Repeating vs One-Off Fast Radio Bursts: What We Know

Astronomers know that fast radio bursts (FRBs) are millisecond-long flashes of radio energy from deep space, and they come in two distinct types: repeaters, which flash multiple times from the same source, and one-offs, which have only been detected once. The key difference is that repeating FRBs are known to originate from a single, persistent source that can flash again, while one-off FRBs may also repeat but have not been caught doing so, possibly due to observational limits. This distinction is shaping how scientists think about what produces these enigmatic signals.

What Are Fast Radio Bursts?

Fast radio bursts are intense pulses of radio waves that last only a few milliseconds, yet they can outshine entire galaxies in radio frequencies. They were first discovered in 2007 when astronomer Duncan Lorimer and his student David Narkevic found a bright, dispersed burst in archival data from the Parkes Observatory in Australia. Since then, hundreds of FRBs have been cataloged, and the number continues to grow as dedicated instruments like the Canadian Hydrogen Intensity Mapping Experiment (CHIME) and the Australian Square Kilometre Array Pathfinder (ASKAP) scan the skies.

Most FRBs are detected at frequencies between 400 MHz and 8 GHz, and they show a characteristic sweep called dispersion: higher frequencies arrive slightly earlier than lower frequencies. This delay happens because radio waves travel through ionized gas in space, and the amount of delay tells astronomers roughly how far the burst traveled. Most FRBs are extragalactic, originating from distances of millions to billions of light-years.

The Discovery of Repeating FRBs

The first repeating FRB, designated FRB 121102, was discovered in 2012 by the Arecibo Observatory in Puerto Rico, and it was found to repeat in 2016. This was a landmark moment because it proved that at least some FRBs are not one-off cataclysmic events, but rather sources that can produce multiple bursts over time. FRB 121102 was later localized to a dwarf galaxy about 3 billion light-years away, and it is associated with a persistent radio source and a region of star formation.

Since then, a handful of other repeaters have been confirmed, including FRB 180916, which repeats on a roughly 16-day cycle, and FRB 20201124A, which shows complex, non-periodic behavior. The number of known repeaters is still small—only a few dozen out of hundreds of FRBs—but their existence has forced astronomers to rethink models that assumed all FRBs were destructive events like neutron star mergers or black hole formation.

Key Differences Between Repeaters and One-Offs

The most obvious difference is the behavior: repeaters flash multiple times, sometimes with irregular intervals, while one-offs have only been seen once. But there are other subtle distinctions that have emerged from observations:

• Duration and bandwidth: Repeating FRBs tend to have slightly longer durations (a few milliseconds) and narrower bandwidths compared to one-offs, which often show a single, broad-band spike.
• Polarization: Some repeaters show complex polarization patterns that suggest they are in magnetized environments, while one-offs show a wider variety of polarization properties.
• Host galaxies: The few repeaters that have been localized reside in galaxies with active star formation, whereas some one-offs have been traced to older, quiescent galaxies.
• Energy distribution: Repeaters often show a power-law distribution of burst energies, meaning they emit many faint bursts and few bright ones. One-offs, by definition, have only one observed burst, so their energy distribution is unknown.

These differences hint that the two classes may have different origins, but astronomers caution that the sample sizes are tiny and selection effects could be at play. A one-off burst might actually be a repeater that we simply haven’t observed long enough to see burst again.

Possible Sources: Magnetars and More

The leading theory for FRBs is that they are produced by magnetars—highly magnetized neutron stars. Magnetars have magnetic fields a thousand times stronger than typical neutron stars, and they can release enormous amounts of energy in flares. In 2020, a magnetar in our own galaxy, SGR 1935+2154, emitted a burst that resembled a faint FRB, providing strong evidence that magnetars can produce such signals.

For repeating FRBs, the magnetar model works well: a young, active magnetar could produce multiple bursts as its magnetic field evolves. For one-offs, the picture is less clear. Some one-offs might be magnetars that undergo a catastrophic event, like a starquake or a collision with an asteroid, that destroys the source. Others might be mergers of neutron stars or black holes, which would naturally produce a single burst.

Another possibility is that all FRBs are repeaters, but the intervals between bursts are so long that we haven’t caught them again. The CHIME telescope, which has been operating since 2018, has detected many one-offs, but it only observes each patch of sky for a few minutes per day. If a source repeats every few months, it could easily be missed.

