Spectroscopic Discovery of the Broad-Lined Type Ic Supernova 2010bh Associated with the Low-Redshift GRB 100316D

TL;DR

This paper reports the spectroscopic discovery of a broad-lined Type Ic supernova (SN 2010bh) associated with a nearby gamma-ray burst, revealing high ejecta velocities and a stripped helium envelope, with implications for explosion energy and progenitor characteristics.

Contribution

First spectroscopic detection of SN 2010bh associated with GRB 100316D, showing unprecedented ejecta velocities and insights into progenitor properties.

Findings

SN 2010bh has ejecta velocities ~26,000 km/s at 21 days.

The progenitor was largely stripped of helium.

Host galaxy is low luminosity and low metallicity.

Abstract

We present the spectroscopic discovery of a broad-lined Type Ic supernova (SN 2010bh) associated with the nearby long-duration gamma-ray burst (GRB) 100316D. At z = 0.0593, this is the third-nearest GRB-SN. Nightly optical spectra obtained with the Magellan telescopes during the first week after explosion reveal the gradual emergence of very broad spectral features superposed on a blue continuum. The supernova features are typical of broad-lined SNe Ic and are generally consistent with previous supernovae associated with low-redshift GRBs. However, the inferred velocities of SN 2010bh at 21 days after explosion are a factor of ~2 times larger than those of the prototypical SN 1998bw at similar epochs, with v ~ 26,000 km/s, indicating a larger explosion energy or a different ejecta structure. A near-infrared spectrum taken 13.8 days after explosion shows no strong evidence for He I at 1.083 microns, implying that the progenitor was largely stripped of its helium envelope. The host galaxy is of low luminosity (M_R ~ -18.5 mag) and low metallicity (Z < 0.4 Z_solar), similar to the hosts of other low-redshift GRB-SNe.