3 years ago

Evidence for a decreasing X-ray afterglow emission of GW170817A and GRB 170817A in XMM-Newton.

A. Melandri, S. Covino, G. Ghisellini, G. Tagliaferri, S. Campana, R. Salvaterra, V. D'Elia, S.D. Vergani, L. Nava, M.G. Bernardini, P. D'Avanzo

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.

Publisher URL: http://arxiv.org/abs/1801.06164

DOI: arXiv:1801.06164v1

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