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

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.

💡 Research Summary

We present a comprehensive spectroscopic study of the broad‑lined Type Ic supernova SN 2010bh, which was discovered in association with the nearby long‑duration gamma‑ray burst GRB 100316D (z = 0.0593). This event is the third‑closest GRB‑SN known, providing an exceptional opportunity to probe the early evolution of a GRB‑linked supernova.

Observations were carried out with the Magellan 6.5‑m telescopes (Clay and Baade) obtaining nightly optical spectra from the first night after the burst through the first week. A near‑infrared (NIR) spectrum was obtained with Gemini‑South/GNIRS at 13.8 days post‑explosion. Standard reduction procedures, including flux calibration with spectrophotometric standards and telluric correction, yielded wavelength‑calibrated spectra with an accuracy of ≈ ±2 Å.

The early optical spectra are dominated by a blue continuum with extremely broad absorption features. As the supernova evolves, the depth and width of the Si II λ6355, Ca II NIR triplet, and blended Fe II lines increase. By 21 days after explosion the Si II absorption minimum corresponds to a velocity of ~26,000 km s⁻¹, roughly twice that measured for the prototypical GRB‑SN SN 1998bw at comparable epochs (≈ 13,000 km s⁻¹). This high velocity implies either a larger kinetic energy (∼10⁵² erg) or a different ejecta density structure, such as a thinner outer layer or a more asymmetric explosion.

The NIR spectrum shows no significant He I 1.083 µm absorption, indicating that the progenitor star had been almost completely stripped of its helium envelope, consistent with the classification as a Type Ic event and with the prevailing picture that GRB‑associated supernovae arise from highly stripped massive stars.

Host‑galaxy analysis, based on archival imaging and emission‑line diagnostics, reveals a low‑luminosity system (M_R ≈ ‑18.5 mag) with sub‑solar metallicity (Z < 0.4 Z_⊙). These properties echo those of other low‑redshift GRB‑SN hosts (e.g., GRB 980425/SN 1998bw, GRB 060218/SN 2006aj) and support theoretical expectations that low metallicity favors rapid rotation and jet formation in massive stars.

We discuss the implications of the unusually high velocities. Simple analytical models suggest an ejected mass of 3–5 M_⊙ and a kinetic energy of (2–3) × 10⁵² erg, indicating a more energetic explosion than SN 1998bw, albeit with a comparable or slightly lower ejecta mass. The data are also compatible with a strongly aspherical explosion, where a relativistic jet deposits additional energy into the outer layers, producing the observed broad lines.

In summary, SN 2010bh adds a valuable data point to the small sample of nearby GRB‑SN events. Its extreme line velocities, lack of helium, and occurrence in a low‑metallicity, low‑luminosity host reinforce the connection between stripped massive progenitors, high‑energy jet production, and the environments that enable long‑duration GRBs. Future work, including spectropolarimetry, high‑resolution radio imaging, and three‑dimensional radiative‑transfer modeling, will be essential to disentangle the geometry of the ejecta and to quantify the role of asymmetry in shaping the observed properties.