Abigail Serrano – Andover High School, MA, USA
The First Burst
When the first fast radio burst, or FRB, was found in 2007 by D.R. Lorimer (Lorimer et al. 2007) through archival data, it came as a shock to the astronomy community. The radio burst was many orders of magnitude brighter than previously known radio sources. Its characteristic frequency of emission also drifted down to lower values over time, a tell-tale sign of the burst having propagated a long way through a medium of scattering electrons. The burst appeared to be of extragalactic origins. This was clearly a new radio phenomenon.
Fig. 1 The so-called Lorimer burst as observed at radio frequencies. Its characteristic frequency of emission was seen to decrease with increasing time, a consequence of electrons along the path of propagation scattering away the higher-frequency emission first. This frequency drift could be the result of the source being located at great distance and/or in an electron-rich environment. The Lorimer burst was so strong that it completely reset the background level, as seen in the upper plot. (Credit: from Petroff et al. [2019])
FRBs are now defined as very luminous and very fast (durations of milliseconds or less) bursts of radio emission (Petroff et al., 2019, p. 3). Pulses of such short durations have been known since 1982, when the first millisecond pulsar was discovered. Pulsars are rapidly rotating neutron stars, the collapsed core of a massive star gone supernova, that emit regular radio pulses. These objects were quickly ruled out as the source of Lorimer’s burst, as this FRB did not repeat and also had a higher inferred energy output than a pulsar (Lorimer et al., 2007, p. 2). The only known radio sources that could produce a similar burst of what Lorimer found were rotating radio transients and giant pulses (Lorimer et al., 2007, p. 6). Rotating radio transients are pulsars with sudden brightenings so sporadic that one cannot easily find the pulsar’s rotational period. Giant pulses are extremely bright radio bursts emitted by an energetic, typically young pulsar, such as one which has a rotation period on the scale of milliseconds. Again, these sources were ruled out. So if not a pulsar, what could FRBs possibly be?
The FRB Population
To learn the origins of FRBs, we must first understand the properties and different types of FRBs that we have observed thus far. Since Lorimer’s burst, numerous FRBs have been found with varying properties. Though the majority of FRBs are one-time events, several have been found to repeat (Ng, 2023, p. 2). Repeaters have two states: active, where bright bursts are emitted, and quiet. Some repeaters also have periodic windows of activity (Ng, 2023, p. 2). Most are non-periodic, making it difficult to know when to search for another burst. This could result in a repeating FRB being mistaken for a non-repeater. In the end, FRBs fall into the two main camps of repeaters and non-repeaters. The two types have behavioral differences, exhibiting a wide range of observable properties despite all being bright, fast, and mysterious radio sources. Telescopes have been able to locate several FRBs within host galaxies, which are almost all star-forming spiral galaxies with some variability in their properties (Ng, 2023, p. 2). For example, although an FRB might be from a star-forming galaxy it does not mean it is in a star-forming region. These diverse environments and variations in properties could point toward a diversity of FRBs. Many physical sources have been proposed ranging from ones with clear-cut evidence to only pure speculation.
Little Green Men
The most exciting proposition for the source of FRBs is that they originate from advanced extragalactic intelligence. When FRBs were first discovered, it was considered that they might be radio frequency interference, or RFI, from Earth. However, several FRBs have since been located within host galaxies and therefore confirmed to not be interference (Ng, 2023, p. 2). But this does not mean that they are not of artificial origin. According to two astronomers, the parameters for a beam powering a large light sail aligns with those of FRBs (Lingam & Loeb, 2017). Light sails, although far outside current human technology, could be used by highly advanced alien intelligences to transport objects at near the speed of light. This explanation of extragalactic origin is suggested as an alternative to the idea that an alien civilization would emit such a beam as a “beacon” signaling their presence to other civilizations. It provides a simpler explanation that does not require extensive questioning of possible motives. While the possibility of at least some FRBs originating from extraterrestrial intelligence cannot be ruled out completely, it is a highly speculative idea, admitted even by those who proposed it.
One main issue is that the dispersion measure, or DM of FRBs indicates that they likely are of extragalactic origin. DM, is related to the density of electrons from the source to the observer. These electrons cause the lower frequency radio waves to arrive later than those of higher Frequency (see Fig. 1). The greater the distance between the observer and the source, the greater the number of intervening free electrons, and therefore the greater the DM. FRBs tend to have a higher DM than other radio sources, indicating a cosmological distance (Thornton et al., 2013). If alien civilizations did produce FRBs, one would expect to detect them within the Milky Way galaxy as well as fainter sources from greater distances. Despite this reality, such speculation may be worth looking further into as the pulse structure of an FRB from an artificial source and a natural source would be distinct enough to allow an artificial source to be ruled out (Lingam & Loeb, 2017).
Angry Neutron Stars
On April 28, 2020, The Canadian Hydrogen Intensity Mapping Experiment (CHIME) FRB project (see Fig. 2) detected an intense radio burst from SGR 1935+2154, a magnetar in our galaxy (Andersen et al., 2020, p. 2).
Fig. 2 The CHIME radio observatory located at the Dominion Radio Astronomy Observatory in British Columbia (Credit: National Research Council of Canada
Magnetars represent extreme forms of the already extreme neutron stars. They are young and highly magnetized, which leads to violent activity producing gamma-ray and x-ray bursts, along with some also producing radio pulses (Andersen et al., 2020, p. 2). The burst detected from SGR 1935+2154 was extremely luminous, much brighter than any other radio burst seen from other galactic magnetars. In fact, had the burst been seen from a nearby galaxy it would appear identical to an FRB (Andersen et al., 2020, p. 2). Before this discovery it had been a leading theory that repeating FRBs could originate from extragalactic magnetars. However, until this burst from SGR 1935+2154, the radio luminosities of galactic magnetars had been several orders of magnitude too low (Andersen et al., 2020, p. 2). SGR 1935+2154 finally provided the needed evidence to show that at least some FRBs originate from magnetars (see Fig. 3).
