GCN Circular 29344

D. A. Kann (HETH/IAA-CSIC), A. Nicuesa Guelbenzu (TLS Tautenburg), A. de 
Ugarte Postigo (HETH/IAA-CSIC, DARK/NBI), C. C. Thoene (HETH), M. 
Blazek, J. F. Agui Fernandez (both HETH/IAA-CSIC), A. Rau (MPE 
Garching), S. Klose (TLS Tautenburg), and J. I. Vico Linares (CAHA) 
report:

We obtained further observations of the GRB-less afterglow 
ZTF21aaeyldq/AT2021any
Not found
 
 
 (Ho et al., GCN #29305; de Ugarte Postigo et al., 
GCN #29307) with CAFOS at the 2.2m telescope at Calar Alto, Almeria, 
Spain, in the Rc band at 2.8 days post-discovery, and with GROND mounted 
at the 2.2m MPG telescope at the ESO La Silla Observatory (Chile) at 3.9 
days after the trigger. The afterglow is clearly detected in each 
stacked image.

Further to the observations and analysis described in Kann et al. (GCN 
#29321, with data from Ho et al., GCN #29305; de Ugarte Postigo et al., 
GCN #29307; and Zhu et al., GCN #29310) and Nicuesa Guelbenzu et al. 
(GCN #29330), we fit the combined data set, also leaving the host-galaxy 
magnitudes free, as the GROND data shows a characteristic flattening.

We find that a single power-law fit does not describe the data well 
(chi^2/d.o.f. = 3.7), overestimating the CAHA data and not fitting the 
curvature seen in the GROND data. However, a broken power-law fit yields 
a significantly improved result (chi^2/d.o.f. = 0.12) with fit 
parameters alpha_1 = 0.95 +/- 0.03, alpha_2 = 2.30 +/- 0.76 and t_b = 
0.82 +/- 0.08 days. This fit may be improved or modified with further 
data/observations, but the break signature is clear. It therefore 
confirms the initial suggestion of Kann et al. (GCN #29321). This is a 
typical feature in GRB afterglows and a further indicator that the 
nature of this transient is a GRB afterglow.

Ho et al., GCN #29305, report the first detection at 
2021-01-16T06:59:45.6, and a deep non-detection 22 minutes earlier. 
Antia et al., GCN #29340, and Nadella et al., GCN #29342, report the 
detection of a bright GRB 210116A with AstroSat LAXPC and CZTI at ~05:53 
UR on the same day, about 44 minutes before the ZTF non-detection. 
Judging from typical GRB afterglow behavior, this makes it unlikely that 
the two events are associated with each other but does not rule it out. 
An IPN localization of GRB 210116A could confirm or rule out the 
association.