The Redshift of GRB 190829A/SN 2019oyw: A Case Study of GRB-SN Evolution

The nearby long gamma-ray burst (GRB) 190829A was observed using the Hubble Space Telescope/Wide Field Camera 3/infrared grisms about four weeks to 500 days after the burst. We find the spectral features of its associated supernova, SN 2019oyw, are redshifted by several thousand km s^‑1 compared to the redshift of the large spiral galaxy on which it is superposed. This velocity offset is seen in several features but most clearly in Ca II near-infrared triplet λλ8498, 8542, 8662 (CaIR3). We also analyze Very Large Telescope/FOcal Reducer and low dispersion Spectrograph and X-shooter spectra of the supernova (SN) and find strong evolution with time of its P-Cygni features of CaIR3 from the blue to the red. However, comparison with a large sample of Type Ic-BL and Ic SNe shows no other object with the CaIR3 line as red as that of SN 2019oyw were it at the z = 0.0785 redshift of the disk galaxy. This implies that SN 2019oyw is either a highly unusual SN or is moving rapidly with respect to its apparent host. Indeed, using CaIR3 we find the redshift of SN 2019oyw is 0.0944 ≤ z ≤ 0.1156. The GRB-SN is superposed on a particularly dusty region of the massive spiral galaxy; therefore, while we see no sign of a small host galaxy behind the spiral, it could be obscured. Our work provides a surprising result on the origins of GRB 190829A, as well as insights into the time evolution of GRB-SNe spectra and a method for directly determining the redshift of a GRB-SN using the evolution of strong spectral features such as CaIR3.