The Very Early Light Curve of SN 2015F in NGC 2442: A Possible Detection
  of Shock-Heated Cooling Emission and Constraints on SN Ia Progenitor System
  The main progenitor candidate of Type Ia supernovae (SNe Ia) is white dwarfs
in binary systems where the companion star is another white dwarf (double
degenerate system) or a less evolved non-degenerate star with R* >~ 0.1 Rsun
(single degenerate system), but no direct observational evidence exists that
tells which progenitor system is more common. Recent studies suggest that the
light curve of a supernova shortly after its explosion can be used to set a
limit on the progenitor size, R*. Here, we report a high cadence monitoring
observation of SN 2015F, a normal SN Ia, in the galaxy NGC 2442 starting about
84 days before the first light time. With our daily cadence data, we catch the
emergence of the radioactively powered light curve, but more importantly detect
with a > 97.4% confidence a possible dim precursor emission that appears at
roughly 1.5 days before the rise of the radioactively powered emission. The
signal is consistent with theoretical expectations for a progenitor system
involving a companion star with R* = ~0.1 -- 1 Rsun or a prompt explosion of a
double degenerate system, but inconsistent with a typically invoked size of
white dwarf progenitor of R* ~ 0.01 Rsun. Upper limits on the precursor
emission also constrain the progenitor size to be R* < 0.1 Rsun, and a
companion star size of R* < ~1.0 Rsun, excluding a very large companion star in
the progenitor system. Additionally, we find that the distance to SN 2015F is
23.9 +-0.4 Mpc.
