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Selection bias obfuscates the discovery of fast radio burst sources

Nature volume 634, pages 1065–1069 (2024)Cite this article

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Abstract

Fast radio bursts (FRBs) are a newly discovered class of extragalactic radio transients characterized by their high energy and short duration (from microseconds to milliseconds)1. The physical origin of these FRBs remains unknown and is a subject of ongoing research, with magnetars emerging as leading candidates2,3. Previous studies have used various methodologies to address the problem of FRB origin, including demographic analyses of FRB host galaxies and their local environments4,5,6, assessments of FRB rate evolution with redshift7,8,9 and searches for proposed multi-messenger FRB counterparts10. However, these studies are susceptible to substantial biases stemming from unaccounted radio and optical selection effects. Here we present empirical evidence for a substantial selection bias against detecting FRBs in galaxies with large inclination angles (edge-on) using a sample of hosts identified for FRBs discovered by untargeted surveys. This inclination-related bias probably leads to a significant underestimation (by about a factor of two) of the FRB rates reported in the literature and disfavours globular clusters as the dominant origin of FRB sources, as previously speculated6. These conclusions have important implications for FRB progenitor models and targeted FRB follow-up strategies. We further investigate the impact of this bias on the relative rate of FRBs in different host environments. Our analysis suggests that scattering in FRB hosts is probably responsible for the observed bias11,12. However, a larger sample of localized FRBs is required to robustly quantify the contribution of scattering to the inclination-related selection bias.

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Fig. 1: CDF of inclination angles for FRB hosts and SDSS galaxies.
Fig. 2: FRB host galaxy properties across bins with different inclination angles.

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Data availability

The data that support the plots in this paper and other findings of this study are available from the corresponding author upon reasonable request. The Pan-STARRS DR1, SDSS DR16 and DESI optical r-band imaging data used in this analysis can be accessed from the publicly available MAST SDSS Data Archive Server, MAST PS1 Science Archive Server and DESI Legacy Imaging Survey interface, respectively, or can be downloaded from GitHub (https://github.com/Astronomer-Mohit/Bhardwajetal_2024_nature_inclination_angle).

Code availability

The following software packages were used to analyse the data presented in this paper: APLpy69, astropy70, AutoProf19, Photutils20 and Scipy71. The codes used for data processing and producing the figures are available from GitHub (https://github.com/Astronomer-Mohit/Bhardwajetal_2024_nature_inclination_angle) or from the corresponding author upon reasonable request.

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Acknowledgements

We thank D. Lang for the discussions. We also thank C. Stone for assisting with the profile-fitting routines of AutoProf. This research used Photutils, an Astropy package for the detection and photometry of astronomical sources. M.B. is a McWilliams fellow and an International Astronomical Union Gruber fellow. M.B. receives support from the McWilliams seed grant.

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Author notes

  1. These authors contributed equally: Mohit Bhardwaj, Jimin Lee

Authors and Affiliations

  1. McWilliams Center for Cosmology, Department of Physics, Carnegie Mellon University, Pittsburgh, PA, USA

    Mohit Bhardwaj, Jimin Lee & Kevin Ji

Authors
  1. Mohit Bhardwaj
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  2. Jimin Lee
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Contributions

M.B. conceived the methodology and framework of this work. K.J. conducted the initial analysis for this project under the supervision of M.B. However, all the results presented in this work were refined by J.L. with the assistance of M.B. M.B. led the writing of this paper, with J.L. contributing to the Methods section.

Corresponding author

Correspondence to Mohit Bhardwaj.

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Nature thanks James Cordes, Jason Hessels and Maura McLaughlin for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Fig. 1 Parametric profile fitting results for 23 FRB host galaxies using the AutoProf package.

For each galaxy, we show three images - (left) r-band image of the host, (center) best-fitted galaxy model, and (right) resultant residual. The numbers in the top left of each r-band image correspond to the host number and the estimated inclination angle (in degrees, quoted in parentheses), as shown in Table 1. The X-axis and Y-axis of all plots represent right ascension and declination, respectively.

Extended Data Fig. 2 Non-parametric profile fitting results for 23 FRB host galaxies using the Photutils package.

For each galaxy, we show three images - (left) r-band image of the host, (center) best-fitted galaxy model, and (right) resultant residual. The numbers in the top left of each r-band image correspond to the host number and the estimated inclination angle (in degrees, quoted in parentheses), as shown in Table 1. The X-axis and Y-axis of all plots represent right ascension and declination, respectively.

Extended Data Fig. 3 Histogram of Cosine of Inclination Angles for SDSS-DR16 Galaxies.

This histogram illustrates the distribution of cos(i) values for galaxies sampled from the SDSS-DR16 catalog, following the selection criteria described in the Methods section.

Extended Data Fig. 4 Comparison of Cosine of Inclination Angle (cos(i)) CDFs for SDSS Galaxies and FRB Hosts.

The top plot shows the mean \(\cos (i)\) CDF for randomly sampled SDSS galaxies, employing a methodology similar to that used for Fig. 1, albeit with variations in u − r color thresholds. Similarly, the bottom left plot illustrates the impact on the mean \(\cos (i)\) CDF of SDSS galaxies for different mr thresholds. Finally, the bottom right plot displays the \(\cos (i)\) CDF where SDSS galaxies are sampled in each iteration to match the mr distribution of our FRB host sample. The blue shaded region represents the 68% credible bound on the SDSS galaxy CDF to account for the small sample size. In all cases, the SDSS galaxy CDFs are found to be statistically different from the \(\cos (i)\) CDF of FRB hosts in our sample, depicted in red in all three plots.

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Bhardwaj, M., Lee, J. & Ji, K. Selection bias obfuscates the discovery of fast radio burst sources. Nature 634, 1065–1069 (2024). https://doi.org/10.1038/s41586-024-08065-w

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