XIPE

The X-ray Imaging Polarimetry Explorer (XIPE) an international satellite observatory with participation from Tübingen, which is currently being planned and evaluated. It is focused on the study of X-ray polarization of astronomical sources and will help to understand radiation processes involving matter under extreme conditions, using neutron stars and black holes as a laboratory to probe fundamental physics.

XIPE will be the first dedicated observatory to explore the polarization of radiation in the X-ray band, emitted by astronomical objects. It will thus provide a major breakthrough in the understanding of physical processes in high energy astrophysics and open a new window on the extreme conditions that are prevalent whenever X-rays are involved.

X-ray Polarimetry

An electromagnetic wave consists of a coupled oscillating electric field and magnetic field which are always perpendicular to each other and propagating in a certain direction. By convention the polarization of an electromagnetic wave refers to the oscillation direction of the electric field. Most astronomical sources of light are classified as unpolarized because they emit a random mixture of waves having different polarizations with no preferred oscillation direction.

Sources of electromagnetic radiation are polarized with a degree of polarization and an angle of polarization when a significant fraction of the emitted waves oscillate in a certain orientation. These two observables can help to understand the mechanisms leading to the observed radiation.

A "vertically" polarized wave with wavelength λ, electric field E (red), magnetic field B (blue), propagating in the z-direction. [Credits: P.Wormer [CC BY-SA 3.0 or GFDL], via Wikimedia Commons]

The crab nebula, the only astronomical source of X-ray radiation so far, for which the degree of polarization could be measured. The X-ray emission is colored in purple and shows the pulsar wind nebula. [Credit: NASA/JPL-Caltech]

While optical polarization of astronomical sources has already been studied extensively, only a single source has been observed in X-rays so far: the Crab nebula. This well-known object is the remnant of the explosion of a star in a supernova which was observed by Chinese astronomers in the year 1054. It contains a pulsar in its center which is creating a pulsar wind nebula. In the seventies, the X-ray polarization of the source was observed for the first time by several independent missions. The measurements revealed a degree of polarization of about 20%. To date, only limits are known for the degree of polarization of other sources.

XIPE would be launched 8-10 years after funding was provided and it would grant the ability to resolve X-ray polarization in time, space and spectrally and will thus dramatically improve the sensitivity of X-ray polarization measurements. After 40 years without further X-ray polarimetry measurements, XIPE is now expected to increase the number of X-ray sources with known polarization from one to several hundreds.

(a) XIPE in the VEGA launcher fairing, (b) the complete XIPE spacecraft with attached solar panels, (c) the three main elements of the XIPE spacecraft. From top to bottom: the focal plane assembly, the telescope tube and the mirror assembly (d) the telescope supporting structure together with the service module and the payload. [Credits: INAF-IAPS]

The XIPE Observatory

The XIPE observatory consists of three main elements: the mirror assembly, the focal plane assembly and the instrument control electronics.

The Mirror Assembly

The mirror assembly consists of three identical Wolter-I telescopes. As X-rays have a very high energy, they would pass through the mirrors instead of being reflected by them. However, at a very small grazing angle around 1° they can still be reflected. Therefore, X-ray mirrors consists mostly of concentric mirror tubes on the inside of which X-rays can be focused.

The Focal Plane Assembly

The focal plane assembly is the location in which the polarization is detected. It is based on three identical Gas Pixel Detectors (GPDs). When incoming X-ray photons are absorbed by the gas, they transfer energy and impulse to an electron in the detector. The electron leaves an ionization track in the gas which is then drifted to an electron multiplier, through which it is amplified and analyzed. The polarization of the X-ray photon can be reconstructed by the direction of the electron track because the electrons are preferentially emitted in the direction of the electric field of the incoming photon.

Prototype board for the control electronics which is developed in Tübingen. [Credits: A. Guzman, IAAT]

Instrument Control Unit

The functions of the instrument control unit can be divided in the following categories: data processing, instrument control and power distribution. It is developed at the institute in Tübingen. Data from the instruments are processed and stored and so-called Quick Look Analysis Data are generated. In addition, the state and health status of the instruments is monitored and housekeeping data, following the evolution of the most important parameters like temperature, current and voltage, are generated.

IAAT Participation

Design of the ICU - box. [Credits: S. Diebold, IAAT]

The Institute for Astronomy and Astrophysics in Tübingen (IAAT) participates in XIPE as follows:

Development of the instrument control unit (ICU)
Development of the XIPE Electrical Ground Support Equipment
Participation to tests and calibration of the detector and electronics prototypes
Simulations and data analysis
Contributions to the science working groups
Preparation of the XIPE Science Data Center

Links & Sources

Last Update 08/2018: Eva Laplace, Inga Saathoff, Chris Tenzer