How Astronomers Study Them

Astronomers study FRBs using radio telescopes that can detect the faint, dispersed pulses. The key steps are:

1. Detection: Telescopes like CHIME and ASKAP scan large areas of sky and trigger on bright, dispersed pulses in real time.
2. Localization: To pinpoint the host galaxy, astronomers use interferometry—combining signals from multiple antennas—to get a precise position. This has been done for about 20 FRBs so far.
3. Follow-up: Once a source is localized, telescopes monitor it for repeat bursts. This is how repeaters are identified.
4. Multi-wavelength observations: Astronomers look for X-ray, optical, or gamma-ray counterparts to understand the environment around the source.

One of the biggest challenges is the sheer volume of data. CHIME detects thousands of bursts per year, but most are one-offs with no repeat. To find repeaters, astronomers must re-observe the same sky positions, which is time-consuming and competitive.

What the Burst Rate Tells Us

The burst rate—how often a source emits—is a crucial observable. For repeaters, the rate can vary dramatically. FRB 121102 has been seen to burst dozens of times in a single hour, while FRB 180916 has a 16-day period where it is active for about 4 days and then silent for 12. The calculator above lets you estimate the average burst rate per day based on the number of bursts and the time span you observe. For example, if you see 5 bursts in 10 minutes, that corresponds to 720 bursts per day, which is extremely active. In contrast, a source that bursts once every 10 days has a rate of 0.1 bursts per day.

This rate is not just a curiosity—it helps constrain the energy budget of the source. If a magnetar emits a burst every few minutes, the total energy over time must be replenished somehow, perhaps by magnetic field decay or accretion from a companion. For one-offs, the rate is effectively zero, so the energy must come from a single event.

Common Misconceptions

One common misconception is that one-off FRBs are definitely non-repeating. In reality, we simply haven’t seen them repeat. The distinction is observational, not physical. Another misconception is that FRBs are related to alien signals. While the idea is popular, there is no evidence for that, and natural explanations like magnetars are well-supported.

Another point of confusion is the term “fast”—it refers to the duration of the burst, not the speed of the signal. All radio waves travel at the speed of light, but the dispersion sweep makes the burst appear to be spread out in time.

Frequently Asked Questions

How many repeating FRBs have been found?

As of 2024, astronomers have confirmed roughly 30 repeating FRBs out of over 600 detected FRBs. The number is growing as more telescopes dedicate time to follow-up observations of known sources.

Can a one-off FRB become a repeater?

Yes, it’s possible. Some sources initially classified as one-offs have later been observed to repeat. For example, FRB 121102 was first seen as a single burst in 2012, but when it was re-observed in 2016, it flashed again. So the classification is provisional.

What causes the 16-day period in some repeaters?

FRB 180916 shows a periodic activity cycle of 16.35 days, but the burst itself is not periodic—it’s just that the source is active during a window of about 4 days. This could be due to the orbital motion of a binary companion, precession of the neutron star’s beam, or a stellar wind that periodically blocks the signal.

Are all FRBs extragalactic?

Yes, all FRBs detected so far are from outside our galaxy, except for the 2020 burst from the magnetar SGR 1935+2154, which is in the Milky Way. That burst was much fainter than typical FRBs, suggesting that extragalactic FRBs are simply brighter versions of the same phenomenon.

Why are one-offs more common than repeaters?

It could be that most FRB sources are short-lived and only burst once, or that repeaters are rare. But it’s also possible that many one-offs are actually repeaters with long intervals that we haven’t caught. The CHIME telescope has found that about 3% of FRBs repeat, but this fraction is likely an underestimate.

What is the dispersion measure and why does it matter?

The dispersion measure (DM) is the column density of free electrons along the line of sight. It causes the frequency-dependent delay in the burst. A higher DM means the burst traveled through more ionized gas, which usually implies a greater distance. However, DM can also be affected by the local environment of the source, so it’s not a perfect distance indicator.

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in t | Time between bursts | min | 10 | 0.001 | 100000
in n | Number of bursts | - | 5 | 2 | 1000
out rate | Burst rate | bursts per day | (n / t) * 1440 | 2