D'Avanzo, P.P.D'AvanzoCampana, S.S.CampanaSalafia, O. S.O. S.SalafiaGhirlanda, G.G.GhirlandaGhisellini, G.G.GhiselliniMelandri, A.A.MelandriBernardini, M. G.M. G.BernardiniBranchesi, M.M.BranchesiChassande-Mottin, E.E.Chassande-MottinCovino, S.S.CovinoD'Elia, ValerioValerioD'EliaNava, L.L.NavaSalvaterra, R.R.SalvaterraTagliaferri, G.G.TagliaferriVergani, S. D.S. D.Vergani2020-09-172020-09-172018-05-01https://hdl.handle.net/20.500.13025/5611Astronomy & Astrophysics, Volume 613, id.L1, 5 pp.We report our observation of the short GRB 170817A, associated to the binary neutron star merger event GW 170817, perfomed in the X-ray band with XMM-Newton 135 d after the event (on the 29th December 2017). We find that the X-ray light curve started to decrease with respect to the previous steadily brightening trend. This is also supported by a nearly simultaneous optical Hubble Space Telescope observation. Since the optical-to-X-ray spectral slope did not change with respect to previous observations, we exclude that the decrease in flux is due to the passage of the cooling frequency its origin must be geometric or dynamical. We interpret all the existing afterglow data with two models i) a structured jet and ii) a jet-less isotropic fireball with some stratification in its radial velocity structure. Both models fit the data and predict that the radio flux must decrease simultaneously with the optical and the X-ray one, making hard to distinguish between them at the present stage. Polarimetric measures and the rate of short GRB-GW association in future LIGO/Virgo runs will be key to disentangle these two geometrically different scenarios.Astrophysics - High Energy Astrophysical Phenomena Comment Letter submitted to Astronomy & Astrophysics. Other authors will be added to the present author list as soon as they have permission from the respective collaborationsjournal article10.1051/0004-6361/201832664arXiv:1801.06164https://ui.adsabs.harvard.edu/abs/2018A%26A...613L...1D/abstracthttps://www.aanda.org/articles/aa/full_html/2018/05/aa32664-18/aa32664-18.html2018A&A...613L...1D