TITLE: GCN CIRCULAR NUMBER: 16936 SUBJECT: GRB 141004A: La Palma late-time observations and possible brightening DATE: 14/10/20 21:09:31 GMT FROM: Steve Schulze at U of Iceland S. Schulze (PUC, MAS), A. de Ugarte Postigo (IAA-CSIC, DARK/NBI), K. Wiersema (U. Leicester), D. Xu (DARK/NBI), N. R. Tanvir (U. Leicester), D. Malesani, J. P. U. Fynbo (DARK/NBI), P. Jakobsson (U. Iceland), V. Nascimbeni (INAF/OAPd), J. Harmanen (NOT), and O. Vaduvescu (ING) report on behalf of a larger collaboration: We monitored the optical counterpart of GRB 141004A (D'Elia et al., GCN 16878; Mereghetti et al., GCN 16879) with the 2.5-m Nordic Optical Telescope (NOT), the 4.2-m William Herschel Telescope (WHT), and the 10.4-m Gran Telescopio Canarias (GTC). The latest NOT observation was secured with ALFOSC on October 14.17 (9.19 days after the burst) and consisted of 9x240-s in i' band. In the same night we obtained a deep 8x300-s image with ACAM mounted at the WHT in r' band starting at October 14.17. A GTC observation was secured on October 18.24 (13.27 days after the burst) and consisted of 3x180 s in r' band and 4x120 s in i' band. We applied differential photometry to check for variability using six unsaturated nearby stars. The transient did not change its brightness in r’ band by more than ~0.10 mag, but in the i' band our photometry indicates a tentative brightening by 0.46 +/- 0.27 mag. While the detected variability is only marginally significant, this behaviour is consistent with the emergence of a SN similar to SN 1998bw, both in terms of brightness and color evolution. We encourage further follow-up to test for the presence of an emerging SN.