{
  "bibcode": "2021GCN.29344....1K",
  "body": "D. A. Kann (HETH/IAA-CSIC), A. Nicuesa Guelbenzu (TLS Tautenburg), A. de \nUgarte Postigo (HETH/IAA-CSIC, DARK/NBI), C. C. Thoene (HETH), M. \nBlazek, J. F. Agui Fernandez (both HETH/IAA-CSIC), A. Rau (MPE \nGarching), S. Klose (TLS Tautenburg), and J. I. Vico Linares (CAHA) \nreport:\n\nWe obtained further observations of the GRB-less afterglow \nZTF21aaeyldq/AT2021any (Ho et al., GCN #29305; de Ugarte Postigo et al., \nGCN #29307) with CAFOS at the 2.2m telescope at Calar Alto, Almeria, \nSpain, in the Rc band at 2.8 days post-discovery, and with GROND mounted \nat the 2.2m MPG telescope at the ESO La Silla Observatory (Chile) at 3.9 \ndays after the trigger. The afterglow is clearly detected in each \nstacked image.\n\nFurther to the observations and analysis described in Kann et al. (GCN \n#29321, with data from Ho et al., GCN #29305; de Ugarte Postigo et al., \nGCN #29307; and Zhu et al., GCN #29310) and Nicuesa Guelbenzu et al. \n(GCN #29330), we fit the combined data set, also leaving the host-galaxy \nmagnitudes free, as the GROND data shows a characteristic flattening.\n\nWe find that a single power-law fit does not describe the data well \n(chi^2/d.o.f. = 3.7), overestimating the CAHA data and not fitting the \ncurvature seen in the GROND data. However, a broken power-law fit yields \na significantly improved result (chi^2/d.o.f. = 0.12) with fit \nparameters alpha_1 = 0.95 +/- 0.03, alpha_2 = 2.30 +/- 0.76 and t_b = \n0.82 +/- 0.08 days. This fit may be improved or modified with further \ndata/observations, but the break signature is clear. It therefore \nconfirms the initial suggestion of Kann et al. (GCN #29321). This is a \ntypical feature in GRB afterglows and a further indicator that the \nnature of this transient is a GRB afterglow.\n\nHo et al., GCN #29305, report the first detection at \n2021-01-16T06:59:45.6, and a deep non-detection 22 minutes earlier. \nAntia et al., GCN #29340, and Nadella et al., GCN #29342, report the \ndetection of a bright GRB 210116A with AstroSat LAXPC and CZTI at ~05:53 \nUR on the same day, about 44 minutes before the ZTF non-detection. \nJudging from typical GRB afterglow behavior, this makes it unlikely that \nthe two events are associated with each other but does not rule it out. \nAn IPN localization of GRB 210116A could confirm or rule out the \nassociation.",
  "circularId": 29344,
  "createdOn": 1611337067000,
  "email": "kann@iaa.es",
  "subject": "ZTF21aaeyldq: GROND and CAHA jet break confirmation",
  "submitter": "Alexander Kann at IAA-CSIC  <kann@iaa.es>",
  "eventId": "ZTF21aaeyldq"
}