The HaloSat and PolarLight CubeSat Missions for X-ray Astrophysics
  Astronomical observations in the X-ray band are subject to atmospheric
attenuation and have to be performed in the space. CubeSats offer a cost
effective means for space-based X-ray astrophysics but allow only limited mass
and volume. In this article, we describe two successful CubeSat-based missions,
HaloSat and PolarLight, both sensitive in the keV energy range. HaloSat was a
6U CubeSat equipped with silicon drift detectors. It conducted an all-sky
survey of oxygen line emission and revealed the clumpy nature of the
circumgalactic medium surrounding the Milky Way. PolarLight is a dedicated
X-ray polarimeter performing photoelectron tracking using a gas pixel detector
in a 1U payload. It observed the brightest X-ray objects and helped constrain
their magnetic field or accretion geometry. On-orbit operation of both missions
for multiple years demonstrates the capability of CubeSats as an effective
astronomical platforms. The rapid time scales for development and construction
of the missions makes them particularly attractive for student training.
