TITLE: GCN CIRCULAR NUMBER: 28152 SUBJECT: GRB 200716C: CAHA optical observation // Anomalous light curve behavior? DATE: 20/07/21 21:36:27 GMT FROM: Alexander Kann at IAA-CSIC D. A. Kann (HETH/IAA-CSIC), M. Jelinek (ASU CAS Ondrejov), L. Izzo (DARK/NBI), A. de Ugarte Postigo (HETH/IAA-CSIC, DARK/NBI), C. C. Thoene, M. Blazek, J. F. Agui Fernandez (all HETH/IAA-CSIC), B. Arroyo, G. Bergond, and S. Pedraz (all CAHA) report: We observed the position of the afterglow (Ukwatta et al., GCN 28124; Lipunov et al., GCN 28125; Hu et al., GCN 28126; Kumar et al., GCN 28138; Gokuldass et al., GCN 28146; Jelinek et al., GCN 28149; Pozanenko et al., GCN 28151) and potential host galaxy (D'Avanzo, GCN 28132) of the bright GRB 200716C (Swift detection: Ukwatta et al., GCN 28124; Fermi GBM/LAT detections: Veres & Meegan, GCN 28135/Ohno et al., GCN 28130; AGILE detection: Ursi et al., GCN 28133; CALET detection: Torii et al., GCN 28139; Insight-HXMT/HE detection: Xue et al., GCN 28145) with the 2.2m telescope at Calar Alto, Almeria, Spain, equipped with CAFOS. We obtained a 180 s image in SDSS r', starting 2020-07-17 20:37:13 UT, (mid-time 0.903495 days after the GRB trigger), at high airmass. The host galaxy is clearly visible, but there is no clear detection of the afterglow. Calibrating against four nearby SDSS stars, we measure r' = 19.67 +/- 0.07 mag (AB mag). This value is fainter than those given by Pozanenko et al., GCN 28151, but we caution that an uneven background nearby may be influencing our magnitude measurement somewhat. Therefore, we are in agreement with Pozanenko et al. that the afterglow was not distinguishable from the host galaxy anymore at this point in time. Combining the above-mentioned sources and the automatically reduced UVOT data (but excluding the point from Kumar et al.), we find the afterglow, starting with the second Swift orbit, decays according to a broken power-law with decay slopes alpha_1 = 0.80 +/- 0.04, alpha_2 = 5.5 +/- 1.3, and break time 0.44 +/- 0.03 days. Hereby, we estimated host galaxy magnitudes in UVOT ubv based on the magnitudes given by D'Avanzo, GCN 28132, and assumed a significantly fainter host in the UV filters. Even compared to this extreme fit, the data point given by Kumar et al. is nearly two magnitudes too bright, as the steep decay should have set on already here assuming it is achromatic. While we caution the second decay slope depends strongly on the assumed host magnitudes, even without a host there is clearly a break to a very steep decay seen in uvw2, b, white, and partially v. This behavior is in strong contrast to the X-rays, which show an unbroken decay at alpha_X = 1.56 across this time span (Page & Evans, GCN 28131). Combined with indications from the Konus-Wind analysis that this may actually be a non-collapsar event (Frederiks et al., GCN 28148), this points to this being an event of interest. Further follow-up is warranted. [GCN OPS NOTE(21jul20): Due to some non-printing characters in the Subject-line of the submission, the MIME processor left a large chunk of the header of the submission email to appear as more of the body of the email. This has been removed from the archive copies.]