  It is now accepted that long duration gamma-ray bursts (GRBs) are produced
during the collapse of a massive star. The standard "collapsar" model predicts
that a broad-lined and luminous Type Ic core-collapse supernova (SN)
accompanies every long-duration GRB. This association has been confirmed in
observations of several nearby GRBs. Here we present observations of two nearby
long-duration GRBs that challenge this simple view. In the GRBs 060505 and
060614 we demonstrate that no SN emission accompanied these long-duration
bursts, down to limits hundreds of times fainter than the archetypal SN 1998bw
that accompanied GRB 980425, and fainter than any Type Ic SN ever observed.
Multi-band observations of the early afterglows, as well as spectroscopy of the
host galaxies, exclude the possibility of significant dust obscuration and show
that the bursts originated in star-forming regions. The absence of a SN to such
deep limits is qualitatively different from all previous nearby long GRBs and
suggests a new phenomenological type of massive stellar death. From the
supplementary material: Now we have observed SN-less GRBs in star-forming
regions, suggesting that a non-detection of a SN does not preclude a massive
progenitor. The position of the GRB, i.e. in a star-forming region or in an
older component, may be the only way to discriminate between merging compact
objects and massive stars as progenitors. In fact, several host galaxies for
short GRBs have been found to be as actively star-forming as some host galaxies
of long-duration GRBs. The GRB labels "long" and "short" have become synonymous
with "massive stars" and "other progenitors". These distinctions may need to be
relaxed.
