FRB 20180916B Bursts Back Onto the Scene at the Lowest Frequencies to Date

Title: Chromatic periodic activity down to 120 MHz in a Fast Radio Burst

Authors: Inés Pastor-Marazuela, Liam Connor, Joeri van Leeuwen, Yogesh Maan, Sander ter Veen, Anna Bilous, Leon Oostrum, Emily Petroff, Samayra Straal, Dany Vohl, Jisk Attema, Oliver M. Boersma, Eric Kooistra, Daniel van der Schuur, Alessio Sclocco, Roy Smits, Elizabeth A. K. Adams, Björn Adebahr, Willem J.G. de Blok, Arthur H. W. M. Coolen, Sieds Damstra, Helga Dénes, Kelley M. Hess, Thijs van der Hulst, Boudewijn Hut, V. Marianna Ivashina, Alexander Kutkin, G. Marcel Loose, Danielle M. Lucero, Ágnes Mika, Vanessa A. Moss, Henk Mulder, Menno J. Norden, Tom Oosterloo, Emanuela Orrú, Mark Ruiter, Stefan J. Wijnholds

First Author’s Institution: Anton Pannekoek Institute, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands

Status: Submitted, open access on arXiv, 

We have learned a lot about fast radio bursts (FRBs) since the first one was discovered in 2007. These enigmatic bursts of radio emission, which are both extremely short, lasting only milliseconds, and extremely energetic, with fluxes of 10s to 100s of Jansky (or a few thousand times brighter than most pulsars), are one of the fastest growing fields in astronomy. While most bursts are one-off events, astronomers have now found not only one, but multiple FRBs that seem to repeat. A few have been localized to host galaxies, and a burst very similar to an FRB has been observed coming from a magnetar in our own Milky Way! New radio telescopes like the Canadian Hydrogen Intensity Mapping Experiment (CHIME) are already observing hundreds of FRBs, and telescopes in development like the Square Kilometer Array (SKA) will see many more!

But there is still so much we don’t know about FRBs, like what causes them (though there are many theories). But the authors of today’s paper have made a discovery that may give clues about the sources of FRBs: the lowest frequency FRB detection to date at 120 MHz. This frequency is almost low enough to tune into it with the FM radio on your car! The FRB in question is FRB 20180916B, which is a repeating FRB known to be active and emit bursts every 16.3 days or so. This low frequency detection is exciting because it can tell us about the environment around whatever is producing the FRB, which may lead to clues about its source.

The low frequency bursts

Image showing two FRB burst detections as a function of radio frequency and time. One burst seen at APERTIF at around 1400 MHz is very narrow and not scene at low frequencies around 150 MHz by LOFAR. The other burst seen at low frequencies by LOFAR but not at higher frequencies by APERTIF appears to be smeared out a bit, characteristic of pulse scattering by the interstellar medium.
Figure 1: Example bursts from FRB 20180916B detected by APERTIF and LOFAR. Left: A high frequency burst detected by APERTIF, visible in blue in the upper left panel spanning a large high frequency range over a short time, which is not detected by LOFAR. Right: A low frequency burst detected by LOFAR visible in blue in the lower right panel, and not detected by APERTIF. This low frequency burst spans a wider time and appears to be smeared out due to effects from the interstellar medium. Figure 1 in the paper.

Since FRB 20180916B is known to repeat, the authors turned two telescopes towards it. One is the Low Frequency Array (LOFAR), which observes at very low radio frequencies, and the other is the Westerbork Synthesis Radio Telescope APERture Tile In Focus (WSRT-APERTIF), which observes at higher radio frequencies where FRB 20180916B has been observed before. The reason for doing this is not only to try to detect these bursts, but also to see if any bursts that are detected are observed at both frequencies at the same time, which would provide clues about the physical process behind the bursts.

Low and behold, the authors detected bursts with both telescopes at both frequencies, but found that they were only seen with one telescope or the other, not both, as shown in Figure 1. Even more interestingly, the low frequency bursts detected with LOFAR were all found within an expected activity window where no higher frequency bursts were detected with APERTIF, as shown in Figure 2. This seems to suggest that whatever mechanism produces the bursts causes them to emit at different frequencies at different times.

