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Conversations with Di Li: On deciphering mysterious Fast Radio Bursts.

Fast Radio Bursts (FRBs) are enigmatic, short-lived radio explosions from space, whose origins remain mysterious. The Commensal Radio Astronomy FAST Survey (CRAFTS), under the Five-hundred-meter Spherical radio Telescope (FAST) program, discovered the first ever consistently repeating FRB, titled FRB 20190520B. This unique FRB provides vital information regarding the origins of such phenomena.

Dr. Li Di’s international team which included researchers at the National Astronomical Observatories of the Chinese Academy of Sciences, monitored this FRB using the Parkes telescope in Australia and the Green Bank Telescope in the U.S., finding an unusual field reversal around the source. Unlike other FRBs, FRB 20190520B always bursts when observed.

Data analysis from both telescopes showed dramatic changes in its Faraday rotation measure, suggesting a magnetic field reversal. This might be due to its signal passing through turbulent magnetic fields, potentially from a nearby black hole or a star. This discovery offers crucial insights into the nature of FRBs.

Dr. Li took some time to discuss his paper which was featured in Science with SCINQ.

Given the brief and sporadic nature of FRBs, what initially drew you and your team to investigate these elusive cosmic events, and how has your understanding of their significance evolved over time?

I obtained my PhD in radio astronomy from Cornell and was trained at the Arecibo Observatory. The Arecibo was the world’s largest antenna for over 50 years, until FAST came online in 2016. Among Arecibo’s landmark discoveries was the first repeating fast radio burst in 2015, namely FRB 121102. This facilitated the localization of such sources in galaxies and confirmed their cosmological origin.

I started planning FAST’s FRB endeavors following this discovery, with a particular focus on repeaters. The unprecedented sensitivity of FAST holds great potential. For example, we published 1652 bursts (more than all previous publications combined) from FRB 121102, all coming within one active epoch spanning about 50 days. Further inspection revealed many surprising features, including the bimodality in its energy distribution, which starts to paint an actively evolving picture of repeating FRBs.

What have been the most influential previous discoveries or theories in the field of FRBs, and how have these shaped the approach and focus of your current research on FRB 20190520B?

I would list two: FRB 121102 mentioned above and FRB 20200428, the first and only FRB event known to be located within the Milky Way. FAST has a relatively small field of view, so these well-localized sources are ideal targets for FAST to delve deep into. They also incentivize our efforts to discover new sources in their categories. FRB 190520 is the first new repeater in our Commensal Radio Astronomy FAST survey (CRAFTS) and turned out to be the first persistently active FRB, with many intriguing properties.

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Can you explain how the discovery and study of FRB 20190520B differ from past investigations into Fast Radio Bursts (FRBs), and how this has provided new insights into the origins of these cosmic phenomena?

Most FRBs were only seen once. A few can be fairly active, such as FRB 121102, which was seen to produce hundreds of bursts per hour by FAST. However, even 121102 frequently turns off, sometimes for as long as 8 months. So far, only FRB 190520 remains persistently active, in that each time we trained our telescope toward FRB 190520, even for as short as 20 minutes, we always detected pulses. FRB 190520 has also been detected by other major radio observatories, such as JVLA, GBT, and Parkes.

The reversal in Faraday rotation measure (RM) has been highlighted as a significant finding in this study. Can you elaborate on the implications of this discovery and how it contributes to our understanding of the FRB’s environment? Also, you’ve compared the magnetic field around repeating FRBs to a ball of wool. Could you expand on this analogy and describe the conditions you believe are likely to exist around these bursts?

The so-called rotation measure (RM) can be approximated by the product of the magnetic field and electron density. In another Science paper (Feng et al. 2022), we demonstrated that FRB 190520 has a complex plasma environment. In that paper, we were able to derive the so-called sigma_RM, corresponding to the local variation of RM for all active repeaters known.

Lo and behold, FRB 190520 has the largest sigma_RM among all repeaters, reflecting that it is shrouded in “a ball of wool” of magneto-ionic clouds. The changing RM can be attributed to either a changing density or changing magnetic field (strength as well as direction) or both.

The new insight in this paper is the extreme reversal of the sign of RM. Since there is no negative electron density and RM’s being the product of density and field, the orientation of the magnetic field has to flip. Such a flip happened a couple of times on our watch spanning 15 months, likely due to orbital motion altering the viewing geometry.

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The Parkes telescope was able to detect an unprecedented number of bursts from FRB 20190520B. Could you discuss the value of this reliably repeating FRB in the context of ongoing and future observational studies? Also, you suggest that FRBs that repeat might all come from similar places but have differences due to their local surroundings. Could you tell us more about what this means and how it could change our view of these cosmic radio bursts?

We are continually monitoring FRB 190520, not least because it is the only persistently active FRB known so far. Such reliability elevates the efficiency and productivity of observations. In a series of papers following its discovery, we have been able to localize FRB 190520 in the periphery of a metal-poor dwarf galaxy, confirm it has the largest DM local to the source, the largest delta_RM corresponding to a complex environment, and now its possible origin in a binary system. The next up on the menu would be any systematic trend of these observables, thus for the first time, revealing the ongoing evolution of any FRB.


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