[vsnet-grb-info 28340] GRB 210704A: NOT optical observations and color change

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  30443
SUBJECT: GRB 210704A: NOT optical observations and color change
DATE:    21/07/11 20:07:25 GMT
FROM:    Luca Izzo at DARK/NBI  <luca.izzo at gmail.com>

D. A. Kann (HETH/IAA-CSIC), L. Izzo (DARK/NBI), F. J. Galindo Guil, and A. Kasikov (NOT) report:

We observed the optical afterglow (Kim et al., GCN #30384; D'Avanzo et al., GCN #30385; Kann et al., GCN #30391; de Ugarte Postigo et al., GCN #30392; Kann et al., GCN #30401; Sun et al., GCN #30411; D'Avanzo et al., GCN #30432; Rastinejad et al., GCN #30433; Watson et al., GCN #30436; Pankov et al., GCN #30440; Troja et al., GCN #30442) 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; Malacaria et al. GCN #30380; Ridnaia et al., GCN #30388) with the AlFOSC imager mounted on the Nordic Optical Telescope (NOT) located in La Palma, Spain.

We have obtained 3x300s images in the g' and r' filters, at a mean epoch of 6.085 days and 6.096 days after the GRB, respectively. The optical afterglow is still detected in each stacked image, and we measure g' = 23.78 +/- 0.11 mag, r' = 23.27 +/- 0.09 mag against Pan-STARRS comparison stars (AB system).

This suggests that the optical transient has hardly changed in brightness for several days compared to earlier measurements (D'Avanzo et al., GCN #30432; Watson et al., GCN #30346), while remaining two magnitudes brighter than the potential host galaxy (Watson et al., GCN #30436).

On the other hand, the g' - r' color has increased compared to the measurement of Kann et al., GCN #30401, a behavior expected both for a SN component and a KN component, but not an afterglow. As the temporal evolution is also peculiar for a fast-evolving KN, the interpretation as a rising SN gains credence (Watson et al., GCN #30436). The magnitude, however, may be indicative of a redshift exceeding those indicated by a nearby galaxy and somewhat more distant galaxy cluster (Levan et al., GCN #30381).