Fast radio bursts (FRBs) emit broad band radio wave radiation that may, in rare cases, encode atomic hydrogen (HI) absorption signals produced as they traverse the interstellar medium of their host galaxies. Combining such signals with high resolution HI emission maps offers a unique opportunity to probe the dynamics of neutral gas at cosmological distances through constraints of the HI excitation temperature, which characterises the balance of neutral gas phases and the underlying thermal processes within these galactic environments. While no absorption signal has been recorded in an FRB to date, Roxburgh et al. demonstrate a proof of concept with the bright (F = 35 Jy ms) and narrow (0.2 ms) FRB 20211127I detected by ASKAP. While this test case provides little constraining power, the team find that narrow, non-repeating FRBs observed with all dishes with the current MeerKAT, ASKAP, or DSA telescopes can probe integrated optical depths below 5 km s⁻¹. Furthermore, they highlight that utilising FAST’s incredible sensitivity to stack thousands of bursts from hyperactive repeaters also provides a plausible avenue through which HI absorption, and hence spin temperature, can be measured. Finally, they discuss how HI absorption can address several modern challenges in FRB science, providing a physical anchor for locating bursts within their host galaxies and helping to disentangle the host contribution to dispersion and scattering.

The figure above shows at left the ASKAP Dynamic spectrum of FRB 20211127I. The right-hand panel displays the spectrum of the FRB averaged across its temporal pulse width. The location of the redshifted HI line is shown by the horizontal dashed line, and the band-averaged flux density is denoted by the vertical dashed line in the right panel.  On the right is a VLT i-band image of the host galaxy of FRB 20211127I overlaid with its HI emission as seen by MeerKAT (white contours) and the localisation region in green. The array beam is shown in the bottom right.