TITLE:   GCN CIRCULAR
NUMBER:  32695
SUBJECT: GRB 221009A: NuSTAR Detection 
DATE:    22/10/11 19:38:52 GMT
FROM:    Ryan Chornock at UC Berkeley  <chornock@berkeley.edu>

Daniel Brethauer (UC Berkeley), Brian Grefenstette (Caltech), Judith Racusin (NASA/GSFC), Raffaella Margutti (UC Berkeley), 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), and Tanmoy Laskar (Utah) report:

NuSTAR began a Target of Opportunity observation of GRB 221009A (GCN #32632) on October 11, 2022 at 03:10:07 UTC, approximately 38 hours after the GBM Trigger (GCN #32642), with an exposure of 23.4 ks (PIs Racusin and Margutti).

The 3-79 keV spectrum is well fit by a power law with a photon index of Gamma= 1.81 +/- 0.01, which is consistent with the value inferred from Swift-XRT observations acquired during the same time window.  The corresponding unabsorbed flux is (3.37 +/- 0.02) e-10 erg/cm2/s (3-79 keV). Over the NuSTAR observation, the source X-ray flux declines by about 30%. However, preliminary analysis does not indicate a significant evolution of the 10-20 keV / 3-6 keV hardness ratio.

These findings are consistent with the hard X-ray afterglow observed by INTEGRAL JEM-X and ISGRI (GCN #32691) and support an absorbed simple power-law spectrum extending from soft X-ray to hard X-ray energies. 

Three additional NuSTAR monitoring observations are planned and are anticipated to occur on October 15th, 20th, and November 2nd.  We thank the NuSTAR SOC for promptly implementing these observations.