Constraining the Evolution of the HI Spin Temperature with Fast Radio Bursts

TL;DR

The study proposes using HI absorption in FRB signals to constrain the HI spin temperature $T_{\text{spin}}$ in FRB host galaxies by comparing absorption signals with high-resolution HI emission maps. It develops the theoretical framework, linking $T_{\text{spin}}$ to observable quantities via $N_{\text{HI}} = 1.823\times 10^{18} T_{\text{spin}} \int \tau(\nu) d\nu$ and the sensitivity limit $L_{3\sigma}$ for FRB absorption detections, and tests the concept with FRB 20211127I, obtaining a 3σ upper limit on the integrated optical depth of $\approx 33$ km s$^{-1}$ and a 3σ lower limit on $T_{\text{spin}}$ of about $26$ K. The authors evaluate detectability with current facilities (ASKAP, DSA, MeerKAT, FAST) and forecast gains for future instruments (SKA-Mid, expanded DSA), showing that bright non-repeating FRBs could probe $\int \tau$ down to $\sim 5$ km s$^{-1}$, while stacking thousands of bursts from repeating FRBs with FAST could reach much tighter limits. This framework offers a physical anchor for locating FRBs within their hosts, aids in disentangling host Dispersion Measure contributions, and enables a redshift-dependent census of $T_{\text{spin}}$ across cosmic time, with substantial implications for understanding the multi-phase ISM in galaxy evolution.

Abstract

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 $T_{spin}$, 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, we demonstrate a proof of concept with the bright (F = 35 Jy ms) and narrow (0.2 ms) FRB 20211127I detected by ASKAP. We find a 3$σ$ upper limit on the integrated optical depth in the pulse-averaged spectrum of 33 km s$^{-1}$, and, based on the HI emission observed in a 3 hr MeerKAT L-band observation, subsequently find a lower limit on $T_{spin}$ of 26 K. While this test case provides little constraining power, we find that narrow, non-repeating FRBs with fluences greater than 20/70/150 Jy ms observed with all dishes with the current MeerKAT/ASKAP/DSA telescopes can probe integrated optical depths below 5 km s$^{-1}$. Furthermore, we 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 $T_{spin}$, can be measured. Finally, we 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.

Figures (4)

• Figure 1: (Left) Dynamic spectrum of FRB 20211127I. The right panel displays the spectrum of the FRB averaged across its temporal pulse width. The location of the redshifted H i 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. (Right) VLT $i$-band image of the host galaxy of FRB 20211127I overlaid with its H i emission as seen by MeerKAT (white contours) and the localisation region in green. Contours increase as [1,1.25,1.5,...] $\times$ 5.34 $\times 10^{20}$ cm$^{-2}$ and the beam is shown in the bottom right.
• Figure 2: (Top) Normalised H i emission measured by MeerKAT at the pixel closest to the localisation region. The raw data is overlaid with a Gaussian profile fit to the binned data. (Bottom) Flattened and normalised FRB spectrum at the location of the H i line. For visual purposes, the spectrum is overlaid with a binned spectrum at a velocity resolution of 11 km s$^{-1}$ in the rest frame and the 2$\sigma$ deviation from unity is shown. Due to scintillation, the SNR of the spectrum increases towards the left.
• Figure 3: 3$\sigma$ limits on the integrated H i optical depth detectable in the pulse-averaged spectra of non-repeating FRBs observed by various telescopes. The black dashed lines in each panel indicate a limit of 5 km s$^{-1}$. These limits assume maximal sensitivity (i.e. with full antenna configuration) and coherent beamforming, an H i linewidth of 50 km s$^{-1}$, and a flat FRB spectrum. Overlaid are samples of reported FRBs with known fluences and widths: ASKAP Shannon2024, DSA Law2024, FAST Zhu2020Niu2021Zhou2023, and MeerKAT Rajwade2022Jankowski2023Driessen2024. ASKAP is divided in colour based on the central frequency of the band used to observe the FRB. In all panels, points with red borders indicate FRBs with known localisations to galaxies with redshifts below 0.1.
• Figure 4: Average burst fluence required for FAST to detect various integrated H i optical depths as a function of the number of bursts stacked from a repeating FRB.