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GCN Circular 22201

LIGO/Virgo G298048: Late-time detection of the X-ray afterglow with Chandra
2017-12-07T17:34:57Z (6 years ago)
Eleonora Troja at GSFC <>
E. Troja (UMD/GSFC), L. Piro (INAF/IAPS), G. Ryan (UMD),
H. van Eeerten (U. Bath), T. Sakamoto (AGU), and S. B. Cenko (NASA/GSFC)

The Chandra X-ray Observatory re-observed the field of GW170817/AT2017gfo
(PI: B. Wilkes) starting on 2017-12-03 (~109 days after the LVC trigger)
for a total exposure of 74 ks. A second shorter observation was performed on
2017-12-06 for a total exposure of 25 ks.

Our preliminary analysis of the data clearly detects the X-ray counterpart
of AT2017gfo at a flux level of ~1.5E-14 erg/cm^2/s (0.5-8.0 keV).
The observed value is three times higher than the flux measured at
earlier times (~5E-15 erg/cm^2/s at 15 days; Troja, E., et al. 2017,
Nature, 551, 71) and implies a continued rise of the X-ray afterglow.
We do not measure any significant spectral variation between these
observations and our observations at earlier times.

This behavior is hard to reconcile with the simple top-hat jet model and
suggests a more complex structure of the GRB jet.

We thank Belinda Wilkes, Andrea Prestwich and the entire Chandra for
performing these observations.
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