TITLE: GCN CIRCULAR NUMBER: 25643 SUBJECT: Liverpool Telescope observations of ZTF19abvizsw, a candidate untriggered GRB afterglow DATE: 19/09/04 04:50:36 GMT FROM: Daniel Perley at Liverpool JMU D. A. Perley (LJMU), A. Y. Q. Ho (Caltech), and C. M. Copperwheat (LJMU) report on behalf of the GROWTH collaboration: We obtained Liverpool Telescope IO:O imaging observations of ZTF19abvizsw/AT2019pim (Kool et al., GCN 25616), initially reported as a candidate counterpart to S190901ap but recently shown (Burdge et al., GCN 25639) to be an unrelated luminous transient at high redshift. Three r-band images of 150 seconds each were acquired between UT 20:28 and 20:35, followed by a griz sequence (3x150s per filter) between 22:33 and 23:08. The transient is well-detected in all four bands, but has faded substantially since the ZTF discovery epoch (see also Wei et al., GCN 15640). We report the following photometry (times given as UT midpoints): Time (UT) filter mag(AB) unc ------------------- ------ ------ ---- 2019-09-03 22:37:50 r 21.62 0.09 2019-09-03 22:46:33 g 22.02 0.10 2019-09-03 22:55:16 i 21.16 0.07 2019-09-03 23:03:57 z 20.87 0.12 After correcting for Galactic extinction (Schlafly et al. 2011, ApJ 737 103) the SED is well-fit by to a power-law in frequency with spectral index (F_nu ~ nu^beta) of beta ~ -1.4. As noted in GCN 25616, this field was imaged by ZTF every night during the 30 days prior to the ZTF discovery. No source was detected in any of these observations, with the most recent nondetection corresponding to 2019-09-01 UT 07:35, 0.86 days prior to the ZTF discovery observation. The fast rise, red power-law spectrum, and fast decline are consistent with observations of known GRB optical afterglows. However, we searched the Fermi GBM subthreshold GRB catalog over the window between the last ZTF detection and the discovery observation and found no GRB consistent with the location of the transient over time window. Further observations, including multiwavelength observations, have been requested. ________________________________ Important Notice: the information in this email and any attachments is for the sole use of the intended recipient(s). If you are not an intended recipient, or a person responsible for delivering it to an intended recipient, you should delete it from your system immediately without disclosing its contents elsewhere and advise the sender by returning the email or by telephoning a number contained in the body of the email. No responsibility is accepted for loss or damage arising from viruses or changes made to this message after it was sent. The views contained in this email are those of the author and not necessarily those of Liverpool John Moores University.