GCN Circular 21765

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
Not found
 
 
) 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
Not found
 
 
; Margutti et al., LVC GCN 21648
Not found
 
 
) 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
Not found
 
 
;
Savchenko et al., LVC GCN 21507
Not found
 
 
), 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
Not found
 
 
)
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
Not found
 
 
), and Cenko et al. (LVC GCN 21572
Not found
 
 
).

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.