IXPE (Imaging X-ray Polarimetry Explorer) is a small 300-kilogram observatory, which, despite its size, is aimed at studying the most complex objects in the Universe: collapsing neutron stars and black holes. IXPE will measure the intensity of magnetic fields and determine how this affects the X-rays emitted by objects. But first of all, scientists will try to understand how these stars and black holes, in principle, emit radiation.

X-rays are generated when gas heats up to hundreds of millions of degrees Celsius and ionizes to form plasma – a swirling mixture of electrons and ions. Usually, the magnetic and electric fields of photons oscillate perpendicular to their path, but in random directions. However, the magnetic conditions, when they appear or interact on their way, can polarize the photons, placing the vibrations in the same plane.

Orbiting Solar Observatory 8, launched in 1975, detected small polarized X-rays from one source, the Crab Nebula. Since then, there have been many more questions that such X-rays can answer, but only IXPE will finally fly out to look for answers. Prior to this, it was believed that the radiation may be too small and missions for their study are inappropriate.

IXPE will study cosmic X-ray sources for at least two years. The observatory has three identical telescopes in its arsenal – such a solution turned out to be cheaper than one large device, and besides, this way they are better protected from failures. The telescopes are held four meters from the detectors by a mast that extends after launch.

Each telescope is a cylinder with 24 concentric shells that focus X-rays through grazing reflections (while the X-rays pass through standard telescope mirrors). The sensors, provided by the Italian Space Agency, capture X-rays and polarization in a layer of dimethyl ether gas. An X-ray beam hits a gas atom and knocks out an electron, which, in turn, tends to fly out in the direction of polarization, leaving a visible trace. Images of multiple traces and their propagation paths show how polarized the light is and in which direction.

Pulsars are the main target of IXPE. They have a diameter of up to 30 km and rotate very quickly: sometimes a hundred times per second, emitting radio waves, X-rays and other radiation. And all this flies past the Earth.

Competing theories about the origin of radiation assume different parts of the pulsar as the source – the entire surface, poles, or atmosphere. Each version assumes that the X-ray polarization is over its entire surface, at its poles, or in its atmosphere. Each theory predicts that the polarization signal varies over time, but IXPE is able to distinguish between them.

Still in the field of view of the mission are magnetars – stellar remnants similar to pulsars, but with even more powerful magnetic fields (100 million times stronger than any magnet created on Earth). The lines of force of the magnetic field displace fast-moving electrons onto a spiral path, forcing them to emit polarized X-rays, known as synchrotron radiation. By analyzing the change in the polarization of X-rays as the magnetar rotates, scientists will be able to “sketch” the field surrounding a celestial body and detect flares.

Polarization can also show how “hungry” the supermassive black hole at the center of the Milky Way was. Gas and dust swirling around an active and material-storing black hole glow brightly with X-rays as it warms up by gravitational forces near the event horizon. X-rays coming directly from the Milky Way’s black hole are faint, suggesting that it is stationary.

However, X-rays, emitted earlier, but lost in distant gas clouds, can also reach us. This scattering leaves a trail of polarization on the X-rays, which means they came from the center of the galaxy, not from the cloud. Moreover, the brightness of these rays will show whether the black hole was more active tens of thousands of years ago.

While the IXPE’s primary target is pulsars, the biggest advance could be understanding the mechanics of powerful jets (relativistic jets) emitted by supermassive black holes in distant galaxies. The jets throw matter into space up to 10 million light-years away – 100 times the diameter of the Milky Way. The researchers believe that the fields created by the mixing of charged particles in the accretion disk combine with the black hole’s magnetic field and inject material and lines of force into the jets. And they are already erupting from both poles of the black hole.

Other X-ray telescopes have detected X-rays emitted close to the base of the jet, and this is probably synchrotron radiation. But what makes electrons travel close to the speed of light when they revolve around the lines of force?

One of the explanations is the shock waves of a fast-moving plasma. Another is magnetic reconnection: the lines of force of the tense field break and reconnect, releasing energy, which accelerates the electrons. The polarization measurements obtained by IXPE will give an idea of ​​how large and chaotic this region of radiation is and what accelerates the electrons. Moreover, it may even turn out that both theories are true.