GCN Circular 31626
Subject
ZTF22aaajecb/AT 2022cmc: CAHA 2.2m/CAFOS detection, luminous transient
Event
Date
2022-02-21T21:16:30Z (4 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.