[vsnet-grb-info 26052] GRB 200716C: CAHA optical observation // Anomalous light curve

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  28152
SUBJECT: GRB 200716C: CAHA optical observation // Anomalous light curve 
DATE:    20/07/21 21:36:27 GMT
FROM:    Alexander Kann at IAA-CSIC  <kann at iaa.es>

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Date: Tue, 21 Jul 2020 23:36:25 +0200
From: David Alexander Kann <kann at iaa.es>
To: gcncirc at capella.gsfc.nasa.gov, scott at milkyway.gsfc.nasa.gov
Subject: GRB 200716C: CAHA optical observation // Anomalous light curve
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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.