Institute for Astronomy and Astrophysics

eXTP

Sketch of eXTP and the most important instruments. [Credits: eXTP]

eXTP (enhanced X-ray Timing and Polarimetry mission) is a mission supported by the Chinese Academy of Sciences (CAS). It is performed by an international scientific consortium led by the Institute of High Energy Physics of the CAS.

eXTP is designed to study the state of matter under extreme conditions of density, gravity and magnetism. Primary goals are the determination of the equation of state of matter at supra-nuclear density, the measurement of QED effects in highly magnetized star, and the study of accretion in the strong-field regime of gravity. In addition, eXTP is a powerful innovative observatory at high energies, since it carries a unique and unprecedented suite of state-of-the-art scientific instruments enabling for the first time ever the simultaneous spectral-timing-polarimetry studies of cosmic sources in the energy range from 0.5-30 keV (and beyond). Primary targets include isolated and binary neutron stars, strong magnetic field systems like magnetars, and stellar-mass and supermassive black holes.

The eXTP international consortium includes major institutions of the Chinese Academy of Sciences and Universities in China, as well as major institutions in several European countries and the United States. The predecessor of eXTP, the XTP mission concept, has been selected and funded as one of the so-called background missions in the Strategic Priority Space Science Program of the Chinese Academy of Sciences since 2011. The strong European participation has significantly enhanced the scientific capabilities of eXTP. The planned launch date of the mission is earlier than 2025.

eXTP Instruments

The current baseline of the scientific payload includes four science instruments: the Spectroscopy Focusing Array (SFA), the Large Area Detector (LAD), the Polarimetry Focusing Array (PFA) and the Wide Field Monitor (WFM).

  1. SFA: 9 telescopes based on Nickel technology optics, which have a TRL between 7 and 9; the baseline detector is a 19-pixel Silicon Drift Detector (SDD), featuring high TRL (heritage of XMM-Newton, Bepi-Colombo, …).
  2. LAD: 40 modules based on SDD detectors, capillary-plate collimator, FEE and digital electronics. It is a direct heritage of the LOFT mission.
  3. PFA: 4 telescopes based on Nickel technology optics with Carbon layer, and has a TRL between 7 and 9; the baseline detector is a Gas Pixel Detector active in the 2-8 keV range. It is a direct heritage of the XIPE mission.
  4. WFM: 6 cameras, based on silicon drift detectors and coded masks. It is a direct heritage of the LOFT mission.
Sketch of the four instruments of eXTP. [Credits: ISDC/eXTP]

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]

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 upper limits are known for the degree of polarization of other sources.

eXTP will be launched in 2025 and 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, eXTP is now expected to increase the number of X-ray sources with known polarization from one to several hundreds.

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]

IAAT Participation

The high energy astrophysics group of the Institute of Astronomy and Astrophysics in Tübingen (IAAT) is part of the international eXTP Collaboration. Our contribution will consist of electronics components for the LAD and the WFM. The group also plays an important political role in the leadership of the consortium and takes an active part in the science working groups for eXTP.

Links & Resources

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