Figure 2: Timeline of observation of FRB 20180916B by APERTIF, LOFAR, and other radio telescopes. Each dot is a detected burst, with the color of the dot denoting which telescope detected it, when it was detected, noted on the x-axis, and how bright the burst was, noted by the signal-to-noise ratio (SNR) on the y-axis. The grey shaded regions indicate an expected activity window every 16.3 days for FRB 20180916B. In the lower panel, each mark indicates an observation of FRB 20180916B by a telescope indicated by the color of the mark. Areas where the green (APERTIF) and purple (LOFAR) observation overlap indicate near-simultaneous observations from this work. In the one activity window where bursts were detected with LOFAR, no bursts were detected by any other telescope. Figure 3 in the paper.

What can we learn from these new bursts?

Histograms showing how many bursts from FRB 20180916B were detected by different telescopes at different radio frequencies over the activity window of FRB 20180916B. One histogram from APERTIF at 1370 MHz has most bursts earlier in the window, from a phase of around 0.3 to 0.5. The bursts from CHIME around 600 MHz span a larger range of 0.4 to 0.7 or so in phase. The bursts from LOFAR at 150 MHz are to the right of both the other histograms, ranging from about 0.6 to 0.8 in the activity phase window, though there are only nine bursts detected by LOFAR.
Figure 3: Histograms showing how many bursts from three different telescopes, APERTIF (green), CHIME (orange), and LOFAR (red) arrive within the 16.3 day activity window (here shown as a phase of 0 to 1, where 0.5 is around the expected activity window) for FRB 20180916B. From the plot it seems like higher frequency bursts arrive earlier during the activity window than lower frequency bursts, which CHIME bursts spanning the early and middle of the window, and low frequency bursts from LOFAR at the tail end. However, very few LOFAR bursts have been detected, so more bursts are needed to determine if this is a physical effect or a selection effect. Figure 4 in the paper.

Quite a lot it turns out. The authors split up the times of arrival of the FRB by frequency, comparing when the high frequency APERTIF bursts arrived within the activity window to the LOFAR bursts as well as previously detected bursts from FRB 20180916B with CHIME, which operates from 400-800 MHz, between APERTIF and LOFAR. The histograms of these arrival times, shown in Figure 3, presents an interesting picture, suggesting that higher frequency bursts are emitted earlier in the activity window and lower frequency bursts are emitted later on. What causes this is still unknown, though the authors suggest it could be either the pulses drifting to lower frequencies with time, or some long-scale frequency dependent delay, such as dispersion in pulsars. More low frequency bursts will be necessary to pin this down.

In addition to this apparent frequency drift in time, just the fact that these low frequency bursts were detected at all tells us about the environment around FRB 20180916B. Some FRB models predict a wind or some other dense environment around the source which would absorb this low frequency emission. This detection rules that model out. Additionally, at low frequencies, the interstellar medium, or the gas and dust between us and the burst, can smear out the signal completely, in what is known as pulse scattering, and is visible in the apparent smearing of the right hand burst in Figure 1. But the authors find that the bursts detected by LOFAR are not scattered by much more than we would expect if they had been emitted within the Milky Way. This means the environment around whatever is emitting the bursts that make up FRB 20180916B must be fairly empty, which is in contention with some FRB production models.

What exactly this means about the source of these bursts is not clear yet, but it does rule out most of the current models of what causes the periodic bursts of FRB 20180916B. Clearly there is still much to learn about FRBs, so stay tuned. We are learning new things about them almost everyday!

Astrobite edited by: Jason Hinkle

Featured image credit: Adapted from Figure 2 of the above discussed paper, Pastor-Marazuela, I, et al. 2020

About Brent Shapiro-Albert

I'm a fourth year graduate student at West Virginia University studying various aspects of pulsars. I'm a member of the NANOGrav collaboration which uses pulsar timing arrays to detect gravitational waves. In particular I study how the interstellar medium affects the pulsar emission. Other than research I enjoy reading, hiking, and video games.

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