The Carnegie Hubble Program: The Leavitt Law at 3.6 and 4.5 micron in
  the Milky Way
  The Carnegie Hubble Program (CHP) is designed to calibrate the extragalactic
distance scale using data from the post-cryogenic era of the Spitzer Space
Telescope. The ultimate goal of the CHP is a systematic improvement in the
distance scale leading to a determination of the Hubble Constant to within an
accuracy of 2%. This paper focuses on the measurement and calibration of the
Galactic Cepheid Period-Luminosity (Leavitt) Relation using the warm Spitzer
IRAC 1 and 2 bands at 3.6 and 4.5 \mu m. We present photometric measurements
covering the period range 4 - 70 days for 37 Galactic Cepheids. Data at 24
phase points were collected for each star.
  Three PL relations of the form M=a(Log(P)-1)+b are derived. The method
adopted here takes the slope a to be -3.31, as determined from the Spitzer LMC
data of Scowcroft et al. (2012). Using the geometric HST guide-star distances
to ten Galactic Cepheids we find a calibrated 3.6 micron PL zero-point of
-5.80\pm0.03. Together with our value for the LMC zero-point we determine a
reddening-corrected distance modulus of 18.48\pm0.04 mag to the LMC.
  The mid-IR Period-Color diagram and the [3.6] - [4.5] color variation with
phase are interpreted in terms of CO absorption at 4.5 \mu m. This situation
compromises the use of the 4.5 \mu m data for distance determinations.
