The monopole and quadrupole moments of the epoch of reionization (EoR) 21-cm bispectrum.

We study the monopole (⁠|$\bar{B}^0_0$|⁠) and quadrupole (⁠|$\bar{B}^0_2$|⁠) moments of the 21-cm bispectrum (BS) from EoR simulations and present results for squeezed and stretched triangles. Both |$\bar{B}^0_0$| and |$\bar{B}^0_2$| are positive at the early stage of EoR where the mean neutral hydrogen (⁠|${\rm H\, {\small I}}$|⁠) density fraction |$\bar{x}_{\rm {\rm H\, {\small I}}}\approx 0.99$|⁠. The subsequent evolution of |$\bar{B}^0_0$| and |$\bar{B}^0_2$| at large and intermediate scales (k  = 0.29 and |$0.56 \, {\rm Mpc}^{-1}$|⁠ , respectively) is punctuated by two sign changes which mark transitions in the |${\rm H\, {\small I}}$| distribution. The first sign flip where |$\bar{B}^0_0$| becomes negative occurs in the intermediate stages of EoR |$(\bar{x}_{\rm {\rm H\, {\small I}}}\gt 0.5)$|⁠ , at large scale first followed by the intermediate scale. This marks the emergence of distinct ionized bubbles in the neutral background. |$\bar{B}^0_2$| is relatively less affected by this transition, and it mostly remains positive even when |$\bar{B}^0_0$| becomes negative. The second sign flip, which affects both |$\bar{B}^0_0$| and |$\bar{B}^0_2$|⁠ , occurs at the late stage of EoR |$(\bar{x}_{\rm {\rm H\, {\small I}}}\lt 0.5)$|⁠. This marks a transition in the topology of the |${\rm H\, {\small I}}$| distribution, after which we have distinct |${\rm H\, {\small I}}$| islands in an ionized background. This causes |$\bar{B}^0_0$| to become positive. The negative |$\bar{B}^0_2$| is a definite indication that the |${\rm H\, {\small I}}$| islands survive only in underdense regions. [ABSTRACT FROM AUTHOR]