TITLE:   GCN CIRCULAR
NUMBER:  30433
SUBJECT: GRB 210704A: Gemini-North Infrared Source Detection
DATE:    21/07/09 21:48:13 GMT
FROM:    Jillian Rastinejad at Northwestern Univ.  <jillianrastinejad2024@u.northwestern.edu>

J. Rastinejad (Northwestern), A. Levan (Radboud U.), W. Fong (Northwestern), N. R. Tanvir (U. Leicester) and A. Rouco Escorial (Northwestern) report on behalf of a larger collaboration:

We observed the location of 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 14x60-sec imaging in K-band at a mid-time of 2021 July 9.265 UT (4.45 days post-burst) at an average airmass of 2.1. We detect a faint source consistent with the location of the optical afterglow (Kim et al., GCN 30384; D'Avanzo et al., GCN 30385; Troja et al., GCN 30390; Kann et al., GCN 30391, 30401; Sun et al., GCN 30411). Calibrated to 2MASS and applying the standard AB conversion, we measure a brightness of K_AB = 22.9 +/- 0.3 mag. At present, it is not possible to discern whether the source is point-like or extended, and the degree to which any underlying host galaxy may be contributing (c.f., D'Avanzo et al., GCN 30432). 

For comparison, we note that the expected approximate brightness for a kilonova of the same luminosity as GW170817/AT2017gfo at z~0.08 (Levan et al., GCN 30381) is K_AB ~ 22.5 mag at this epoch. Scaled to the nearby galaxy cluster redshift of z~0.2 (Levan et al., GCN 30381; Dichiara et al. GCN 30383), the expected magnitude for such a kilonova becomes K_AB ~ 24.6 mag, considerably fainter than the source detected here. On the other hand, it is plausible that the detected NIR source is a background galaxy at higher redshift (de Ugarte Postigo et al., GCN 30392; D'Avanzo et al., GCN 30432). Concurrent and continued multi-band follow-up is encouraged to discern the nature of this source.

Further NIR observations are planned to assess any variability of the source. We thank the Gemini staff for the rapid scheduling and execution of these observations.