The cepheid period-luminosity relation (the leavitt law) at mid-infrared wavelengths. II. Second-epoch LMC data

We present revised and improved mid-infrared (mid-IR) period-luminosity (PL) relations for Large Magellanic Cloud (LMC) Cepheids based on double-epoch data of 70 Cepheids observed by Spitzer at 3.6, 4.5, 5.8, and 8.0μm. The observed scatter at all wavelengths is found to decrease from ±0.17 mag to ±0.14 mag, which is fully consistent with the prediction that the total scatter is made up of roughly equal contributions from random sampling of the light curve and nearly uniform samplings of stars across the instability strip. It is calculated that the Cepheids in this sample have a full amplitude of about 0.4 mag and that their fully sampled, time-averaged magnitudes should eventually reveal mid-IR PL relations that each have intrinsic scatter at most at the ±0.12 mag level, and as low as ± 0.08 mag after correcting for the tilt of the LMC.