The Epoch of Reionization 21 cm Bispectrum at $z=8.2$ from MWA data II: Smooth Component Filtering

The 21 cm bispectrum (BS) offers a powerful probe of the Epoch of Reionization (EoR), but its observational access is severely hindered by dominant astrophysical foregrounds. Considering Murchison Widefield Array (MWA) observations at $154.2~\mathrm{MHz}$ ($z=8.2$), we mitigate the foregrounds with Smooth Component Filtering (SCF) and estimate the 21 cm BS. We validate the pipeline using a simulated 21 cm signal and show that the input BS is recovered for modes $k_{\parallel} \ge [k_\parallel]f=0.135~{\rm Mpc}^{-1}$. Applied to actual data, the SCF produces substantial foreground suppression, reducing the amplitude of the cylindrical BS $B(k{1\perp},k_{2\perp},k_{3\perp},k_{1\parallel},k_{2\parallel})$ by $3-4$ orders of magnitude. The artifacts due to the missing frequency channels in the data are also suppressed. The resulting EoR window is significantly cleaner at small $k_{\perp}$. We adopt the region $(k_{1 \perp},k_{2 \perp},k_{3 \perp})\leq 0.026~{\rm Mpc}^{-1}$ and $(k_{1\parallel},k_{2\parallel},k_{3\parallel})>0.135~{\rm Mpc}^{-1}$ to evaluate the 3D spherical BS and constrain the EoR signal. By combining estimates over all triangle shapes, we place the lower and upper limits on the mean cube brightness temperature fluctuations $Δ^3$. The estimates are consistent with statistical fluctuations from system noise. The most stringent lower limit $Δ^3_{\rm LL}=-(1.25\times 10^4)^3~{\rm mK}^3$ and upper limit $Δ^3_{\rm UL}=(1.22\times 10^4)^3~{\rm mK}^3$ are obtained at $k_1=0.281~{\rm Mpc}^{-1}$. Additional observing time will reduce the noise level and enable substantially tighter constraints on the EoR signal.

💡 Research Summary

This paper presents the first detailed measurement of the three‑dimensional 21 cm bispectrum (BS) from Murchison Widefield Array (MWA) Phase II drift‑scan data at a central frequency of 154.2 MHz (corresponding to redshift z = 8.2). The bispectrum, as the lowest‑order statistic sensitive to non‑Gaussianity, can capture mode‑coupling information that the power spectrum (PS) alone cannot, making it a powerful probe of the ionisation topology during the Epoch of Reionization (EoR). However, foreground emission from Galactic synchrotron radiation and extragalactic radio sources is 4–5 orders of magnitude brighter than the expected 21 cm signal, and the MWA data suffer from a periodic pattern of missing frequency channels (five flagged channels every 1.28 MHz), which together cause severe foreground leakage into the so‑called “EoR window”.

To mitigate these issues the authors adopt Smooth Component Filtering (SCF), a technique previously introduced for power‑spectrum analyses (Elahi et al. 2025). After gridding the calibrated visibilities onto a regular u‑v plane, each visibility spectrum Vg(ν) is convolved with a Hann window of width 2 MHz (N = 50 channels). This extracts a smooth component Vs,g(ν) that captures the slowly varying foreground contribution. The filtered visibility is then defined as Vf,g(ν) = Vg(ν) − Vs,g(ν). The convolution is normalized by the flagging function to avoid bias from missing channels, and the first and last N channels are discarded to eliminate edge effects, leaving an effective bandwidth of 26.72 MHz.

The pipeline (dubbed AMBER) proceeds in three steps: (I) gridding of visibilities, (II) estimation of the multi‑frequency angular bispectrum (MABS) from three‑point visibility correlations, and (III) a 2‑D Fourier transform of the MABS to obtain the 3‑D cylindrical bispectrum B(k1⊥,k2⊥,k3⊥,k1‖,k2‖). A Blackman–Nuttall window is applied in the frequency domain to suppress ringing caused by the finite bandwidth. The authors first validate the SCF+AMBER pipeline using simulated 21 cm data injected into realistic MWA noise and flagging patterns. They demonstrate that for line‑of‑sight wavenumbers k‖ ≥