Chandra observations of GW170817 2.5 years since merger (epoch 1 of 4)
Abstract
We initiated a new round of observations of the NS merger GW170817 with the Chandra X-ray Observatory (CXO) to continue following its evolution at ~ 2.5 years after the merger. Four observations will be acquired between March 9 and March 15, for a total exposure time of 100 ks. Here we report on the first observation (ObsID 21323, PI Margutti; program 20500299), which was acquired on March 09, 2020 at 22:04:48 UT (~935 days since merger) with an exposure time of 24.29 ks. X-ray emission is detected at the location of GW170817 with a significance of ~ 3-sigma (Gaussian equivalent) and a 0.5-8 keV count-rate of (1.2 +/- 0.7)e-04 c/s, and a total of ~ 2.8 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. Hajela et al. 2019, ApJL, 886L, 17H), and following the same spectra-extraction procedure as in Hajela et al. 2019, the count-rate above translates into an unabsorbed flux of ~ (3.2 +/- 1.9)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 X-ray flux measurement is nominally above, yet statistically consistent with the off-axis jet model predictions given the large flux uncertainties. Further CXO observations comprising the remaining exposure time of ~75 ks are scheduled in the next few days and will help quantify the statistical evidence for the possible departure from the off-axis jet model (which might represent the onset of the kilonova afterglow). An updated plot with the most recent results can be found at this link: https://sites.google.com/u.northwestern.edu/xray-observations-of-gw170817/home We thank the entire Chandra team for making these observations possible.
- Publication:
-
GRB Coordinates Network
- Pub Date:
- March 2020
- Bibcode:
- 2020GCN.27357....1H