TITLE:   GCN CIRCULAR
NUMBER:  29321
SUBJECT: ZTF21aaeyldq CAHA 2.2m detection and power-law decay
DATE:    21/01/18 18:06:36 GMT
FROM:    Alexander Kann at IAA-CSIC  <kann@iaa.es>

D. A. Kann (HETH/IAA-CSIC), A. de Ugarte Postigo (HETH/IAA-CSIC, 
DARK/NBI), C. C. Thoene (HETH), M. Blazek, J. F. Agui Fernandez (both 
HETH/IAA-CSIC), and J. I. Vico Linares (CAHA) report:

We observed the position of the GRB-less afterglow ZTF21aaeyldq (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. We 
obtained 10 x 360 s exposures, centered at 1.74430 days after the 
transient discovery, under fair conditions but mediocre seeing.

The afterglow (Kumar et al., GCNs #29308; Ahumada et al., GCN #29309; 
Zhu et al., GCN #29310; Hu et al., GCN #29312) is clearly detected. We 
measure Rc = 22.89 +/- 0.12 mag. (AB mag, vs. the same SDSS star as in 
our GTC observation, GCN #29307 converted to Rc following the Lupton 
transformations, then transformed back to AB mag).

We take data from Ho et al., GCN #29305; de Ugarte Postigo et al., GCN 
#29307; and Zhu et al., GCN #29310 (all in r'), and here assume a 
trigger time 22 minutes before the discovery (at the last 
non-detection). We find this data is fit well with a single power-law, a 
straight decay slope alpha ~ 0.91 +/- 0.02. The magnitude we report now 
lies slightly (0.3 mag) below the extrapolation of the afterglow decay. 
Further observations are needed to determine whether this represents a 
jet break.