The spectroscopic classification and explosion properties of SN2009NZ associated with GRB091127 at z = 0.490

Abstract

We present spectroscopic observations of GRB 091127 (z = 0.490) at the peak of the putative associated supernova SN 2009nz. Subtracting a late-time spectrum of the host galaxy, we isolate the contribution of SN 2009nz and uncover broad features typical of nearby GRB-SNe. This establishes unambiguously that GRB 091127 was accompanied by a broad-lined Type Ic SN, and links a cosmological long burst with a standard energy release (Eγ,iso ≈ 1.1 × 1052 erg) to a massive star progenitor. The spectrum of SN 2009nz closely resembles that of SN 2006aj, with SN 2003dh 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 IIλ6355 absorption feature, vph ≈ 17,000 km s−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 explosion parameters: MNi ∼ 0.35 M⊙, EK ∼ 2.3×1051 erg, and Mej ∼ 1.4 M⊙, similar to those of SN 2006aj. These properties indicate that SN 2009nz follows a trend of lower MNi for GRB-SNe with lower EK and Mej. Equally important, since GRB 091127 is a typical cosmological burst, the similarity of SN 2009nz to SN 2006aj either casts doubt on the claim that XRF 060218/SN 2006aj 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, including proper subtraction of the host galaxy contribution, 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.