Trigger detected by Fermi, Integral (though only ~ 3 sigma Integral (GCN 21507) I doubt this was a GCN socket trigger). Ligo Virgo Collaboration reported Aug 17 12:41:06 GMT 2017 (GCN at 13:21).

The EM counterpart was discovered by the One-Meter Two-Hemisphere (1M2H) collaboration, GRB Coordinates Network 21529 (2017), and D. A. Coulter, et al., Science, doi:10.1126/science.aap9811 (2017).

By 18:55 (GCN21516) they nailed it down to 15 galaxies in the GLADE catalog GLADE website: http://aquarius.elte.hu/glade/. The GLADE folks say (https://arxiv.org/pdf/1804.05709.pdf) that EM signals from NS-NS GW events are most likely from galaxies of high B luminosity (star formation) or from high stellar mass, so some weighted combination of these properties makes for the best ranking of targets.

Discovery optical Observations ~ 11 hr after trigger.

First optical spectra 11.75 hrs after spectra Shappee+17; Earliest NIR spectrum 1.5 d, CHornock+17.

Spectra

All about the Spectra in Shappee+17 https://arxiv.org/pdf/1710.05432.pdf

Arcavi18 https://iopscience.iop.org/article/10.3847/2041-8213/aab267/pdf says:

• UV in the first ~ 1-4 hr is critical in distinguishing models because the later models are degenerate (1)
• The discovery was postponed to 5 hr because of a “glitch” in one of the GW detectors (but BG notes this depended on weather, availability of SA telescope).

(1) “The combined optical data show an early rise consistent with radioactive decay of low-opacity ejecta as the main emission source, but the subsequent decline is fit well by all models.”