TITLE: GCN CIRCULAR NUMBER: 21972 SUBJECT: GRB 171004A: Correction to RATIR Astrometry and Photometry DATE: 17/10/04 20:04:21 GMT FROM: Alan M. Watson at Instituto de Astronomia UNAM Alan M. Watson (UNAM), Nat Butler (ASU), Alexander Kutyrev (GSFC), William H. Lee (UNAM), Michael G. Richer (UNAM), Ori Fox (STScI), J. Xavier Prochaska (UCSC), Josh Bloom (UCB), Antonino Cucchiara (UVI), Eleonora Troja (GSFC), Owen Littlejohns (ASU), Enrico Ramirez-Ruiz (UCSC), Jesús González (UNAM), Carlos Román-Zúñiga (UNAM), Harvey Moseley (GSFC), John Capone (UMD), V. Zach Golkhou (U. Wash.), and Vicki Toy (UMD) report: The position and magnitudes of the uncataloged source reported by Watson et al. (GCN Circ. 21968) are incorrect. We thank David Alexander Kann for drawing our attention to this error. The correct position is RA, Dec = 139.169462 52.693080 (J2000) or 09:16:40.67 +52:41:35.1. This position lies 0.4 arcsec from the enhanced XRT position (Osborne et al., GCN Circ. 21970). The correct magnitudes are r = 21.17 +/- 0.06 i = 20.87 +/- 0.05 Z = 20.72 +/- 0.10 Y = 20.59 +/- 0.13 J = 20.36 +/- 0.14 H = 20.17 +/- 0.17 A fit to the i-band light curve of the source shows that the flux behaves as t^(-0.9+/-0.4). That is, it is fading at the 2-sigma level. The positional coincidence combined with the weak evidence for fading suggest that this source might well be the afterglow of the GRB.