Introduction to Photoelectric X-ray Polarimeters

X-ray astronomy has been impaired by the lack of access to the unique information contained in the polarization of X-rays emitted by celestial sources, due largely to the insufficient sensitivity of available polarimeters. However, a new generation of polarization-sensitive instruments, developed over the last 20 years and employing micro-pattern gaseous detectors (MPGDs), has demonstrated unprecedented sensitivity that has made X-ray polarimetry a practical astronomical tool. These polarimeters exploit the photoelectric effect, which is both the dominant interaction mechanism for soft X-rays and an ideal polarization analyzer. As the result of photoionization of an atom, the photoelectron is emitted preferentially in the direction of the electric field of the ionizing X-ray. MPGD-based polarimeters form two-dimensional images of such photoelectron tracks with pixel sizes that are small compared to the track length. From these images, the angle of emission, and hence the state of polarization, is estimated for each event. The energy of each X-ray is also measured with typical proportional counter energy resolution. Some implementations also provide focal plane imaging. Here, we discuss in general how MPGD-based polarimeters meet the demanding requirements of X-ray astronomy by efficiently exploiting the polarization sensitivity of the photoelectric effect. In the two chapters that follow, we describe in more detail the two basic implementations, the gas pixel detector (GPD) and the time-projection chamber (TPC) polarimeter.