On 2012 May 17.2 UT , only 1.5 \pm 0.2 d after explosion , we discovered SN 2012cg , a Type Ia supernova ( SN Ia ) in ( catalog NGC 4424 ) ( d \approx 15 Mpc ) .
As a result of the newly modified strategy employed by the Lick Observatory SN Search , a sequence of filtered images was obtained starting 161 s after discovery .
Utilizing recent models describing the interaction of SN ejecta with a companion star , we rule out a \sim 1 M _ { \sun } companion for half of all viewing angles and a red-giant companion for nearly all orientations .
SN 2012cg reached a B -band maximum of 12.09 \pm 0.02 mag on 2012 June 2.0 and took \sim 17.3 d from explosion to reach this , typical for SNe Ia .
Our pre-maximum-brightness photometry shows a narrower-than-average B -band light curve for SN 2012cg , though slightly overluminous at maximum brightness and with normal color evolution ( including some of the earliest SN Ia filtered photometry ever obtained ) .
Spectral fits to SN 2012cg reveal ions typically found in SNe Ia at early times , with expansion velocities \gtrsim 14,000 km s ^ { -1 } at 2.5 d past explosion .
Absorption from C ii is detected early , as well as high-velocity components of both Si ii \lambda 6355 and Ca ii .
Our last spectrum ( 13.5 d past explosion ) resembles that of the somewhat peculiar SN Ia 1999aa .
This suggests that SN 2012cg will have a slower-than-average declining light curve , which may be surprising given the faster-than-average rising light curve .