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

Kornpob Bhirombhakdi; Andrew S. Fruchter; Andrew J. Levan; Elena Pian; Paolo Mazzali; Luca Izzo; Tuomas Kangas; Stefano Benetti; Kyle Medler; Nial Tanvir

arXiv · 2410.09372

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

Kornpob Bhirombhakdi·Andrew S. Fruchter·Andrew J. Levan·Elena Pian·Paolo Mazzali·Luca Izzo·Tuomas Kangas·Stefano Benetti·Kyle Medler·Nial Tanvir

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Abstract

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 find the spectral features of its associated supernova, SN 2019oyw, are redshifted by several thousands km/s 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 NIR triplet $\lambda\lambda$ 8498, 8542, 8662 (CaIR3). We also analyze VLT/FORS and X-shooter spectra of the 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.

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Kornpob Bhirombhakdi, Andrew S. Fruchter, Andrew J. Levan, Elena Pian, Paolo Mazzali, Luca Izzo, Tuomas Kangas, Stefano Benetti, Kyle Medler, Nial Tanvir. 2024-10-12. The Redshift of GRB 190829A/ SN 2019oyw: A Case Study of GRB-SN Evolution. https://arxiv.org/abs/2410.09372

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