A tale of two GRB-SNe at a common redshift of z = 0.54
  We present ground-based and HST optical observations of the optical
transients (OTs) of long-duration Gamma Ray Bursts (GRBs) 060729 and 090618,
both at a redshift of z = 0.54. For GRB 060729, bumps are seen in the optical
light curves (LCs), and the late-time broadband spectral energy distributions
(SEDs) of the OT resemble those of local type Ic supernovae (SNe). For GRB
090618, the dense sampling of our optical observations has allowed us to detect
well-defined bumps in the optical LCs, as well as a change in colour, that are
indicative of light coming from a core-collapse SN. The accompanying SNe for
both events are individually compared with SN1998bw, a known GRB-supernova, and
SN1994I, a typical type Ic supernova without a known GRB counterpart, and in
both cases the brightness and temporal evolution more closely resemble
SN1998bw. We also exploit our extensive optical and radio data for GRB 090618,
as well as the publicly-available SWIFT -XRT data, and discuss the properties
of the afterglow at early times. In the context of a simple jet-like model, the
afterglow of GRB 090618 is best explained by the presence of a jet-break at
t-to > 0.5 days. We then compare the rest-frame, peak V -band absolute
magnitudes of all of the GRB and X-Ray Flash (XRF)-associated SNe with a large
sample of local type Ibc SNe, concluding that, when host extinction is
considered, the peak magnitudes of the GRB/XRF-SNe cannot be distinguished from
the peak magnitudes of non-GRB/XRF SNe.
