Supernova light-curve models for the bump in the optical counterpart of X-ray flash 030723

XRF 030723 is the first X-ray flash (XRF) to show in its optical light curve (LC) a bump that has been interpreted as the signature of a supernova (SN). After subtracting the afterglow component from the observed optical LC of the XRF counterpart, the properties of the putative SN are constrained by means of synthetic LCs of core-collapse SNe. For the redshift range z similar to 0.3-1, all possible models require a rather small mass of synthesized Ni-56, i.e., M(Ni-56) similar to 0.01-0.3 M-.. The models used to describe the energetic SNe Ic associated with gamma-ray bursts (SNe 1998bw and 2003dh) are too massive for the observed LC. If the relation between ejected 56Ni mass and total ejecta mass established from models of various Type Ic SNe also holds for the putative SN in XRF 030723, the ejecta mass is constrained to be similar to 1 - 3 M-. and the kinetic energy less than or similar to1 x 10(52) ergs. This corresponds to a progenitor with 15 M-. less than or similar to M-MS less than or similar to 25 M-.. The SN therefore appears to have properties inter mediate between a normal SN Ic such as SN 1994I and a more energetic object such as SN 2002ap.