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The Real and Pseudo Dispersion Measures of FRB~20220912A

Yi Feng, Dengke Zhou, Y. -K. Zhang, D. Li, Jianhua Fang, Jiaying Xu, Chenyuan Xu, Jintao Xie

Abstract

Fast radio bursts (FRBs) are millisecond-duration radio transients. As they propagate through the interstellar medium, they interact with free electrons, resulting in dispersion. The corresponding dispersion measure (DM) is referred to as the real DM (DM$_{\rm real}$). In practice, however, the dispersion measure derived from modeling (DM$_{\rm model}$) is often contaminated by intrinsic burst morphology, giving rise to a pseudo DM component (DM$_{\rm pseudo} = {\rm DM}_{\rm model} - {\rm DM}_{\rm real}$). In this work, we focus on the highly active repeating FRB~20220912A and utilize its microshots -- extremely short-duration (typically tens of microseconds), broadband emissions -- to investigate its DM$_{\rm real}$ and DM$_{\rm pseudo}$. We adopt two assumptions: first, that FRB~20220912A resides in a non-magneto-ionic environment and that its DM$_{\rm real}$ variation is smaller than $10^{-2}$\,pc\,cm$^{-3}$ over a few years; and second, that microshots have a negligible intrinsic morphological time delay. By identifying two new microshots and combining them with previously reported ones, we find that all four microshots exhibit remarkably consistent DM values over a one-month timescale, with an average of $219.380 \pm 0.004\,\mathrm{pc\,cm^{-3}}$. We define this value as the DM$_{\rm real}$ of FRB~20220912A. We further show that bright, narrow bursts with a width of less than 2\,ms also yield DM estimates consistent with the microshot-based DM$_{\rm real}$. A survey of five repeating FRBs reveals that DM$_{\rm pseudo}$ is a common phenomenon, with variations typically spanning a range of approximately $10\,\mathrm{pc\,cm^{-3}}$ at 1.2\,GHz. These findings highlight the importance of accounting for morphological contributions in DM interpretation and demonstrate that microshots and narrow bursts are powerful tools for probing DM$_{\rm real}$.

The Real and Pseudo Dispersion Measures of FRB~20220912A

Abstract

Fast radio bursts (FRBs) are millisecond-duration radio transients. As they propagate through the interstellar medium, they interact with free electrons, resulting in dispersion. The corresponding dispersion measure (DM) is referred to as the real DM (DM). In practice, however, the dispersion measure derived from modeling (DM) is often contaminated by intrinsic burst morphology, giving rise to a pseudo DM component (DM). In this work, we focus on the highly active repeating FRB~20220912A and utilize its microshots -- extremely short-duration (typically tens of microseconds), broadband emissions -- to investigate its DM and DM. We adopt two assumptions: first, that FRB~20220912A resides in a non-magneto-ionic environment and that its DM variation is smaller than \,pc\,cm over a few years; and second, that microshots have a negligible intrinsic morphological time delay. By identifying two new microshots and combining them with previously reported ones, we find that all four microshots exhibit remarkably consistent DM values over a one-month timescale, with an average of . We define this value as the DM of FRB~20220912A. We further show that bright, narrow bursts with a width of less than 2\,ms also yield DM estimates consistent with the microshot-based DM. A survey of five repeating FRBs reveals that DM is a common phenomenon, with variations typically spanning a range of approximately at 1.2\,GHz. These findings highlight the importance of accounting for morphological contributions in DM interpretation and demonstrate that microshots and narrow bursts are powerful tools for probing DM.

Paper Structure

This paper contains 6 sections, 2 equations, 3 figures, 2 tables.

Table of Contents

  1. Introduction
  2. DM$_{\text{real}}$ Determination
  3. DISCUSSIONS
  4. narrow bursts
  5. DM$_{pseudo}$ in other repeating FRBs
  6. CONCLUSIONS

Figures (3)

  • Figure 1: Two microshots are shown side by side. For each event, the top panel shows the frequency-averaged pulse profile, the middle panel displays the dynamic spectrum (frequency versus time), and the bottom panel presents the DM–S/N curve. The red line indicates the best-fit model consisting of a Gaussian plus a constant baseline, while the green dashed line marks the best-fitting DM value.
  • Figure 2: DM$_{\text{model}}$ versus MJD for observations with GBT (squares), FAST (circles), and NRT (triangles). Green and blue markers indicate microshots and narrow bursts, respectively. Error bars denote 1$\sigma$ uncertainties. The dashed red horizontal line represents the weighted mean DM of the microshots, which we adopt as the DM$_{\text{real}}$. Right-hand black shaded distributions show the DM probability density functions for each day, estimated via Gaussian kernel density estimation from the measurements of 2023ApJ...955..142Z. Gray dashed lines with circular markers indicate the daily mean DM from the same reference. Only measurements from 2023ApJ...955..142Z with DM uncertainties $<$ 0.5 pc cm$^{-3}$ are included, yielding 766 data points.
  • Figure 3: $\mathrm{DM_{model}}$ distributions for five repeating FRBs on their burst-active days: FRB 20190520B (MJD 59208, 2026SciBu..71...76N), FRB 20201124A (MJD 59314, 2022Natur.609..685X), FRB 20121102A (MJD 58757, 2021Natur.598..267L), FRB 20220912A (MJD 59882, 2023ApJ...955..142Z), and FRB 20240114A (MJD 60382, 2025arXiv250714707Z). Each panel shows the one-dimensional kernel density estimate of $\mathrm{DM_{model}}$ from bursts detected within a single day, including only those with DM uncertainties $<0.5$ pc cm$^{-3}$. The black curve traces the kernel density estimate profile over the $\mathrm{DM_{model}}$ range. In each panel, the vertical axis denotes DM (in pc cm$^{-3}$), while the horizontal extent encodes the normalized density amplitude.