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 $HST$/WFC3/IR grisms about four weeks to 500 days after the burst. We obtained spectra of its associated supernova (SN)~2019oyw. However, we find that the spectral features associated with the SN are redshifted by several thousands km/s compared to the redshift of the large spiral on which it is superposed. This velocity offset is seen most clearly in the \CaIRfull~(CaIR3) emission of the SN, but is also visible in other features, and in particular \HeIfull.

To better understand this, we also analyzed spectra of the SN observed with the VLT/FORS and X-shooter, and found an evolution with time of the P-Cygni features of CaIR3 from the blue to the red. However, the 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 truly at the redshift of the disk galaxy.

This implies that SN~2019oyw, if hosted by the spiral galaxy at redshift $z = 0.0785$, was either a highly unusual SN, or was moving at several thousands km/s with respect to its host. If instead the GRB was in a more distant galaxy behind the spiral, we can constrain its redshift, using the CaIR3 evolution, to a range of $0.0944 \leq z^* \leq 0.1156$. While we see no sign of a distant galaxy, the GRB is superposed on a particularly dusty region of the massive spiral galaxy, and thus a small host galaxy could be obscured. Our work provides not only a surprising result on the origins of the nearby GRB~190829A, but also insights into the time evolution of GRB-SNe and a potential method for directly determining the redshift of a GRB-SN with accuracy at the order of 1,000 km/s.