E. Pian (INAF-IASF Bo), V. D'Elia (INAF-ASDC), S. Piranomonte (INAF-OAR), M. Branchesi (GSSI), S. Campana (INAF-OAB), E. Cappellaro (INAF-OAPD), S. Covino (INAF-OAB), P. D'Avanzo (INAF-OAB), A. Grado (INAF-OAC), G. Greco (Urbino University/INFN Firenze), A. Melandri (INAF-OAB), E. Palazzi (INAF-IASF Bo), G. Stratta (Urbino University/INFN Firenze), L . Tomasella (INAF-OAPD), L. Amati (INAF-IASF Bo), L. A. Antonelli, (INAF-OAR), S. Ascenzi (INAF-OAR), S. Benetti (INAF-OAPD), M.T. Botticella (INAF-OAC), D. Fugazza, F. Getman (INAF-OAC), L. Limatola (INAF-OAC), M. Lisi (INAF-OAR), L. Nicastro (INAF-IASF Bo), L. Pulone (INAF-OAR), A. Rossi (INAF-IASF Bo), P. Schipani (INAF-OAC), G. Tagliaferri (INAF-OAB), V. Testa (INAF-OAR), S. Yang (INAF-OAPD), L. Sbordone (ESO) and E. Brocato (INAF-OAR) on behalf of GRavitational Wave Inaf TeAm and the team of the ESO VLT program 099.D-0382(A) report:
We observed object SSS17a (Coulter et al., GCN 21529; Allam et al., GCN 21530; Yang et al., GCN 21531, Melandri et al., GCN 21532) possibly associated with the LIGO/Virgo event G298048 (GCN 21509), with the ESO Very Large Telescope UT 2 (Kueyen) equipped with the X-shooter spectrograph, covering the wavelength range 3000-25000 AA. Observations started at 23:16 UT on 2017-08-18, during the twilight, roughly 1.5 days after the burst and consisted of 4 exposures of 600 s each.
The transient is bright, well detected in the acquisition image, and clearly visible in all the spectral range. After a preliminary reduction the continuum appears to be similar to that predicted by kilonova models (Tanaka & Hotokezaka 2013, ApJ, 775, 113; Kasen et al. 2015, MNRAS, 450, 1777, Rosswog et al. 2016, arxiv1611.09822). We do not detect any obvious emission lines throughout the spectrum.
Further X-shooter observations are planned.
We acknowledge excellent support from the ESO observing staff in Paranal, in particular N. Jimenez and S. Brillant.
[GCN OPS NOTE(20aug17): Per author's request, (a) the 1st & 2nd authors exchanged order, (b) a couple other changes in the rest of the author list, and (c) LS was removed the 4th paragraph.]
[GCN OPS NOTE(21aug17): Per author's request (D'Elia), G. Greco was added to the author list.]
[GCN OPS NOTE(16sep17): Per author's request (Pian), (a) the phrase "and tentative redshift of SSS17a" was removed from the Subject-line, and (b) the last 4 sentences in the second paragraph were removed.]