Hot Rocks Survey III. A deep eclipse for LHS 1140c and a new Gaussian process method to account for correlated noise in individual pixels

Abstract

Time-series photometry at mid-infrared wavelengths is becoming a common technique to search for atmospheres around rocky exoplanets. This method constrains the brightness temperature of the planet to determine whether heat redistribution is taking place, which would be indicative of the presence of an atmosphere, or whether the heat is reradiated from a low-albedo bare rock. By observing at 15µm, we are also highly sensitive to CO2 absorption. We observed three eclipses of the rocky super-Earth LHS 1140c, using MIRI/Imaging with the F1500W filter. We found a significant variation in the initial settling ramp for these observations and identified a potential trend between the detector settling and the previous filter used by MIRI. We analysed our data using aperture photometry, however, we also developed a novel approach, which performs a joint fit of the pixel light curves using a shared eclipse model and a flexible multi-dimensional Gaussian process which can model changes in the PSF over time. Using simulated data, we demonstrate that our method has the ability to weight away from particular pixels that exhibit increased systematics, allowing for the recovery of eclipse depths in a more robust and precise way. Both methods, as well as an independent analysis, have detected the eclipse at >5σ, while recovering an eclipse depth consistent with a low-albedo bare rock. We measured a dayside brightness temperature of Tday = 561 ± 44 K, close to the theoretical maximum of Tday; max = 537 ± 9 K. We rule out a wide range of atmospheric forward models to >3σ, including pure CO2 atmospheres with surface pressure ≥10 mbar and pure H2O atmospheres with surface pressure ≥1 bar. Our strict constraints on potential atmospheric composition, in combination with future observations of the exciting outer planet LHS 1140b, could provide a powerful benchmark for understanding atmospheric escape around M dwarfs.