GCN Circular 22608

A. Hajela, R. Margutti (Northwestern University), C. Guidorzi (U Ferrara),
A. Kamble (Harvard), A. MacFadyen (NYU), D. Milisavljevic (Purdue), J.
Parrent (Harvard), A. Zauderer (NSF) report:

"We started deep X-ray follow-up observations of GRB161219B with the
Chandra X-ray Observatory on 2017, August 8 UT (dt ~ 232 days after
explosion, exposure time of 15 ks), PI Margutti, program 18500396. An X-ray
source is detected at the location of GRB161219B with count-rate of ~
0.0024 cts/s (0.5 - 8 keV) and significance of 16 - sigma. The spectrum is
well modeled with an absorbed simple power law with best-fitting photon
index, Gamma = 1.95 +/- 0.38 and Galactic absorption, N_h = 2.8e+20 cm-2.
We do not find any evidence of intrinsic absorption. For these parameters,
the unabsorbed flux is (3.41 +/- 0.76) x 10^-14 ergs cm-2 s-1 (0.3 - 10
keV).

A second Chandra observation was acquired between 2018, January 16th and
January 18th (dt ~ 393-395 days since explosion, exposure time of 32 ks).
GRB161219B is detected with the count-rate of ~ 0.0013 cts/s (0.5 - 8 keV)
and a significance of 9 -sigma. The best-fitting photon-index is Gamma =
2.05 +/- 0.34 . The corresponding unabsorbed flux is (1.72 +/- 0.28) x
10^-14 ergs cm-2 s-1 (0.3 - 10 keV).

We find that GRB161219B continues to decay following a power-law with
index, alpha = 1.22 +/- 0.04 , which is consistent with the decay
inferred from XRT data at t < 115 days.

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

[GCN OPS NOTE(06apr18):  Per author's request, the last two paragraphs
were added.]