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We present an optical to near-infrared transmission spectrum of the hotJupiter HAT-P-1b, based on HST observations, covering the spectral regime from0.29 to 1.027{\mu}m with STIS, which is coupled with a recent WFC3 transit(1.087 to 1.687{\mu}m). We derive refined physical parameters of the HAT-P-1system, including an improved orbital ephemeris. The transmission spectrumshows a strong absorption signature shortward of 0.55{\mu}m, with a strongblueward slope into the near-ultraviolet. We detect atmospheric sodiumabsorption at a 3.3{\sigma} significance level, but find no evidence for thepotassium feature. The red data implies a marginally flat spectrum with atentative absorption enhancement at wavelength longer than ~0.85{\mu}m. TheSTIS and WFC3 spectra differ significantly in absolute radius level (4.3 +/-1.6 pressure scale heights), implying strong optical absorption in theatmosphere of HAT-P-1b. The optical to near-infrared difference cannot beexplained by stellar activity, as simulta- neous stellar activity monitoring ofthe G0V HAT-P-1b host star and its identical companion show no significantactivity that could explain the result. We compare the complete STIS and WFC3transmission spectrum with theoretical atmospheric mod- els which include haze,sodium and an extra optical absorber. We find that both an optical absorber anda super-solar sodium to water abundance ratio might be a sce- nario explainingthe HAT-P-1b observations. Our results suggest that strong optical absorbersmay be a dominant atmospheric feature in some hot Jupiter exoplanets.

HST hot Jupiter Transmission Spectral Survey: A detection of Na and strong optical absorption in HAT-P-1b