Difference: GCNCIRCULAR21581 ( vs. 1)

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.

-- ShaonGhosh - 19 Aug 2017