Difference: GCNCIRCULAR21592 ( vs. 1)

V. D'Elia (INAF-ASDC), E. Pian (SNS-Pisa), S. Piranomonte (INAF-OAR), M. Branchesi (GSSI), E. Brocato (INAF-OAR), S. Campana (INAF-OAB), E. Cappellaro (INAF-OAPD), S. Covino (INAF-OAB), P. D'Avanzo (INAF-OAB), G. Greco (Urbino University/INFN Firenze), A. Melandri (INAF-OAB), E. Palazzi (INAF-IAFS 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) on behalf of GRavitational Wave Inaf TeAm 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. Two faint absorption lines are detected at ~6640 AA, while another one at ~5200 AA. These features are tentatively identified with Halpha NII, and MgI, respectively, at a redshift of 0.01069, corresponding to a luminosity distance of 46.4 Mpc. We note that the redshift of the closeby galaxy is 0.009787, which corresponds to 42.5 Mpc. The distance difference of 3.9 Mpc is remarkable, but can be explained either by the kick velocity of the transient or by an absorbing gas moving in the direction of the observer.

Further X-shooter observations are planned.

We acknowledge excellent support from the ESO observing staff in Paranal, in particular L. Sbordone, N. Jimenez and S. Brillant.

-- ShaonGhosh - 19 Aug 2017