GRB 221009A: NICER follow-up observations

We report on initial NICER observations of the exceptionally bright GRB 221009A, at a redshift of 0.1505 (GCN #32648, #32686) and observed from radio to TeV energies (GCNs #32632, #32635, #32636, #32641, #32658, #32661, #32668, #32677, and ATels #15653, #15655, #15656, #15660, #15661). NICER observed GRB 221009A intermittently from 2022 Oct 9 17:11 to Oct 11 00:12 UT, or 14.7 ksec to 126 ksec after the Fermi/GBM trigger time (GCN #32636).

During this period, NICER made 12 observations with exposure times ranging from 40 to 400 sec each. The initial count rate registered with NICER is 1400 counts/s which declined to about 38 counts/s at the time of the last observation reported here. From a preliminary analysis, we find that the decline follows a power law with an index of -1.6.

The 1-10 keV spectrum of each observation is well reproduced by an absorbed power-law model with a spectral index of about 2.0. We used the tbabs model with wilms abundance in XSPEC (Wilms, Allen & McCray 2000) for an assumed Galactic absorption of 5.4 x 1021 cm-2 (Willingale et al. 2013; GCN #32651).

The average column density of these observations with the ztbabs model is 1.1 x 1022 cm-2 at a redshift of 0.151 (GCN #32648). The absorbed (unabsorbed) flux in the 0.3 - 10 keV band declined from 6.1 x 10-9 (1.3 x 10-8) to 1.8 x 10-10 (3.3 x 10-10) erg/sec/cm2. NICER initially received notification of the GRB through OHMAN (On-orbit Hookup of MAXI and NICER) at 14:10:57 UT on Oct 9, but poor visibility delayed a prompt follow-up.

OHMAN is software on an International Space Station laptop computer that provides a new automated triggering capability, monitoring live MAXI data and communicating new transient alerts to NICER for follow-up within minutes, visibility permitting. NICER is continuing to monitor GRB 221009A. Detailed temporal and spectral analysis is ongoing.

The NICER schedule can be found at https://heasarc.gsfc.nasa.gov/docs/nicer/schedule/nicer_sts_current.html. NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.