Thermal Eclipse Observation of the Young Hot Neptune AU Mic b with Spitzer

We present the observation of a secondary eclipse of the young hot Neptune, AU Mic b, in the infrared using the Spitzer Space Telescope. Using a primary transit from Spitzer to constrain the system parameters, we tentatively detect an eclipse centered at $BJD=2458740.848893^{+0.00010}_{-0.000099}$ with an observed depth of $171\pm{29}$ ppm given an uninformed prior. This corresponds to a dayside brightness temperature of $T=1031\pm{58}$ K, which exceeds the calculated equilibrium temperature of $606\pm{19}$ K. We explore some possible explanations for these results, including inefficient heat redistribution, gravitational contraction, stellar pulsations, instrument systematics and choice of eclipse depth prior, but find none of these to be likely explanations for the observed eclipse parameters. We also explore the impact of correlated noise in the systematic trends, and we find that splitting the systematics into low-pass (smoothing) and high-pass trends is required to reach an optimal minimization of the low-frequency systematics in the resulting detrended light curve. Future observations with JWST are needed to confirm our eclipse detection with Spitzer.