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GCN Circular 31626

Subject
ZTF22aaajecb/AT 2022cmc: CAHA 2.2m/CAFOS detection, luminous transient
Date
2022-02-21T21:16:30Z (2 years ago)
From
Alexander Kann at IAA-CSIC <kann@iaa.es>
D. A. Kann (HETH/IAA-CSIC), A. de Ugarte Postigo (Obs. Cote d'Azur), C. 
C. Thoene (HETH/ASU CAS Ondrejov), M. Blazek (HETH), J. F. Agui 
Fernandez (HETH/IAA-CSIC), I. Vico, and A. Guijarro (both CAHA) report:

We observed the red ZTF transient ZTF22aaajecp/AT 2022cmc (Andreoni et 
al., GCN #31590), likely a GRB afterglow at z = 1.193 (Tanvir et al., 
GCN #31602; Lundquist et al., GCN #31612) with CAFOS, mounted on the 
2.2m telescope, at the Calar Alto Observatory (Almeria, Spain). The 
observation started at 04:54:04 UT on 18 February 2022 (6.76 days after 
the first ZTF detection) and consisted of 12 x 120 s integrations in the 
r' and i' bands, each. Observations were hampered by the full Moon, but 
otherwise conditions were very good (1".3 seeing, very good 
transparency). The transient is clearly detected in each of the stacked 
images.

Compared to a nearby Pan-STARRS comparison star, we measure r' = 21.12 
+/- 0.03 mag (AB) 6.772 days after first detection.

Gathering data from other GCNs (Andreoni et al., GCN #31590; Lipunov et 
al., GCN #31600; Kumar et al., GCN #31597; Pankov et al., GCNs #31593, 
#31625; Perley, GCN #31594), and assuming a GRB time one hour before the 
first detection (note the exact choice of time has little influence on 
the late decay slope), we find the transient is described by an 
achromatic decay with slope alpha = 1.05 +/- 0.05. The SED is described, 
similar to the result Perley (GCN #31594) found, by a simple power-law 
with slope beta = 1.29 +/- 0.22. This is slightly steeper than usual for 
synchrotron radiation from a typical GRB afterglow, indicating some 
extra extinction may play a role, as might be expected from the high 
X-ray column density found by NICER (Pasham et al., GCN #31601).

Using the known redshift and the spectral slope, we shift the afterglow 
into the z = 1 frame (following Kann, Klose & Zeh 2006, ApJ, 641, 993). 
We find that at 6.2 days (z = 1 frame), this transient is among the ten 
most luminous GRB afterglows, yielding further evidence that it itself 
is a bona fide GRB afterglow. The long rise indicates an off-axis 
origin, and the luminous afterglow makes it likely the initial GRB was 
very energetic. So far no viable candidates have been reported by the 
IPN (it is not positionally coincident with the short GRB 220211A, 
Andreoni et al., GCN #31590; also we note a short GRB at such a high 
redshift would not be expected to have such a luminous afterglow), 
leading to the conclusion that the off-axis angle was large enough to 
strongly suppress the prompt emission - a true orphan afterglow.
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