Fig. 3 Artist’s conception of a magnetar bursting at gamma-ray, X-ray, and radio frequencies. The burst travels through a dense environment of electrons, thus explaining the large dispersion measures (DMs) that have been observed. (Credit: European Southern Observatory) This does not mean that the mystery is completely solved. Although the burst from SGR 1935+2154 had a similar level of radio energy as some FRBs, there are far brighter FRBs at much greater distances. It is unknown whether magnetars could produce such energetic bursts (Andersen et al., 2020, p. 6). The magnetar model certainly explains some FRBs, whose large DMs can be understood as arising from the electron-rich nebulae that surround them (Zhou, 2022), but as discussed before, there are many different types of FRBs out there. FRBs with periodic properties for example, cannot be explained by the magnetar flare model (Ng, 2023, p. 2). Research into magnetars as sources of FRBs continues, with the magnetar model still leading most other explanations, but with other promising contenders waiting in the wings..
So Many Theories!
With no one clear answer, many other theories for FRBs remain. Some theories of neutron stars predict FRBs to be produced when they collide with other dense objects such as another neutron star or a white dwarf (Petroff et al., 2019, pp. 56-57). Other suggestions involve black holes. It has long been theorized that evaporating black holes would produce radio pulses, even before FRBs were first detected. Black holes interacting with their environments could also result in FRB-like events such as the collapse of a supermassive neutron star into a black hole or the collisions of clumps in the jets formed during accretion (Petroff et al., 2019, pp. 57). Other ideas involving white dwarfs and other compact stellar objects as well as exotic theories not fitting into any of the previously described categories also have been proposed (Petroff et al., 2019, pp. 57-58). “Fast Radio Bursts” by E. Petroff, J.W.T. Hessels & D.R. Lorimer describes many of the possible theories for FRBs and is a good resource to learn about them in more detail. It is quite possible that many of these theories are correct given the diverse population of FRBs, however, the vast majority of explanations do involve neutron stars, making them the most likely sources of FRBs.
The Research Ahead
There remain many theories and explanations, some with more credibility and evidence than others. The most promising, the magnetar model, still requires more research to determine how well it can explain FRBs as a whole. Due to the continued uncertainty surrounding FRBs, they remain an active area of research in radio astronomy. In order to confirm the magnetar model, or perhaps another theory, more detections will be needed. Finding more bursts from galactic magnetars such as that of SGR 1935+2154 for example, would help to back up the magnetar theory and might provide more insight into the processes that cause such large bursts in magnetars. Repeating FRBs also need to be studied much closer, and are a huge question in current FRB research. It is unknown whether or not all FRBs repeat, as it can be difficult to detect the next burst when you don’t know when to look. Just about every aspect of FRBs is still being studied. With each new FRB found, we can learn something new about its properties and the populations they represent. In turn, each new FRB detection could support or contradict a theory and lead us closer to the truth. Despite having come a long way since the mysterious new radio source that Lorimer found in 2007, we still have much to learn.
References
Andersen, B., Bandura, K., Bhardwaj, M., Bij, A., Boyce, M., Boyle, P., Brar, C., Cassanelli, T.,Chawla, P., Chen, T., Cliche, J.-F., Cook, A., Cubranic, D., Curtin, A., Denman, N., Dobbs, M., Dong, F., Fandino, M., Fonseca, E., . . . Kaspi, V. (2020). “A bright millisecond-duration radio burst from a galactic magnetar.” Nature, 587(7832), 54-58.
https://doi.org/10.1038/s41586-020-2863-y
Lingam, M., & Loeb, A. (2017). “Fast radio bursts from extragalactic light sails.” The Astrophysical Journal Letters, 837(2), L23. https://doi.org/10.3847/2041-8213/aa633e
Lorimer, D. R., Bailes, M., McLaughlin, M. A., Narkevic, D. J., & Crawford, F. (2007). “A bright millisecond radio burst of extragalactic origin.” Science, 318(5851), 777-780.
https://doi.org/10.1126/science.1147532
Ng, C. (n.d.) (2023) “A brief review on fast radio bursts.” arXiv e-prints.
https://doi.org/10.48550/ARXIV.2311.01899
Petroff, E., Hessels, J. W. T., & Lorimer, D. R. (2019). “Fast radio bursts.” Astronomy and Astrophysics Review, 27(1). https://doi.org/10.1007/s00159-019-0116-6
Thornton, D., Stappers, B., Bailes, M., Barsdell, B., Bates, S., Bhat, N. D. R., Burgay, M.,Burke-Spolaor, S., Champion, D. J., Coster, P., D’Amico, N., Jameson, A., Johnston, S., Keith,M., Kramer, M., Levin, L., Milia, S., Ng, C., Possenti, A., & van Straten, W. (2013). “Apopulation of fast radio bursts at cosmological distances.” Science, 341(6141), 53-56.
https://doi.org/10.1126/science.1236789
Zhou, W. (2022) “Statistical Properties of Fast Radio Bursts from the CHIME/FRB Catalog 1: The Case for Magnetar Wind Nebulae as Likely Sources,” The Galactic Inquirer, https://galacticinquirer.net/2022/09/statistical-properties-of-fast-radio-bursts-from-the-chime-frb-catalog-1-the-case-for-magnetar-wind-nebulae-as-likely-sources/
Abigail Serrano wrote this research report as a 10th grade student at Andover High School in Andover, MA. She cites Dr. Stephen Sanborn, the science and engineering department’s head at AHS, as a reference.