Astronomical Union General Assembly 2022 in Busan, South Korea, in August.

The method could be promising, says astronomer Adam Riess of Johns Hopkins University in Baltimore, Maryland, but a big challenge is determining how much of an FRB’s scattering is due to the relevant matter in intergalactic space and how much is from the host galaxy and the Milky Way. This problem is common to all attempts to use FRBs as probes, but astronomers are working on various methods to estimate how much scattering occurs in each part of the journey, Prochaska says. “I feel optimistic,” he says.

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