TITLE:   GCN CIRCULAR
NUMBER:  23987
SUBJECT: Correction: Chandra observations of GW170817 ~581 days since merger
DATE:    19/03/22 19:04:44 GMT
FROM:    Raffaella Margutti at Northwestern U  <rafmargutti@gmail.com>

A. Hajela, R. Margutti, W. Fong , K. D. Alexander (Northwestern/CIERA), E.
Berger, P. Blanchard (Harvard), R. Chornock (Ohio U.), D. Coppejans
(Northwestern/CIERA), S. Gomez (Harvard), G. Hosseinzadeh  (Harvard), G.
Terreran (Northwestern/CIERA), A. Villar (Harvard) report:

“We report the results from the first 36ks of Chandra observations of
GW170817 (another ~ 64ks will be acquired in the next few days). Chandra
started to observe GW170817 on 21 March 2019 at 07:44 UT (obsID 21322,
Co-PIs Margutti and Fong; program 20500299). X-ray emission from GW170817
is detected at the level of ~3 sigma  (assuming pure Poissonian
statistics), with a 0.5-8 keV count-rate of  (1.5 +\-0.7)e-4 c/s, and a
total of ~5 net photons. Assuming a simple power-law spectrum with photon
index Gamma=1.6 and no intrinsic absorption, as found in previous Chandra
observations (e.g., Margutti et al., 2018; Alexander et al., 2018), the
count-rate above translates into an unabsorbed flux of  (3.9+/-1.8)e-15
erg/s/cm2 (0.3-10 keV). The Galactic neutral hydrogen column density in the
direction of the transient is N_H = 7.8e+20cm-2 (Kalberla et al., 2005).
This flux measurement is consistent with the expectations from the fast
decay of the emission from the jet (e.g., Margutti et al., 2018; Alexander
et al., 2018; Wu et al., 2018). Further Chandra observations are ongoing.

We thank the entire Chandra team for making these observations possible.”