Well at the beginning of the 97 he was named as governor of Germania superior, I think he was there in October when adopted, with his adoption he received imperium proconsulare and tribunicia potestas as well as the tittle of Caesar and the byname Germanicus, see also Panegyricus of Plinius. In 98 he was named as Consul, at 28 Jan 98 when Nerva died he was in Köln (Cologne), he got the news from Hadrian seems to. He came in Rome 2 years later. Nigrinus was in Syria at the same time.
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would read this:
Werner Eck: An Emperor is Made. Senatorial Politics and Trajan’s Adoption by Nerva in 97. In: Gillian Clark, Tessa Rajak (Hrsg.): Philosophy and Power in the Graeco-Roman World. Oxford University Press, Oxford 2002, S. 211–226.
Miriam Griffin: Trajan. In: Alan K. Bowman, Peter Garnsey und Dominic Rathbone (Hrsg.): The Cambridge Ancient History 11. The High Empire, A. D. 70–192. Cambridge University Press, Cambridge 2000, ISBN 0-521-26335-2, S. 96–131.
here is a translation od Dio
http://penelope.uchicago.edu/Thayer/E/Ro...o/68*.html
and some online reading
http://www.luc.edu/roman-emperors/trajan.htm