Difference: GCNCIRCULAR21541 ( vs. 1)

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.

-- ThomasKupfer - 18 Aug 2017