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We have used the 610 MHz receivers of the Giant Metrewave Radio Telescope(GMRT) to detect associated HI 21cm absorption from the $z = 1.2230$ blazarTXS1954+513. The GMRT HI 21cm absorption is likely to arise against either themilli-arcsecond-scale core or the one-sided milli-arcsecond-scale radio jet,and is blueshifted by $\approx 328$ km s$^{-1}$ from the blazar redshift. Thisis consistent with a scenario in which the HI cloud giving rise to theabsorption is being driven outward by the radio jet. The integrated HI 21cmoptical depth is $(0.716 \pm 0.037)$ km s$^{-1}$, implying a high HI columndensity, $N_{\rm HI} = (1.305 \pm 0.067) \times ({\rm T_s/100\: K}) \times10^{20}$ cm$^{-2}$, for an assumed HI spin temperature of 100 K. We use NickelTelescope photometry of TXS1954+513 to infer a high rest-frame 1216 \AA\luminosity of $(4.1 \pm 1.2) \times 10^{23}$ W Hz$^{-1}$. The $z = 1.2230$absorber towards TXS1954+513 is only the fifth case of a detection ofassociated HI 21cm absorption at $z > 1$, and is also the first case of such adetection towards an active galactic nucleus (AGN) with a rest-frameultraviolet luminosity $\gg 10^{23}$ W Hz$^{-1}$, demonstrating that neutralhydrogen can survive in AGN environments in the presence of high ultravioletluminosities.

Giant Metrewave Radio Telescope detection of associated HI 21cm absorption at $z = 1.2230$ towards TXS1954+513