TITLE:   GCN CIRCULAR
NUMBER:  26966
SUBJECT: ZTF Discovery of ZTF20aajnksq: a Rapidly Fading Luminous Transient in the GRB 200128A Localiziation Region
DATE:    20/02/02 21:53:10 GMT
FROM:    Anna Ho at Caltech  <annayqho@gmail.com>

The Zwicky Transient Facility (ZTF; ATel #11266) reports the discovery of
ZTF20aajnksq, a rapidly fading red transient located at
12:47:04.87 +45:12:02.3 (J2000)
191.770292 +45.200626 (J2000)
and detected as part of the ZTF Uniform Depth Survey (Goldstein et al. in
prep).

The source was discovered on UT 2020-01-28T06:43:20.640 at r=19.6 mag,
which was 0.74 days after the last non-detection. Follow-up photometry with
the IO:O optical imager on the Liverpool Telescope (PI: D. Perley) and the
Wafer-Scale Imager for Prime (WASP) on the 200-inch Hale telescope (PI: I.
Andreoni) at Palomar Observatory established a rapid fade rate of 2.5
magnitudes in 1.25 days as well as red colors (g-r=+0.65, from the WASP
observation).

We obtained a target-of-opportunity (PI: M. M. Kasliwal) spectrum with the
Low Resolution Imaging Spectrometer (LRIS) on the Keck I 10-m telescope,
which showed features consistent with the Lyman break and Lyman alpha
absorption at z=2.9.

We obtained a target-of-opportunity observation with the Neil Gehrels Swift
Observatory (Swift) and detected X-ray emission at 0.0039 ct/s. Assuming a
neutral hydrogen column density n_H = 1.77E20/cm2 and a power-law spectrum
with photon index 2, the unabsorbed flux is 1.5E-13 erg/cm^2/s,
corresponding to a luminosity of 1.1E46 erg/s.

We searched the Fermi/GBM catalog and identified a reported likely short
GRB 24 degrees away: GRB 200128A (GCN #26909). ZTF20aajnksq is located
along the 90% contour of the localization region. The trigger time of GRB
200128A is three hours before the first optical detection.

We are grateful to the staff of Palomar and Keck for rapidly scheduling our
observations, and to M. Heida (ESO Garching) and J. Rhoads (Goddard) for
accommodating the interruption to their scheduled programs. Thank you to
the Swift team for rapidly scheduling and executing our observations.

ZTF is a project led by PI S. R. Kulkarni at Caltech (see ATEL #11266), and
includes IPAC; WIS, Israel; OKC, Sweden; JSI/UMd, USA; UW,USA; DESY,
Germany; NRC, Taiwan; UW Milwaukee, USA and LANL USA. ZTF acknowledges the
generous support of the NSF under AST MSIP Grant No 1440341. Alert
distribution service provided by DIRAC@UW. Alert filtering is being
undertaken by the GROWTH marshal system, supported by NSF PIRE grant
1545949.