We investigate the detectability of leading-order relativistic effects in the bispectrum of future 21cm intensity mapping surveys. The relativistic signal arises from Doppler and other line-of-sight effects in redshift space. In the power spectrum of a single tracer, these effects are suppressed by a factor Script H2/k2. By contrast, in the bispectrum the relativistic signal couples to short-scale modes, leading to an imaginary contribution that scales as Script H/k, thus increasing the possibility of detection. Previous work has shown that this relativistic signal is detectable in a Stage IV Hα galaxy survey. We show that the signal is also detectable by next-generation 21cm intensity maps, but typically with a lower signal-to-noise, due to foreground and telescope beam effects.
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