A new method to correct for host star variability in multiepoch observations of exoplanet transmission spectra

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