TITLE:   GCN CIRCULAR
NUMBER:  21626
SUBJECT: LIGO/Virgo G298048: NuSTAR limits on X-ray emission
DATE:    17/08/21 04:33:27 GMT
FROM:    Mansi M. Kasliwal at Caltech  <mansi@astro.caltech.edu>

F. A. Harrison, K. Forster, J. Garcia, B. W. Grefenstette, M. Heida, M. M.
Kasliwal, K. K. Madsen,  H. Miyasaka, S. Pike, V. Rana, Y. Xu (Caltech)

We obtained X-ray imaging of the optical counterpart of SSS17a (Coulter et
al.; GCN21529), the candidate optical counterpart of G298048 (LVC; GCN
21509) in the 3 – 79 keV band with NuSTAR.  Observations began at
2017-08-18T05:26:09 UT and ended at 2017-08-18T19:46:09, with a total
exposure of 24 ksec.  We did not detect any X-ray emission in a circular
50’’ extraction region centered on the position of the optical counterpart,
with a 3-sigma threshold of 1.4 x 10-14 erg/cm^2/s in the 3-10 keV band,
assuming an X-ray spectrum with a photon power law index of -2.   No source
was detected in the 10-60 keV band, however additional analysis is ongoing
to place limits on possible line emission from nuclear decays. We note that
the NuSTAR hard X-ray band is not sensitive to X-ray emission from star
formation in the host galaxy, and there is no known AGN activity in this
source.

Further NuSTAR observations are planned.