GRB 221009A or Swift J1913.1+1946: PRIME near-infrared detection

We observed the field of the transient Swift J1913.1+1946 or GRB 221009A (Dichiara et al., GCN 32632; Kennea & Williams, GCN 32635; Veres et al., GCN 32636; Bissaldi et al., GCN 32637; Svinkin et al., GCN 32641; Ursi et al., GCN Circ. 32650) with the PRIME near-infrared camera mounted on the 1.8m Telescope at SAAO. Observations were carried out on 2022-10-09 around 18:50 UT, i.e. about 4.7 hours from the Swift/BAT detection (Dichiara et al., GCN 32632), 5.5 hours after the Fermi/GBM detection (Veres et al., GCN 32636), and 1 day after the PRIME instrument first light. We detected a near-infrared source spatially consistent with the bright optical transient previously reported, with preliminary photometry H~12.2 mag, calibrated against nearby 2MASS sources. Our result is consistent with the REM detection (Brivio et al. GCN 32652) and suggests no significant fading between 5.5 hr and 10.5 hr after the GBM trigger. Archival deep near-infrared images of the field taken with WFCAM during the UKIRT Galactic plane survey (Lawrence et al., 2007) do not show any persistent source in J and H bands at the transient location. These results are based on data obtained from PRIME at the South African Astronomical Observatory (SAAO), Sutherland, South Africa.