TITLE:   GCN CIRCULAR
NUMBER:  21541
SUBJECT: LIGO/VIRGO G298048: The Potential Optical Counterpart in NGC 4993 Has Afterglow-Like Colors
DATE:    17/08/18 04:44:32 GMT
FROM:    Philip Cowperthwaite at Harvard U  <pcowperthwaite@cfa.harvard.edu>

M. Nicholl (Harvard), P. S. Cowperthwaite (Harvard), S. Allam (Fermilab),
J. Annis (Fermilab), E. Berger (Harvard), D. J. Brout (UPenn), D. Brown
(Syracuse), R. E. Butler (Fermilab), H.-Y. Chen (Harvard), R. Chornock
(Ohio University), E. Cook (TAMU), , H. T. Diehl (Fermilab), A.
Drlica-Wagner (Fermilab), M. R. Drout (Carnegie), R. J. Foley (UCSC), W.
Fong (Northwestern), D. Fox (Penn State), J. Frieman (Fermilab/UChicago),
M.S.S. Gill (Stanford), R. Gruendl (NCSA), K. Herner (Fermilab), D. Holz
(UChicago), R. Kessler (UChicago), R. Margutti (Northwestern), J. Marshall
(TAMU), E. Neilsen (Fermilab), F. Paz-Chincon (NCSA), A. Rest (STScI), M.
Sako (UPenn), N. Smith (Arizona), M. Soares-Santos (BrandeisU), D. Tucker
(Fermilab), V. A. Villar (Harvard), A. Walker (NOAO), P. K. G. Williams
(Harvard), B. Yanny (Fermilab), P. Lopes (UFRJ), F. Durret (IAP), A.
Lourenço (UFRJ)

On behalf of the DESGW+community team:

We report preliminary optical i- and z-band photometry from DECam imaging of
the possible optical counterpart to G298048 first identified in Coulter et
al. (LVC GCN 21529),  and further reported on by Allam et al. (LVC GCN
21530), Valenti et al. (LVC GCN 21531), Melandri et al. (LVC GCN 21532) and
Arcavi et al. (GCN 21538). Observations were obtained on 2017-08-18 at
00:04 UTC.

We find preliminary PSF magnitudes of

i = 17.5 +/- 0.1 mag
z = 17.5 +/- 0.1 mag

This suggests an i-z color of 0 mag, which is consistent with the expected
colors of a Short GRB afterglow, but not for lanthanide dominated kilonova
emission (e.g., Barnes & Kasen 2013, Barnes et al. 2016, Wollaeger et al.
2017).

These magnitudes are determined from difference PSF photometry of DECam
images relative to Pan-STARRs 3pi catalogs.

Further analysis is ongoing.

[GCN OPC NOTE(18aug17):  Per author's request, the trigger number
in the Subject-line was changed from 298045 to 298048.]