Volume 613, May 2018
|Number of page(s)||5|
|Section||Letters to the Editor|
|Published online||18 May 2018|
Letter to the Editor
The evolution of the X-ray afterglow emission of GW 170817/ GRB 170817A in XMM-Newton observations
INAF – Osservatorio Astronomico di Brera,
Via E. Bianchi 46,
Merate (LC), Italy
2 Dipartimento di Fisica “G. Occhialini”, Università degli Studi di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
3 INFN – Sezione di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
4 Gran Sasso Science Institute, Viale F. Crispi 7, L’Aquila, Italy
5 Laboratoire Univers et Particules de Montpellier, Université Montpellier, CNRS/IN2P3, Montpellier, France
6 INFN, Laboratori Nazionali del Gran Sasso, 67100 L’Aquila, Italy
7 APC, Université Paris Diderot, CNRS/IN2P3, CEA/Irfu, Obs de Paris, Sorbonne Paris Cité, Paris, France
8 Space Science Data Center, ASI, Via del Politecnico, s.n.c., 00133 Roma, Italy
9 INAF, Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, 34143 Trieste, Italy
10 INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano, via E. Bassini 15, 20133 Milano, Italy
11 GEPI, Observatoire de Paris, PSL Research University, CNRS, Place Jules Janssen, 92190 Meudon, France
Accepted: 17 April 2018
We report our observation of the short gamma-ray burst (GRB) GRB 170817A, associated to the binary neutron star merger gravitational wave (GW) event GW 170817, performed in the X-ray band with XMM-Newton 135 d after the event (on 29 December, 2017). We find evidence for a flattening of the X-ray light curve with respect to the previously observed brightening. This is also supported by a nearly simultaneous optical Hubble Space Telescope observation and successive X-ray Chandra and low-frequency radio observations recently reported in the literature. Since the optical-to-X-ray spectral slope did not change with respect to previous observations, we exclude that the change in the temporal evolution of the light curve 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 X-ray emission, making it difficult to distinguish between them at the present stage. Polarimetric measurements and the rate of short GRB-GW associations in future LIGO/Virgo runs will be key to disentangle these two geometrically different scenarios.
Key words: gravitational waves / gamma rays: general
© ESO 2018
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