GCN Circular 28152

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
Not found
 
 
; 
Lipunov et al., GCN 28125
Not found
 
 
; Hu et al., GCN 28126
Not found
 
 
; Kumar et al., GCN 
28138
Not found
 
 
; Gokuldass et al., GCN 28146
Not found
 
 
; Jelinek et al., GCN 28149
Not found
 
 
; Pozanenko 
et al., GCN 28151
Not found
 
 
) and potential host galaxy (D'Avanzo, GCN 28132
Not found
 
 
) of 
the bright GRB 200716C (Swift detection: Ukwatta et al., GCN 28124
Not found
 
 
; 
Fermi GBM/LAT detections: Veres & Meegan, GCN 28135
Not found
 
 
/Ohno et al., GCN 
28130
Not found
 
 
; AGILE detection: Ursi et al., GCN 28133
Not found
 
 
; CALET detection: Torii 
et al., GCN 28139
Not found
 
 
; Insight-HXMT/HE detection: Xue et al., GCN 28145
Not found
 
 
) 
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
Not found
 
 
, 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
Not found
 
 
, 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
Not found
 
 
). Combined with indications from the Konus-Wind analysis that 
this may actually be a non-collapsar event (Frederiks et al., GCN 
28148
Not found
 
 
), 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.]