TITLE: GCN CIRCULAR NUMBER: 32788 SUBJECT: GRB 221009A: Second epoch of NuSTAR data DATE: 22/10/18 03:28:57 GMT FROM: Daniel Brethauer at UC Berkeley daniel_brethauer@berkeley.edu

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.