[vsnet-grb-info 28339] GRB210704A: Gemini near-infrared upper limits

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  30442
SUBJECT: GRB210704A: Gemini near-infrared upper limits
DATE:    21/07/11 17:24:24 GMT
FROM:    Eleonora Troja at NASA/GSFC/UMD  <eleonora at umd.edu>

E. Troja (UMD/GSFC), A. Watson (UNAM), B. O'Connor (UMD/GSFC/GWU) report on
behalf of a larger collaboration:

We observed the field of GRB 210704A (Ursi et al., GCN 30372; Berretta et
al., GCN 30375; Malacaria et al. GCN 30380) with the Near-Infrared Imager
(NIRI) mounted on the 8.1m Gemini North telescope. We obtained 24x60 s
imaging in J-band starting on 2021 July 10 at 06:00 UT (~5.5 days after the
GRB) at an average airmass of 2.0.

At the location of the optical afterglow (Kim et al., GCN 30384) we detect
no source. Preliminary calibration against nearby 2MASS objects yields a
3-sigma upper limit of J>23.5 AB mag.  With respect to the earlier
observations reported by Rastinejad et al. (GCN 30433), our data show a
marginal evidence of fading.

We note that nearly simultaneous Swift/XRT observations still detect the
X-ray afterglow at a flux level of  ~4.4E-14 erg/cm2/s at 4.5 d after the
burst. By using a spectral index of about 0.85, consistent with the
Swift/XRT spectrum (https://www.swift.ac.uk/xrt_spectra/00021454/) and the
observed temporal decay with slope ~1.3, we estimate an
afterglow contribution of ~23.1 AB mag to the nIR emission.  This suggests
that the non-thermal afterglow component could have significantly
contributed to the nIR detection reported in Rastinejad et al. (GCN 30433).

We thank the Gemini staff for scheduling and executing these observations.