HaloSat: The Case of the Missing Baryons

Baryonic matter provides the substance of all the matter in the universe that is familiar, including people, planets, and protons. However, current calculations indicate about one-third of the baryons in the universe to be missing. One possible place baryons can 'hide' from detectors is in the hot halos surrounding galaxies. As the closest galaxy available, the Milky Way Galaxy provides the best halo for study of diffuse x-ray emission. Currently orbiting, HaloSat is the first CubeSat funded by NASA to accomplish an astrophysical mission: determine whether there is a significant presence of baryonic matter in galactic halos in light of the missing baryon problem. At a temperature of about 10⁶ K, the hot halo of the Galaxy provides strong emission lines from O-VII and O-VIII at 561 eV and 653 eV. We adapted off-the-shelf low energy x-ray instrumentation to the low-cost format of a CubeSat with additional modifications to fulfill our mission goals. Recently, HaloSat accomplished full design sensitivity for the measurement of the oxygen lines in the halo and continues to collect data. The in-flight calibration of the instrument will also be discussed, along with background cuts and subtraction methods.