AstroNote 2020-29

The Zwicky Transient Facility (ZTF; ATel #11266) reports the discovery of ZTF20aajnksq, a rapidly fading red transient located at

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 absorption features consistent with the Lyman break and Lyman alpha at z=2.9.

We obtained a target-of-opportunity observation with the Neil Gehrels Swift Observatory, and detected X-ray emission at 0.0039 ct/s. Assuming a neutral hydrogen column density n_H = 1.77E20 cm^-2 and a power-law spectrum with photon index 2, the unabsorbed flux is 1.5E-13 erg cm^-2 s^-1, corresponding to a luminosity of 1.1E46 erg s^-1. 

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.