The CHIME is Now for Fast Radio Bursts

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Title: The First CHIME/FRB Fast Radio Burst Catalog
Authors: The CHIME/FRB Collaboration
Corresponding Author’s Institution: Massachusetts Institute of Technology
Status: Published in ApJS

Fast radio bursts are just as their name suggests — short, millisecond-long bursts observed in radio wavelengths all over the galaxy. But, despite the first burst being found in 2007, astronomers still aren’t sure what type of sources produce them. We do know that there seem to be two types of bursts — those that repeat (repeaters) and those that occur only once (non-repeaters). One of the repeating sources has recently been associated with SGR 1935+2154, a galactic magnetar — a type of neutron star with a very powerful magnetic field. Another repeating source has been associated with the M81 galaxy.

In this article from the CHIME/FRB Collaboration, the authors present a new catalog of fast radio bursts (called “Catalog 1”), which more than doubles the number of known sources. Statistical studies of the entire population are thus much more viable, and as a result, astronomers can begin to address some of the open questions about these mysterious sources.

How Can We Tell Where Fast Radio Bursts Come From?

Although one repeater has been associated with a magnetar in the Milky Way, not all fast radio bursts come from objects within our galaxy. We can see the distribution of fast radio bursts (both repeaters and non-repeaters) in Figure 1.

Figure 1: The distribution of fast radio bursts across the sky seen by the CHIME telescope. CHIME, located in Canada, observes the northern sky and a small band of the southern sky near the celestial equator (declination -11 to 90 degrees). There are 474 unique non-repeating sources (blue circles) and 18 known repeaters (red triangles). [CHIME/FRB Collaboration 2021]

Figure 2: 2D histograms of the amount of radio waves (flux) received in different frequency bands by CHIME for two sample fast radio bursts, with name and dispersion measure in the top right of each panel. These “waterfall plots” show what the telescope observed for each source. The plot on the left has a more pronounced “swoop” pattern and a higher dispersion measure. Plots for the rest of the catalog are here. [Adapted from CHIME/FRB Collaboration 2021]

Fast radio bursts, especially the ones outside our galaxy, travel a long way to get to us on Earth. We can look at the dispersion measure to tell how far the burst has traveled. Space is not empty, and as the burst travels through the interstellar medium it gets scattered off electrons and other particles, which causes the “swoop”-like pattern seen in the data in Figure 2. The dispersion measure is the integrated number density of particles along the path of the waves and is a quantitative way to estimate the distance that the burst has traveled to get to the telescope.

Are Repeaters and Non-Repeaters From the Same Type of Source?

One of the major open questions about fast radio bursts is what type of source produces them. The differences between repeating and non-repeating sources leave open the possibility that the two types of bursts may originate from different sources. The nature of repeating sources rules out cataclysmic scenarios (situations where one or more objects collide or explode, such as a neutron star merger), but some of these might be possible for a non-repeating source.

The authors of the article start by looking at several characteristics (including distribution across the sky, dispersion measure, signal strength, flux, temporal width, and bandwidth) of both repeater and apparent non-repeater bursts to see if it is statistically plausible that they originate from the same distribution. If so, both repeaters and non-repeaters could come from the same underlying source population.

The authors find that repeaters and non-repeaters are distributed similarly across the sky and have similar dispersion measures. Repeaters and non-repeaters have similar flux and fluence distributions, meaning they give off a similar amount of radio waves over the course of the burst (fluence here refers to the integral of the flux over the duration of the burst).

Figure 3: Distribution of (temporal) widths for the bursts. The distribution in blue shows the single non-repeater bursts, while the orange distribution chooses only the first observed burst for each repeater. These distributions do not look at all similar and showcase one of the differences seen between the types of fast radio burst sources. [Adapted from CHIME/FRB Collaboration 2021]

However, repeaters and non-repeaters also show some differences. Their temporal widths and bandwidths appear to differ, which is shown in Figure 3 (as also previously reported with lower statistics in two articles [1] [2]). The fact that the widths differ between the two types of bursts is interesting, because it seems that repeater bursts last longer than non-repeaters on average. The bandwidth (the spread of radio frequencies observed for the burst) is also different between the two types, which is an additional indicator that repeaters and non-repeaters may come from different types of sources.

So, What Is the Major Takeaway From This Article?

One of the most exciting aspects of this article is the increase in the number of sources and bursts now available for fast radio burst research. With higher statistics, studies of the entire population are more meaningful. Differences between repeaters and non-repeaters (with regards to their temporal width and bandwidth) leave open the possibility that these come from different populations. The authors estimate that 820 bursts occur over the full sky per day, which means that statistics will continue to increase over time, more sources may be found to repeat, and more pieces of the fast radio burst puzzle will continue to fall into place.

Original astrobite edited by Konstantin Gerbig and Ali Crisp.

About the author, Jessie Thwaites:

Jessie is a PhD student at the Wisconsin IceCube Particle Astrophysics Center at the University of Wisconsin-Madison. She studies possible astrophysical sources for high-energy neutrinos through multimessenger astrophysics. Outside of physics, she plays horn and enjoys spending time outdoors, especially skiing and biking.