A Spatial Gap in the Sky Distribution of Fast Radio Burst Detections Coinciding with Galactic Plasma Overdensities

TL;DR

This work identifies a prominent detection gap in CHIME/FRB Catalog 2 coincident with the Cygnus X region, and shows that it cannot be explained by exposure or sky temperature alone. Through a combination of geometric gap statistics, sky-model simulations, and comparisons to Planck EM maps and angular broadening measurements, the authors argue that enhanced Galactic scattering in Cygnus X suppresses CHIME-detectable FRBs with a data-driven lower limit of $\tau_{\rm sc,1GHz} \ge 5.59$ ms. The analysis integrates EM and DM considerations against Galactic electron density models, finding consistency with high scattering in regions with elevated EM while highlighting limitations of $n_e$ models. The results establish FRBs as model-independent probes of the warm ionized medium in the Milky Way and point toward future VLBI-enabled Outriggers to map scattering screens and refine ISM models.

Abstract

We analyze the positional and morphological properties of about 3600 unique fast radio burst (FRB) sources reported in the second FRB catalog generated by the Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope. We find a two-dimensional dependence of FRB detections on sky position, and identify a significant absence of detections in a roughly circular region centered at Galactic coordinates (77.7$^\circ$, 0.9$^\circ$), spanning an area of 213.6 deg$^2$. This detection gap spatially coincides with the Cygnus X region $--$ a plasma-rich star-forming region in the Milky Way. This feature is most likely the result of increased sky temperature and strong multi-path scattering by turbulent ionized plasma, which broadens the FRB signals beyond detectability in the CHIME band. Our simulations yield a mean of 6 expected FRB detections within the gap when accounting for the elevated sky temperature in the direction of the detection gap. We infer that a lower limit of the maximum scattering timescale $τ_{\rm sc,\, 1\,GHz} \geq 5.59$ ms, obtained without assuming a model of the Galactic electron distribution, is sufficient to suppress the brightness of all coincident FRBs. A similar suppression is seen in Catalog 2 along other high-emission measure (EM) sightlines (i.e., EM$\geq$2900 pc cm$^{-6}$), further supporting a broader trend of suppression due to Galactic scattering. Future very long baseline interferometry (VLBI) measurements of scattering disks with CHIME Outriggers could help confirm our interpretation. Our work highlights the notion that FRBs can serve as new, model-independent tracers of the warm ionized medium within our Milky Way Galaxy.

Figures (10)

• Figure 1: Positions of 3542 FRBs in our sample overlaid with the Planck emission measure map planckforegrounds16. We qualitatively see an anti-correlation between FRB detections and regions of high EM. The detection gap is highlighted with dashed cyan lines. A zoomed-in view of this region is presented in Figure \ref{['fig:literature_scattering']}.
• Figure 2: Distribution of $R_{\rm LEC}$ estimates obtained from a Delauney-triangulation analysis of the Catalog 2 data (red) and simulations of sky positions of FRBs in $N_{\rm sim} = 10^4$ catalogs derived from the statistics of Catalog 2 detections (blue-filled). The black-dashed vertical line indicates the largest measured $R_{\rm LEC}$ in Catalog 2.
• Figure 3: A comparison of literature values of scattering in and surrounding the Cygnus X region, as described in §\ref{['sec:cygnus']}, overlaid on the Planck emission measure map planckforegrounds16. The angular broadening measurements for quasars and Cygnus X-3 have been converted to scattering timescales using Equation \ref{['eq:tauscat_thetafwhm']}, and scaled to 600 MHz using $\theta \propto \nu^{-2}$. Scattering measurements for pulsars and FRB 20210705 are extrapolated to 600 MHz using $\tau \propto \nu^{-4}$. This shows that sightlines through the Cygnus X region can produce $\sim 10-1,000\,$ms of temporal scattering in CHIME's observing band, which would the hinder detection of an FRB (see §\ref{['sec:scatteringconstraint']}). A white polygon encloses the area with zero FRB detections, formed by Delaunay triangulation as described in §\ref{['sec:void-significance']}.
• Figure 4: Distribution of scattering timescales at 1 GHz that result in zero detectable FRBs in the gap across 10$^6$ simulations, incorporating elevated sky temperature as described in §\ref{['sec:skytemperature']}. Vertical dashed lines indicate scattering predictions from the NE2001 model (black) and YMW16 model (red) for the line of sight along the center of the gap.
• Figure 5: Planck EM map (planckforegrounds16) in Galactic coordinates centered at $l = 80\degree$, $b = 0\degree$ (top) and $l = 180\degree$, $b = 0\degree$ (bottom). Overlaid black contours trace logarithmically-spaced EM 10, 100 and 1,000 $\text{ pc cm}^{-6}$. White solid and dashed circles highlight WISE H II regions wise with angular radii $\theta_\text{IR}$ and $2\theta_\text{IR}$, respectively. The radius of the IR emission $\theta_\text{IR}$ is provided by the WISE Catalog. Lime markers are for FRB LOS with intersecting H II regions and skyblue markers are for non-intersecting FRBs. Cross markers indicate FRBs for which no reliable scattering timescale was measured with total-intensity data, while the radii of circular markers are scaled according to the log of the measured scattering timescale.