  We present the discovery, classification, and radio-to-X-ray follow-up
observations of iPTF17cw, a broad-lined (BL) type Ic supernova (SN) discovered
by the intermediate Palomar Transient Factory (iPTF). Although unrelated to the
gravitational wave trigger, this SN was discovered as a happy by-product of the
extensive observational campaign dedicated to the follow-up of Advanced LIGO
event GW170104. The spectroscopic properties and inferred peak bolometric
luminosity of iPTF17cw are most similar to the gamma-ray burst (GRB) associated
SN 1998bw, while the shape of the r-band light curve is most similar to that of
the relativistic SN 2009bb. Karl G. Jansky Very Large Array (VLA) observations
of the iPTF17cw field reveal a radio counterpart about 10 times less luminous
than SN 1998bw, and with peak radio luminosity comparable to that of SN
2006aj/GRB 060218 and SN 2010bh/GRB 100316D. Our radio observations of iPTF17cw
imply a relativistically expanding outflow. However, further late-time
observations with the VLA in its most extended configuration are needed to
confirm fading of iPTF radio counterpart at all frequencies. X-ray observations
carried out with Chandra reveal the presence of an X-ray counterpart with
luminosity similar to that of SN 2010bh/GRB 100316D. Searching the Fermi
catalog for possible gamma-rays reveals that GRB 161228B is spatially and
temporally compatible with iPTF17cw. The similarity to SN 1998bw and SN 2009bb,
the radio and X-ray detections, and the potential association with GRB 161228B,
all point to iPTF17cw being a new candidate member of the rare sample of
optically-discovered engine-driven BL-Ic SNe associated with relativistic
ejecta.
