In this paper, we describe our investigation of the broad-lined (BL) Type Ic SN 2010ay. Our optical photometric and spectroscopic observations demonstrate that this SN bears a striking resemblance to the recent spectroscopically-confirmed GRB-SN 2010bh with respect to its low host environment metallicity and high photospheric expansion velocity. However, our radio and gamma ray observations place strong upper limits on the relativistic ejecta properties that may have been associated with the SN, effectively ruling out a GRB. This object challenges the importance of progenitor metallicity in producing a GRB and suggests that other parameters also play a key role.
We construct an optical R-band lightcurve using our observations and those reported in the literature. By fitting the template lightcurve of Drout et al. 2011, we determine a peak luminosity of M_R=-20.2 mag, making SN 2010ay one of the brightest SNe Ic ever discovered. This luminosity implies that ~1 solar mass of nickle was synthesized in the explosion.
From our optical spectroscopy at two epochs ~20 days after peak, we measure the photospheric expansion velocity of the SN from the blueshift of the Si II feature. By fitting power laws to the time-evolution of the velocity profile, we find that the high velocity of SN 2010ay has more in common with GRB-SNe than normal SNe Ic-BL.
Nonetheless, our late-time radio non-detections with the EVLA rule out the assocation of an off-axis GRB with relativistic ejecta properties similar to most nearby GRB-SNe. Gamma-ray non-detections from Swift and the Interplanetary Network similarly rule out an associated GRB with fluxes similar to most nearby GRB-SNe.
We measure the metallicity of the host galaxy to be ~0.3 Z_solar, placing the SN progenitor environment in the low-metallicity realm GRB progenitors rather than the high-metallicity environments where typical SNe Ic are found. We investigate the displacement of the host galaxy from the L-Z relation for star-forming galaxies.