TITLE:   GCN CIRCULAR
NUMBER:  30445
SUBJECT: GRB 210704A: Continued Gemini-North NIR K-band Detections
DATE:    21/07/12 02:16:53 GMT
FROM:    Wen-fai Fong at Northwestern U  <wfong@northwestern.edu>

W. Fong and J. Rastinejad (Northwestern) report on behalf of a larger collaboration:

We continue to observe GRB 210704A (Kunzweiler et al., GCN 30369; Ursi et al., GCN 30372; Berretta et al., GCN 30375; Prasad et al., GCN 30378; D'Ai et al., GCN 30379) with the Near-Infrared Imager (NIRI) mounted on Gemini-North under Program GN-2021A-Q-109. We obtained two additional epochs of K-band imaging on 2021-07-10 and 2021-07-11 UT at mid-times of 5.47 days and 6.45 days for 34x60-sec and 43x60-sec, respectively. The infrared source (Rastinejad et al., GCN 30433) is still detected in both epochs, with a brightness of K_AB = 23.2 +/- 0.2 mag at 6.45 days. Performing digital image subtraction between these epochs and our initial imaging at 4.5 days, we find no clear evidence for any significant changes in brightness.

In addition, the source is clearly point-like in our most recent imaging, indicating that there is not a significant contribution from a host galaxy to the NIR flux at this time (consistent with the same conclusion reached in the optical bands, Watson et al., GCN 30436; Kann et al., GCN 30443). The fairly flat temporal evolution of the NIR source over ~4.5-6.5 days is consistent with the contemporaneous optical behavior (D'Avanzo et al., GCN 30432; Kann et al., GCN 30443), and is also at odds with a pure, fading afterglow origin, as previously noted (Watson et al., GCN 30436; Kann et al., GCN 30443).

We thank the Gemini staff for the rapid scheduling and execution of these observations.