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GCN Circular 32788

GRB 221009A: Second epoch of NuSTAR data
2022-10-18T03:28:57Z (2 years ago)
Daniel Brethauer at UC Berkeley <>
Daniel Brethauer (UC Berkeley), Raffaella Margutti (UC Berkeley), Judith
Racusin (NASA/GSFC), Brian Grefenstette (Caltech), Kate D. Alexander
(Arizona), Tom Barclay (NASA/GSFC), Edo Berger (Harvard), Eric Burns (LSU),
Brad Cenko (NASA/GSFC), Yvette Cendes (Harvard), Ryan Chornock (UC
Berkeley), Tarraneh Eftekhari (Northwestern), Jamie Kennea (PSU), Tanmoy
Laskar (Utah)

A second epoch of NuSTAR observations of GRB221009A was obtained on October
15, 2022 starting at 05:21:09 UTC (~5.6 days since trigger; Dichiara et
al., GCN 32632; Veres et al., GCN 32636) with an exposure time of 25 ks
(PIs Racusin and Margutti).

The 3-79 keV spectrum is well fit by a power law with a photon index of
Gamma = 1.91 +/- 0.02 (1 sigma c.l.), which suggests spectrally softer
emission than observed during the first NuSTAR epoch (Brethauer et al., GCN
32695). The inferred NuSTAR power-law index is consistent with the value
inferred from contemporaneous Swift-XRT observations, which indicate Gamma=
1.79 +/- 0.11 and a best-fitting intrinsic absorption NHint=(0.46 +/-
0.10)e22 cm-2 at z=0.151 for an assumed Galactic neutral hydrogen column
density NH_mw=0.563e22 cm-2 (Willingale et al., 2013). The corresponding
unabsorbed flux is ~4.5e-11 ergs/cm^2/s  (3-79 keV), indicating a factor
~10 fading with respect to the previous epoch of NuSTAR data (Brethauer et
al., GCN 32695).

Additional epochs of NuSTAR monitoring are planned on October 20th, and
November 2nd. We thank the entire NuSTAR SOC for promptly implementing
these observations.
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