OVERVIEW

Rockets with polarimeters based on the ’classical’ technique of Bragg diffraction and Thomson scattering were flown in the late sixties and in the early seventies. A rocket experiment in the 1970s hinted at polarized emission from the Crab Nebula and Pulsar. The Orbiting Solar Observatory (OSO-8) X-ray polarimeter confirmed this result with a much higher significance definitely establishing the synchrotron origin of the nebular X-ray emission.

From the advent of X-ray telescopes starting with the Einstein satellite, it was clear that a quantum leap required a focal plane instrument. An experiment based on the classical techniques was devised and built but never flown due to the collapse of the Soviet Unio.

Techniques of microelectronics allowed for designing a detector based on the photoelectric effect in gas in an energy range where the optics are efficient in focalizing X-rays.

Experiment with polarimeters are listed in the table below

THE IXPE PAYLOAD

The IXPE payload consists of a set of three identical telescope systems co-aligned to the pointing axis of the spacecraft and with the Star-Trackers. Each system, while operating independently, comprises of a 4-m-focal length Mirror Module Assembly (MMA) that focuses X-rays onto the respective Detector Units (DUs).

Each Detector Unit (DU) hosts one polarization-sensitive imaging detector, with its own electronics, which communicates with a Detector Service Unit (DSU) interfaced to the spacecraft Integrated Avionics Unit (IAU). The three DUs and the DSU are collectively called the IXPE Instrument. The DUs are mechanically mounted onto the top deck of the spacecraft oriented, as already mentioned above, with a rotation of 120° each with respect to the beam axis.

Each DU is equipped with a multi-function filter and calibration wheel assembly that allows for (i) in-flight calibration of the modulation factor,(ii) the calibration of the gain, (iii) source flux attenuation (iv) background measurements. MMA and DUs are separated by an able to be deployed (3.5 m) boom; the position of the MMA with respect to the DUs can be adjusted after deployment with a Tip/Tilt/Rotate mechanism. Each MMA hosts an X-ray shield to avoid, in combination with a stray-light collimator mounted onto the top of the DU, X-ray photons impinging on the detector active area when arriving from outside the telescope field of view.

The use of a set of telescopes provides many advantages with respect to a configuration based on a single telescope of equivalent collecting area. First of all, since the energy band-pass fixes the ratio between the mirror diameter and the focal length, a multiple telescope configuration is more compact than a single larger telescope; this occurs at the cost of an increase of the measured background which, however, is not a driving requirement for IXPE. Moreover, a multiple system is intrinsically redundant and offers the possibility of comparing independent data and to correct small effects which may mimic a real signal.

The figure below shows the IXPE observatory with key payload elements.

Instrument performances

The instrument performances are listed below.

Updated on November 15, 2021