NICER / ISS Science Nugget
for December 5, 2024

An X-ray view of the closest known source of repeating fast radio bursts

Fast radio bursts (FRBs) are short-duration, coherent radio flashes that are produced by unknown astrophysical objects located in distant galaxies. Uncovering the origins of FRBs is a major goal of modern astrophysics. Although numerous models have been proposed to explain FRBs, those involving magnetars are currently most favored. Magnetars are young neutron stars with enormous magnetic fields (< 10¹⁴ Gauss, where the Earth's field strength is about 1 Gauss). The decay or sudden rearrangement of these large magnetic fields often leads to a wide variety of high-energy phenomena, including giant flares, intermediate flares, and short X-ray bursts. A strong link between FRBs and magnetars was established on 28 April 2020 when an FRB-like radio burst was detected from the magnetar SGR 1935+2154, in our own Galaxy, using the Canadian Hydrogen Intensity Mapping Experiment (CHIME) and STARE2 radio telescopes. A short X-ray burst was also detected simultaneously with this FRB-like burst from SGR 1935, suggesting that some FRBs may be produced by magnetars in other galaxies, and their X-ray emission could be detectable if they are relatively nearby.

In a peer-reviewed paper recently published in Nature Astronomy, A. Pearlman (McGill Univ., Canada) and collaborators describe results from a coordinated X-ray and radio observing campaign of the closest known extragalactic repeating FRB source, FRB 20200120E. In the outskirts of the M81 galaxy, FRB 20200120E resides in a 10 billion-year-old globular cluster - a location that challenges conventional models invoking young magnetars as the source of FRBs, since it is expected that there are no longer any massive stars in such a globular cluster that could form young magnetars via supernovae. Using sensitive X-ray (NICER, Chandra, NuSTAR, and ESA's XMM-Newton) and radio (Effelsberg and CHIME) telescopes, the Pearlman et al. study has placed the deepest X-ray luminosity limits on an FRB source to date in the soft X-ray band, for both persistent and prompt (i.e., coinciding with radio bursts) emission. The new X-ray burst energy limits are four orders of magnitude more constraining than the best previous limits. These prompt X-ray limits rule out an association between FRB 20200120E and magnetar-like giant flares, an SGR 1935-like intermediate flare, and ultra-luminous X-ray bursts (a class of unknown objects that produce bright X-ray bursts and have been traced to other extragalactic globular clusters). Other magnetar-like X-ray events, including some bright intermediate flares and short X-ray bursts, are also excluded based on the results of this study. Additionally, Pearlman et al. rule out scenarios involving ultra-luminous X-ray binaries or a young neutron star embedded in a Crab Nebula-like remnant based on their persistent X-ray limits from FRB 20200120E.

This study marks the first time many of these scenarios have been ruled out for an FRB source, thanks to major contributions from NICER. The study brings us a step closer to uncovering the types of sources responsible for FRB emissions. Pearlman et al. discuss the prospects for detecting X-ray emission from other FRB sources, using existing and planned X-ray telescopes, and show that future observations with NICER will greatly aid in illuminating the origins of FRBs.