[vsnet-grb-info 23544] Liverpool Telescope observations of ZTF19abvizsw, a candidate untriggered GRB afterglow

[GCN Circulars]

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 at ljmu.ac.uk>

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.

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