{
  "bibcode": "2020GCN.28152....1K",
  "body": "D. A. Kann (HETH/IAA-CSIC), M. Jelinek (ASU CAS Ondrejov), L. Izzo \n(DARK/NBI), A. de Ugarte Postigo (HETH/IAA-CSIC, DARK/NBI), C. C. \nThoene, M. Blazek, J. F. Agui Fernandez (all HETH/IAA-CSIC), B. Arroyo, \nG. Bergond, and S. Pedraz (all CAHA) report:\n\nWe observed the position of the afterglow (Ukwatta et al., GCN 28124; \nLipunov et al., GCN 28125; Hu et al., GCN 28126; Kumar et al., GCN \n28138; Gokuldass et al., GCN 28146; Jelinek et al., GCN 28149; Pozanenko \net al., GCN 28151) and potential host galaxy (D'Avanzo, GCN 28132) of \nthe bright GRB 200716C (Swift detection: Ukwatta et al., GCN 28124; \nFermi GBM/LAT detections: Veres & Meegan, GCN 28135/Ohno et al., GCN \n28130; AGILE detection: Ursi et al., GCN 28133; CALET detection: Torii \net al., GCN 28139; Insight-HXMT/HE detection: Xue et al., GCN 28145) \nwith the 2.2m telescope at Calar Alto, Almeria, Spain, equipped with \nCAFOS. We obtained a 180 s image in SDSS r', starting 2020-07-17 \n20:37:13 UT, (mid-time 0.903495 days after the GRB trigger), at high \nairmass.\n\nThe host galaxy is clearly visible, but there is no clear detection of \nthe afterglow. Calibrating against four nearby SDSS stars, we measure r' \n= 19.67 +/- 0.07 mag (AB mag). This value is fainter than those given by \nPozanenko et al., GCN 28151, but we caution that an uneven background \nnearby may be influencing our magnitude measurement somewhat. Therefore, \nwe are in agreement with Pozanenko et al. that the afterglow was not \ndistinguishable from the host galaxy anymore at this point in time.\n\nCombining the above-mentioned sources and the automatically reduced UVOT \ndata (but excluding the point from Kumar et al.), we find the afterglow, \nstarting with the second Swift orbit, decays according to a broken \npower-law with decay slopes alpha_1 = 0.80 +/- 0.04, alpha_2 = 5.5 +/- \n1.3, and break time 0.44 +/- 0.03 days. Hereby, we estimated host galaxy \nmagnitudes in UVOT ubv based on the magnitudes given by D'Avanzo, GCN \n28132, and assumed a significantly fainter host in the UV filters. Even \ncompared to this extreme fit, the data point given by Kumar et al. is \nnearly two magnitudes too bright, as the steep decay should have set on \nalready here assuming it is achromatic. While we caution the second \ndecay slope depends strongly on the assumed host magnitudes, even \nwithout a host there is clearly a break to a very steep decay seen in \nuvw2, b, white, and partially v.\n\nThis behavior is in strong contrast to the X-rays, which show an \nunbroken decay at alpha_X = 1.56 across this time span (Page & Evans, \nGCN 28131). Combined with indications from the Konus-Wind analysis that \nthis may actually be a non-collapsar event (Frederiks et al., GCN \n28148), this points to this being an event of interest. Further \nfollow-up is warranted.\n\n[GCN OPS NOTE(21jul20):  Due to some non-printing characters in the\nSubject-line of the submission, the MIME processor left a large chunk\nof the header of the submission email to appear as more of the body\nof the email. This has been removed from the archive copies.]",
  "circularId": 28152,
  "createdOn": 1595367387000,
  "email": "kann@iaa.es",
  "subject": "GRB 200716C: CAHA optical observation // Anomalous light curve behavior?",
  "submitter": "Alexander Kann at IAA-CSIC  <kann@iaa.es>",
  "eventId": "GRB 200716C"
}