TITLE:   GCN CIRCULAR
NUMBER:  21581
SUBJECT: LIGO/Virgo G298048: Rapid Evolution of Possible Counterpart
DATE:    17/08/19 01:26:07 GMT
FROM:    Iair Arcavi at LCOGT  <iarcavi@lcogt.net>

I. Arcavi, D. A. Howell, C. McCully, G. Hosseinzadeh, S. Vasylyev (UCSB/Las
Cumbres Obs), D. Poznanski, M. Zalzman (TAU), L.P. Singer (NASA/GSFC), S.
Valenti (UC Davis), T. Piran (HUJI), D. Kasen, J. Barnes (UC Berkeley), W.
Fong (Northwestern) and D. Maoz (TAU) report the sudden fading of SSS17a /
DLT17ck, the possible optical counterpart reported by Coulter et al. (LVC
GCN 21529) and others.

In the course of Las Cumbres Observatory LIGO followup we have now
accumulated four epochs of the possible counterpart SSS17a / DLT17ck in the
last 24 hours from our telescopes in the Cerro Tololo Inter-American
Observatory (Chile), the Siding Spring Observatory (Australia) and the
South African Astronomical Observatory (South Africa).

Using aperture photometry in the wide w-band (=g+r+i), we see the transient
first brighten by 0.12 magnitudes (as reported in Arcavi et al. LVC GCN
21565) and then fade by ~0.2 magnitudes:

Time (UT)         Site          Relative w-band Magnitude
2017-08-18 00:15  Chile         0
2017-08-18 09:10  Australia     -0.12 +- 0.02
2017-08-18 17:35  South Africa  +0.19 +- 0.04
2017-08-18 23:13  Chile         +0.22 +- 0.13

Further observations (including other bands) and analysis are ongoing.