Hartoog, O. E.Wiersema, K.Vreeswijk, P. M.Kaper, L.Tanvir, N. R.Savaglio, S.Berger, EdoChornock, RCovino, S.D, V.Flores, H.Fynbo, J. P. U.Goldoni, P.Gomboc, A.Melandri, A.Pozanenko, A.Schaye, J.Postigo, A. d. U.Wijers, R. A. M. J.2017-02-272013Hartoog, O. E., K. Wiersema, P. M. Vreeswijk, L. Kaper, N. R. Tanvir, S. Savaglio, E. Berger, et al. 2013. The Host-Galaxy Response to the Afterglow of GRB 100901A. Monthly Notices of the Royal Astronomical Society 430, no. 4: 2739–2754. doi:10.1093/mnras/stt078.0035-8711http://nrs.harvard.edu/urn-3:HUL.InstRepos:30456075For Gamma-Ray Burst 100901A, we have obtained Gemini-North and Very Large Telescope optical afterglow spectra at four epochs: one hour, one day, three days and one week after the burst, thanks to the afterglow remaining unusually bright at late times. Apart from a wealth of metal resonance lines, we also detect lines arising from fine-structure levels of the ground state of Fe ii, and from metastable levels of Fe ii and Ni ii at the host redshift (z = 1.4084). These lines are found to vary significantly in time. The combination of the data and modelling results shows that we detect the fall of the Ni ii 4F9/2 metastable level population, which to date has not been observed. Assuming that the population of the excited states is due to the UV-radiation of the afterglow, we estimate an absorber distance of a few hundred pc. This appears to be a typical value when compared to similar studies. We detect two intervening absorbers (z = 1.3147, 1.3179). Despite the wide temporal range of the data, we do not see significant variation in the absorption lines of these two intervening systems.en-USgamma-ray burst: individual: GRB 100901Agamma-rays: burstsgalaxies: abundancesgalaxies: ISMgalaxies: distances and redshiftsJournal Article2017-02-2710.1093/mnras/stt078