The spectroscopic classification and explosion properties of SN2009nz associated with GRB091127 at z = 0.490

We present spectroscopic observations of GRB091127 (z = 0.490) at the peak of the putative associated supernova SN2009nz. Subtracting a late-time spectrum of the host galaxy, we isolate the contribution of SN2009nz and uncover broad features typical of nearby gamma-ray-burst-supernovae (GRB-SNe). This establishes unambiguously that GRB091127 was accompanied by a broad-lined Type Ic SN, and links a cosmological long burst with a standard energy release (E _γ,iso ≈ 1.1 × 10⁵²erg) to a massive star progenitor. The spectrum of SN2009nz closely resembles that of SN2006aj, with SN2003dh also providing an acceptable match, but has significantly narrower features than SNe 1998bw and 2010bh, indicative of a lower expansion velocity. The photospheric velocity inferred from the Si II λ6355 absorption feature, v _ph ≈ 17, 000kms^-1, is indeed closer to that of SNe 2006aj and 2003dh than to the other GRB-SNe. Combining the measured velocity with the light curve peak brightness and width, we estimate the following nominal (maximal) explosion parameters: M _Ni≈ 0.35 (0.6) M _⊙, E_K ≈2.3 × 10⁵¹ (8.4 × 10⁵¹)erg, and M _ej≈ 1.4 (3.5) M _⊙, similar to those of SN2006aj. These properties indicate that SN2009nz follows a trend of lower M _Ni for GRB-SNe with lower E_K and M _ej. Equally important, since GRB091127 is a typical cosmological burst, the similarity of SN2009nz to SN2006aj either casts doubt on the claim that XRF060218/SN2006aj was powered by a neutron star or indicates that the nature of the central engine is encoded in the SN properties but not in the prompt emission. Future spectra of GRB-SNe at z ≳ 0.3 will shed light on the full dispersion of SN properties for standard long GRBs, on the relation between SNe associated with sub-energetic and standard GRBs, and on a potential dispersion in the associated SN types.