GCN Circular 28152
Subject
GRB 200716C: CAHA optical observation // Anomalous light curve behavior?
Event
Date
2020-07-21T21:36:27Z (5 years ago)
From
Alexander Kann at IAA-CSIC <kann@iaa.es>
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
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