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Transmission spectra of exoplanets orbiting active stars suffer fromwavelength-dependent effects due to stellar photospheric heterogeneity.WASP-19b, an ultra-hot Jupiter (T$_{eq}$ $\sim$ 2100 K), is one such stronglyirradiated gas-giant orbiting an active solar-type star. We present optical(520-900 nm) transmission spectra of WASP-19b obtained across eight epochsusing the Gemini Multi-Object Spectrograph (GMOS) on the Gemini-Southtelescope. We apply our recently developed Gaussian Processes regression basedmethod to model the transit light curve systematics and extract thetransmission spectrum at each epoch. We find that WASP-19b's transmissionspectrum is affected by stellar variability at individual epochs. We report anobserved anticorrelation between the relative slopes and offsets of the spectraacross all epochs. This anticorrelation is consistent with the predictions fromthe forward transmission models, which account for the effect of unoccultedstellar spots and faculae measured previously for WASP-19. We introduce a newmethod to correct for this stellar variability effect at each epoch by usingthe observed correlation between the transmission spectral slopes and offsets.We compare our stellar variability corrected GMOS transmission spectrum withprevious contradicting MOS measurements for WASP-19b and attempt to reconcilethem. We also measure the amplitude and timescale of broadband stellarvariability of WASP-19 from TESS photometry, which we find to be consistentwith the effect observed in GMOS spectroscopy and ground-based broadbandphotometric long-term monitoring. Our results ultimately caution againstcombining multi-epoch optical transmission spectra of exoplanets orbitingactive stars before correcting each epoch for stellar variability.

A new method to correct for host star variability in multi-epoch observations of exoplanet transmission spectra