The Type Ia supernova ( SN Ia ) 2013dy in NGC 7250 ( d \approx 13.7 Mpc ) was discovered by the Lick Observatory Supernova Search .
Combined with a prediscovery detection by the Italian Supernova Search Project , we are able to constrain the first-light time of SN 2013dy to be only 0.10 \pm 0.05 d ( 2.4 \pm 1.2 hr ) before the first detection .
This makes SN 2013dy the earliest known detection of an SN Ia .
We infer an upper limit on the radius of the progenitor star of R _ { 0 } \lesssim 0.25 { R } _ { \sun } , consistent with that of a white dwarf .
The light curve exhibits a broken power law with exponents of 0.88 and then 1.80 .
A spectrum taken 1.63 d after first light reveals a C ii absorption line comparable in strength to Si ii .
This is the strongest C ii feature ever detected in a normal SN Ia , suggesting that the progenitor star had significant unburned material .
The C ii line in SN 2013dy weakens rapidly and is undetected in a spectrum 7 days later , indicating that C ii is detectable for only a very short time in some SNe Ia .
SN 2013dy reached a B -band maximum of M _ { B } = -18.72 \pm 0.03 mag \sim 17.7 d after first light .