We report ground-based transmission spectroscopy of the highly irradiated and ultra-short period hot-Jupiter WASP-103b covering the wavelength range \approx 400 – 600 nm using the FORS2 instrument on the Very Large Telescope .
The light curves show significant time-correlated noise which is mainly invariant in wavelength and which we model using a Gaussian process .
The precision of our transmission spectrum is improved by applying a common-mode correction derived from the white light curve , reaching typical uncertainties in transit depth of \approx 2 \times 10 ^ { -4 } in wavelength bins of 15 nm .
After correction for flux contamination from a blended companion star , our observations reveal a featureless spectrum across the full range of the FORS2 observations and we are unable to confirm the Na absorption previously inferred using Gemini/GMOS or the strong Rayleigh scattering observed using broad-band light curves .
We performed a Bayesian atmospheric retrieval on the full optical-infrared transmission spectrum using the additional data from Gemini/GMOS , HST/WFC3 and Spitzer observations and recover evidence for H _ { 2 } O absorption at the 4.0 \sigma level .
However , our observations are not able to completely rule out the presence of Na , which is found at 2.0 \sigma in our retrievals .
This may in part be explained by patchy/inhomogeneous clouds or hazes damping any absorption features in our FORS2 spectrum , but an inherently small scale height also makes this feature challenging to probe from the ground .
Our results nonetheless demonstrate the continuing potential of ground-based observations for investigating exoplanet atmospheres and emphasise the need for the application of consistent and robust statistical techniques to low-resolution spectra in the presence of instrumental systematics .