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We discuss the signature of a cosmic string wake in 21cm redshift surveys.Since 21cm surveys probe higher redshifts than optical large-scale structuresurveys, the signatures of cosmic strings are more manifest in 21cm maps thanthey are in optical galaxy surveys. We find that, provided the tension of thecosmic string exceeds a critical value (which depends on both the redshift whenthe string wake is created and the redshift of observation), a cosmic stringwake will generate an emission signal with a brightness temperature whichapproaches a limiting value which at a redshift of $z + 1 = 30$ is close to 400mK in the limit of large string tension. The signal will have a specificsignature in position space: the excess 21cm radiation will be confined to awedge-shaped region whose tip corresponds to the position of the string, whoseplanar dimensions are set by the planar dimensions of the string wake, andwhose thickness (in redshift direction) depends on the string tension. Forwakes created at $z_i + 1 = 10^3$, then at a redshift of $z + 1 = 30$ thecritical value of the string tension $\mu$ is $G \mu = 6 \times 10^{-7}$, andit decreases linearly with redshift (for wakes created at the time of equalmatter and radiation, the critical value is a factor of two lower at the sameredshift). For smaller tensions, cosmic strings lead to an observableabsorption signal with the same wedge geometry.

The 21 cm Signature of Cosmic String Wakes