TITLE:   GCN CIRCULAR
NUMBER:  21580
SUBJECT: LIGO/Virgo G298048: Observed fading of optical counterpart SSS17a
DATE:    17/08/19 00:22:23 GMT
FROM:    Philip Cowperthwaite at Harvard U  <pcowperthwaite@cfa.harvard.edu>

M. Nicholl (Harvard), P. S. Cowperthwaite (Harvard), E. Berger (Harvard),
P. K. G. Williams (Harvard), S. Allam (Fermilab), J. Annis (Fermilab), J.
Garcia Bellido (IFT CSIC/UAM), 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), Z. Doctor (U. Chicago), M. R. Drout
(Carnegie), B. Farr (U Chicago), 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), H. Lin (Fermilab), J. Marriner (Fermilab), R.
Margutti (Northwestern), J. Marshall (TAMU), E. Neilsen (Fermilab), F.
Paz-Chincon (NCSA), A. Rest (STScI), M. Sako (UPenn), D. Scolnic (KICP), N.
Smith (Arizona), M. Soares-Santos (BrandeisU), D. Tucker (Fermilab), V. A.
Villar (Harvard), A. Walker (NOAO), B. Yanny (Fermilab), P. Lopes (UFRJ),
F. Durret (IAP), A. Lourenço (UFRJ)

On behalf of the DESGW+community team:

We report further 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 2017-08-19 at
23:23 UTC.

We find preliminary PSF magnitudes of

i = 17.8 +/- 0.1 mag
z = 17.6 +/- 0.1. mag

In images obtained the previous night (2017-08-18 at 00:04 UT), we measured
i ~ z ~ 17.5 mag (Nicholl et al., LVC GCN 21541). This indicates a fading
of
~0.3 mag in i-band over the course of 1 day. This value is consistent with
that
expected for an afterglow or rapidly-evolving kilonova.

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

Further analysis is ongoing.