  We present a search for an electromagnetic counterpart of the gravitational
wave source GW151226. Using the Pan-STARRS1 telescope we mapped out 290 square
degrees in the optical i_ps filter starting 11.5hr after the LIGO information
release and lasting for a further 28 days. The first observations started
49.5hr after the time of the GW151226 detection. We typically reached
sensitivity limits of i_ps = 20.3-20.8 and covered 26.5% of the LIGO
probability skymap. We supplemented this with ATLAS survey data, reaching 31%
of the probability region to shallower depths of m~19. We found 49
extragalactic transients (that are not obviously AGN), including a faint
transient in a galaxy at 7Mpc (a luminous blue variable outburst) plus a
rapidly decaying M-dwarf flare. Spectral classification of 20 other transient
events showed them all to be supernovae. We found an unusual transient,
PS15dpn, with an explosion date temporally coincident with GW151226 which
evolved into a type Ibn supernova. The redshift of the transient is secure at
z=0.1747 +/- 0.0001 and we find it unlikely to be linked, since the luminosity
distance has a negligible probability of being consistent with that of
GW151226. In the 290 square degrees surveyed we therefore do not find a likely
counterpart. However we show that our survey strategy would be sensitive to
NS-NS mergers producing kilonovae at D < 100 Mpc which is promising for future
LIGO/Virgo searches.
