SN 2010ay IS A LUMINOUS AND BROAD-LINED TYPE Ic SUPERNOVA WITHIN A LOW-METALLICITY HOST GALAXY

We report on our serendipitous pre-discovery detection and follow-up observations of the broad-lined Type Ic supernova (SN Ic) 2010ay at z = 0.067 imaged by the Pan-STARRS1 3π survey just ∼4 days after explosion. The supernova (SN) had a peak luminosity, M R ≈ −20.2 mag, significantly more luminous than known GRB–SNe and one of the most luminous SNe Ib/c ever discovered. The absorption velocity of SN 2010ay is v Si ≈ 19 × 10 3 km s −1 at ∼40 days after explosion, 2–5 times higher than other broad-lined SNe and similar to the GRB–SN 2010bh at comparable epochs. Moreover, the velocity declines ∼2 times slower than other SNe Ic-BL and GRB–SNe. Assuming that the optical emission is powered by radioactive decay, the peak magnitude implies the synthesis of an unusually large mass of 56 Ni, M Ni = 0.9 M ☉ . Applying scaling relations to the light curve, we estimate a total ejecta mass, M ej ≈ 4.7 M ☉ , and total kinetic energy, E K ≈ 11 × 10 51 erg. The ratio of M Ni to M ej is ∼2 times as large for SN 2010ay as typical GRB–SNe and may suggest an additional energy reservoir. The metallicity (log (O/H) PP04 + 12 = 8.19) of the explosion site within the host galaxy places SN 2010ay in the low-metallicity regime populated by GRB–SNe, and ∼0.5(0.2) dex lower than that typically measured for the host environments of normal (broad-lined) SNe Ic. We constrain any gamma-ray emission with E γ ≲ 6 × 10 48 erg (25–150 keV), and our deep radio follow-up observations with the Expanded Very Large Array rule out relativistic ejecta with energy E ≳ 10 48 erg. We therefore rule out the association of a relativistic outflow like those that accompanied SN 1998bw and traditional long-duration gamma-ray bursts (GRBs), but we place less-stringent constraints on a weak afterglow like that seen from XRF 060218. If this SN did not harbor a GRB, these observations challenge the importance of progenitor metallicity for the production of relativistic ejecta and suggest that other parameters also play a key role.