  We report the detection of thermal emission from the hot Jupiter WASP-3b in
the KS band, using a newly developed guiding scheme for the WIRC instrument at
the Palomar Hale 200in telescope. Our new guiding scheme has improved the
telescope guiding precision by a factor of ~5-7, significantly reducing the
correlated systematics in the measured light curves. This results in the
detection of a secondary eclipse with depth of 0.181%\pm0.020% (9-{\sigma}) - a
significant improvement in WIRC's photometric precision and a demonstration of
the capability of Palomar/WIRC to produce high quality measurements of
exoplanetary atmospheres. Our measured eclipse depth cannot be explained by
model atmospheres with heat redistribution but favor a pure radiative
equilibrium case with no redistribution across the surface of the planet. Our
measurement also gives an eclipse phase center of 0.5045\pm0.0020,
corresponding to an ecos{\omega} of 0.0070\pm0.0032. This result is consistent
with a circular orbit, although it also suggests the planet's orbit might be
slightly eccentric. The possible non-zero eccentricity provides insight into
the tidal circularization process of the star-planet system, but also might
have been caused by a second low-mass planet in the system, as suggested by a
previous transit timing variation study. More secondary eclipse observations,
especially at multiple wavelengths, are necessary to determine the
temperature-pressure profile of the planetary atmosphere and shed light on its
orbital eccentricity.
