TITLE: GCN CIRCULAR NUMBER: 21765 SUBJECT: LIGO/Virgo G298048: Discovery of X-ray emission from SSS17a in NGC4993 DATE: 17/08/30 12:07:12 GMT FROM: Eleonora Troja at GSFC/Swift E. Troja (UMD/GSFC), L. Piro (INAF/IAPS), T. Sakamoto (AGU), S. B. Cenko (NASA/GSFC), and A. Lien (UMBC/GSFC) report on behalf of a larger collaboration: We observed the field of SSS17a (Coulter et al., LVC GCN 21529) with the Chandra X-ray Observatory under our approved guest observer program (18500489; PI: E. Troja). Observations were carried out on 2017-08-26 (~9 days after the LVC trigger) for a total exposure of 50 ks. We detect the extended X-ray source visible in previous observations (Evans et al., LVC GCN 21612; Margutti et al., LVC GCN 21648) at a comparable flux level. In addition, we detect an X-ray source at the optical/IR transient position, approximately 10 arcsecond from the centroid of the extended X-ray emission. The probability to find an unrelated X-ray source within the small localization of the optical transient is negligible (<1E-5). Previous candidate kilonovae (e.g. GRB080513, Perley et al. 2009; GRB130603B; Tanvir et al. 2013) were associated to transient X-ray emission, although bright X-rays are not a basic prediction of this model. In our case, the properties of the X-ray emission and the overall spectral energy distribution appear different. The most likely explanation seems that the observed X-rays arise from the afterglow of GRB170817A (Connaughton et al., LVC GCN 21506; Savchenko et al., LVC GCN 21507), thus confirming the spatial association between the short GRB and SSS17a. A newborn spinning-down magnetar could power a long-lived and nearly constant X-ray emission (Zhang & Meszaros, 2001). However, the observed timescale and X-ray luminosity would imply unrealistic values of the initial spin period and magnetic field. The previous lack of detection from Chandra (Margutti et al., LVC GCN 21648) further disfavors this magnetar model. Other models (e.g. Metzger & Piro, 2014) also do not match our observed luminosities and timescales. For a standard on-axis afterglow, the extrapolation of the observed flux would violate the upper limits reported by Evans et al. (LVC GCN 21550), and Cenko et al. (LVC GCN 21572). Our observations are instead consistent with the onset of an off-axis afterglow from the GRB jet. This would explain the low luminosity of the observed gamma-ray emission, and the lack of early afterglow detections. Further observations are planned. We thank Belinda Wilkes and the entire CXC staff for rapidly scheduling these observations.