SN 2007ax: An Extremely Faint Type Ia Supernova

We present multiband photometric and optical spectroscopic observations of SN 2007ax, the faintest and reddest Type Ia supernova (SN Ia) yet observed. With MB = − 15.9 and (B − V)max = 1.2, this SN is over half a magnitude fainter at maximum light than any other SN Ia. Similar to subluminous SN 2005ke, SN 2007ax also appears to show excess in UV emission at late time. Traditionally, Δ m15(B) has been used to parameterize the decline rate for SNe Ia. However, the B-band transition from fast to slow decline occurs sooner than 15 days for faint SNe Ia. Therefore we suggest that a more physically motivated parameter, the time of intersection of the two slopes, be used instead. Only by explaining the faintest (and the brightest) supernovae can we thoroughly understand the physics of thermonuclear explosions. We suggest that future surveys should carefully design their cadence, depth, pointings, and follow-up to find an unbiased sample of extremely faint members of this subclass of faint SNe Ia.