TITLE: GCN CIRCULAR NUMBER: 24000 SUBJECT: Further Chandra observations of GW170817 ~581-583 days since merger DATE: 19/03/25 03:12:22 GMT FROM: Aprajita Hajela at Northwestern U 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 initiated a series of Chandra X-ray observations of GW170817 at t~581 days since merger, for a total exposure time of 100 ks. The results from the first observation with the exposure time of ~36 ks is reported in Hajela et al. (GCN 23987). Here, we report on the remaining ~64ks of Chandra observations of GW170817 acquired starting on 22 March 2019 at 06:17 UT (obsID 22157, exposure time of ~38 ks) and 23 March 2019 at 23:52 UT (obsID 22158, exposure time of ~25 ks, PIs Margutti and Fong). X-ray emission from GW170817 is detected with a significance of >=3 sigma in each of the observations, with a total of ~6 and ~3 photons from the source in obsID 22157 and obsID 22158, respectively (0.5-8 keV energy range). Assuming a simple power-law spectral model with photon index Gamma = 1.6 and NHint = 0, as found in previous Chandra observations (e.g., Margutti et al., 2018; Alexander et al., 2018), we find an unabsorbed flux of (2.7 +\- 1.2)e-15 erg/s/cm2 and (6.3+\- 3.5)e-15 erg/s/cm2 for ObsID 22157 and 22158, respectively (0.3-10 keV). The Galactic neutral hydrogen column density in the direction of the transient is NH = 7.8e+20cm-2 (Kalberla et al., 2005). From a joint spectral fit of the three epochs of data (obsID 21322, 22157, and 22158) we find that the photon index is loosely constrained to Gamma=1.6 +/- 0.8 and there is no evidence for intrinsic absorption. For Gamma=1.6, and NHint=0, the joint fit of the entire 100ks of Chandra observations indicates an unabsorbed flux of (3.6 +/- 0.9)e-15 erg/s/cm2 (t~582 days since merger, 0.3-10 keV). We confirm that 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). All fluxes reported are in the 0.3-10 keV energy range with 1-sigma uncertainties. We thank the entire Chandra team for making these observations possible.”