To better understand the accelerating expansion of the universe, the Canadian Hydrogen Intensity Mapping Experiment (CHIME) constructed an interferometric radio telescope. The signals collected by the instrument since its first light in 2017 are being used to reconstruct a three-dimensional map of hydrogen density, which is one way to measure the universe’s expansion.
The telescope’s wide field of view and broad frequency range of 400–800 MHz also make it particularly adept at discovering fast radio bursts (FRBs)—the bright, brief flashes of extragalactic radio waves first discovered in 2007 and whose origins remain somewhat mysterious. FRBs have on occasion been measured repeatedly at the same location, and researchers have found that they may originate from magnetars or periodically rotating radio pulsars (see Physics Today, January 2021, page 15).
Of the more than 1000 FRBs that CHIME has detected, the vast majority last only a few milliseconds and usually don’t have a periodically repeating pattern. But in December 2019, researchers from the CHIME/FRB Collaboration detected an FRB that seemed to break all the rules. In a new paper, the team reports that the extremely long three-second duration of the FRB, named 20191221A, has a remarkably consistent periodicity of 0.2 seconds.
When an FRB is detected at CHIME, the total intensity of the radio signal as a function of time and frequency is stored in its databases. Only 0.5% of the FRBs detected thus far have five or more distinct frequency components, and FRB 20191221A has at least nine, shown in the pulse profile below. Each component’s peak is marked by a vertical line, and the red line is a best-fit model to the data. (The bottom part of the figure shows the residual data after removing the model from the pulse signal.)
Given the sky position where the FRB was detected, some signal scattering is to be expected. And as implied by the partially overlapped peaks in the signal, FRB 20191221A experienced far more scattering than would be possible if it originated in the Milky Way. Such extreme scattering could occur if the radio waves propagated through a turbulent plasma. The period of FRB 20191221A is remarkably close to the average period of pulsars—that is, rotating neutron stars that emit radio waves.
Like the galactic pulsars from which some FRBs may originate, including a magnetar found last year, an extragalactic neutron star could have emitted repeating radio pulses of exceptional luminosity. For now, it’s the leading possibility for the source of FRB 20191221A. A more conclusive determination will take more data. Thankfully, CHIME is continuing to detect roughly 1000 FRBs every year. (CHIME/FRB Collaboration, Nature 607, 256, 2022.)