(Oxford University Press (OUP), 2013) Hartoog, O. E.; Wiersema, K.; Vreeswijk, P. M.; Kaper, L.; Tanvir, N. R.; Savaglio, S.; Berger, Edo; Chornock, R; Covino, 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.
For 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.
