What do the acronyms in "AT 2017 gfo" mean? About 11 hours after the detection of GW170817 by Ligo/Virgo and the quickly succeeding detected GRB170817, a transient afterglow in the optical-nIR frequency appeared which showed many signs of r-process nucleosynthesis. This transient was given the identification AT 2017 gfo.


  "SSS17a, now with the IAU identification of AT 2017 gfo"


My questions are simple. What do the acronyms "AT" and "gfo" mean? And for that matter, what does the acronymn "SSS" stand for? I tried to do a quick search through the IAU website but all I could find were general instructions on how to come up with your own astronomical identifier. I suspect "AT" means astronomical transient, but I can not confirm.
You are correct. The AT is short for Astronomical Transient. The follows the year and an arbitary sequence of letters to uniquely identify the event, which is assigned by the Transient Name Server

This is the official IAU designation for Supernova Candidates, however true supernovae will be re-designated "SN YYYYX". The neutron star merger was not a supernova, so retains its AT designation

This is described at (source) with more details at Transient Name Server (TNS)