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Context: Research on supernovae (SNe) over the past decade has confirmed thatthere is a distinct class of events which are much more luminous (by $\sim2$mag) than canonical core-collapse SNe (CCSNe). These events with visual peakmagnitudes $\lesssim-21$ are called superluminous SNe (SLSNe). Aims: There area few intermediate events which have luminosities between these two classes.Here we study one such object, SN 2012aa. Methods: The optical photometric andspectroscopic follow-up observations of the event were conducted over a timespan of about 120 days. Results: With V_abs at peak ~-20 mag, the SN is anintermediate-luminosity transient between regular SNe Ibc and SLSNe. It alsoexhibits an unusual secondary bump after the maximum in its light curve. Weinterpret this as a manifestation of SN-shock interaction with the CSM. If wewould assume a $^{56}$Ni-powered ejecta, the bolometric light curve requiresroughly 1.3 M_sun of $^{56}$Ni and an ejected mass of ~14 M_sun. This wouldalso imply a high kinetic energy of the explosion, ~5.4$\times10^{51}$ ergs. Onthe other hand, the unusually broad light curve along with the secondary peakindicate the possibility of interaction with CSM. The third alternative is thepresence of a central engine releasing spin energy that eventually powers thelight curve over a long time. The host of the SN is a star-forming Sa/Sb/Sbcgalaxy. Conclusions: Although the spectral properties and velocity evolution ofSN 2012aa are comparable to those of normal SNe Ibc, its broad light curvealong with a large peak luminosity distinguish it from canonical CCSNe,suggesting the event to be an intermediate-luminosity transient between CCSNeand SLSNe at least in terms of peak luminosity. We argue that SN 2012aa belongsto a subclass where CSM interaction plays a significant role in powering theSN, at least during the initial stages of evolution.

SN 2012aa - a transient between Type Ibc core-collapse and superluminous supernovae