  We present sixteen occultation and three transit light curves for the
ultra-short period hot Jupiter WASP-103 b, in addition to five new radial
velocity measurements. We combine these observations with archival data and
perform a global analysis of the resulting extensive dataset, accounting for
the contamination from a nearby star. We detect the thermal emission of the
planet in both the $z'$ and $K_{\mathrm{S}}$-bands, the measured occultation
depths being 699$\pm$110 ppm (6.4-$\sigma$) and $3567_{-350}^{+400}$ ppm
(10.2-$\sigma$), respectively. We use these two measurements together with
recently published HST/WFC3 data to derive joint constraints on the properties
of WASP-103 b's dayside atmosphere. On one hand, we find that the $z'$-band and
WFC3 data are best fit by an isothermal atmosphere at 2900 K or an atmosphere
with a low H$_2$O abundance. On the other hand, we find an unexpected excess in
the $K_{\mathrm{S}}$-band measured flux compared to these models, which
requires confirmation with additional observations before any interpretation
can be given. From our global data analysis, we also derive a broad-band
optical transmission spectrum that shows a minimum around 700 nm and increasing
values towards both shorter and longer wavelengths. This is in agreement with a
previous study based on a large fraction of the archival transit light curves
used in our analysis. The unusual profile of this transmission spectrum is
poorly matched by theoretical spectra and is not confirmed by more recent
observations at higher spectral resolution. Additional data, both in emission
and transmission, are required to better constrain the atmospheric properties
of WASP-103 b.
