Next week, NASA will launch a small mission with an ambitious task: to peer into some of the universe’s most violent objects for clues to how they work. By detecting how intense magnetic fields in collapsed stars and black holes align, or polarize, the x-rays they emit, the Imaging X-ray Polarimetry Explorer (IXPE) could reveal how those objects spew out radiation in the first place. “We’re just trying to find out how do you produce these x-rays anyway,” says principal investigator Martin Weisskopf of NASA’s Marshall Space Flight Center.
“The community has been waiting for this for a long time,” says x-ray astronomer Feryal Özel of the University of Arizona, who is not part of the project. NASA’s Orbiting Solar Observatory 8, launched in 1975, detected a smattering of polarized x-rays from a single source, the Crab nebula, Weisskopf says. Since then, the questions such x-rays might answer have piled up, but no mission has set out to measure them—in part because the data from that first sensor suggested there might be too few to be worth the effort. Weisskopf says he proposed dedicated x-ray polarimetry missions several times before succeeding with IXPE, a $190 million mission that is set to launch on 9 December from the Kennedy Space Center in Florida on a SpaceX Falcon 9 rocket.
X-rays are emitted when gas is heated up to hundreds of millions of degrees Celsius and ionized to produce plasma, a roiling soup of electrons and ions. Typically, the photons’ own magnetic and electric fields oscillate perpendicularly to their path, but in random directions. However, magnetic conditions at their birth or interactions on their journey can polarize the photons by forcing those oscillations into the same plane.
IXPE will spend at least 2 years scrutinizing cosmic x-ray sources with three identical telescopes—cheaper than one big one and a hedge against failures. Each telescope is a cylinder containing two dozen concentric shells that focus x-rays through grazing reflections (x-rays penetrate standard telescope mirrors). Sensors provided by the Italian Space Agency detect the x-rays and their polarization in a layer of dimethyl ether gas. An x-ray hitting a gas atom knocks out an electron that tends to shoot off in the direction of polarization, leaving a visible trail. Imaging many trails and their spread tells observers how polarized the light is, and in which direction.
Pulsars are a prime target for IXPE. These city-size remnants of dead stars spin frenetically, sometimes hundreds of times per second, emitting beams of radio waves, x-rays, and other radiation that sweep past Earth like a lighthouse beacon. Weisskopf says rival theories of beam generation suggest the x-rays originate in different locations on the pulsar: all across its surface, at its poles, or in its atmosphere. Each theory predicts the x-ray polarization signal should vary with the timing of the pulses in a way IXPE should be able to distinguish. “We will get a result, as long as we launch,” he predicts.
Also in the mission’s sights are magnetars, stellar remnants like pulsars, but with even more powerful magnetic fields, 100 million times stronger than any magnet made on Earth. The magnetic field lines force fast-moving electrons into helical paths, causing them to spray out polarized x-rays known as synchrotron radiation. By measuring how the polarization of the x-rays changes as the magnetar spins, observers will be able to map the geometry of the field across the entire globe—and watch for tangles that lead to eruptive outbursts. “With IXPE we may really see the magnetic field direction, the twisted magnetosphere,” says theorist Matthew Baring of Rice University.
Polarization could also reveal how hungry the supermassive black hole at the center of our Milky Way has been in recent history. Gas and dust swirling around an active, accreting black hole shine brightly with x-rays as they are ferociously heated by gravitational forces close to the event horizon. The x-rays now arriving directly from the Milky Way’s black hole are dim, suggesting it is quiescent. But x-rays emitted longer ago could also be arriving, having followed dogleg paths after being scattered by distant gas clouds. That scattering should imprint a polarization on the x-rays, a sign that they came from the galactic center and not the cloud itself. “If we look at clouds and see polarization, that would be a smoking gun,” says astrophysicist Philip Kaaret of the University of Iowa, who worked on a rival polarimeter proposal that lost out to IXPE. Their brightness might show whether the black hole was more active tens of thousands of years ago.
IXPE’s biggest coup might be in helping understand the mechanics of immensely powerful jets launched by supermassive black holes in distant galaxies. These jets blast material up to 10 million light-years out into space—that’s 100 times the diameter of the Milky Way. Researchers believe that fields generated by churning charged particles in the accretion disk combine with the black hole’s own magnetic field to funnel material and field lines into the jets, which spew from both poles.
Other x-ray telescopes have seen x-rays emitted close to the root of the jet, and researchers believe that synchrotron radiation is responsible. But what powers the electrons to near-light-speeds as they spiral around the field lines? One possibility is shockwaves of fast-moving plasma. Another is magnetic reconnection, when stressed field lines snap and reconnect, releasing energy that can accelerate electrons.
Polarization measurements by IXPE could offer insight into how big and chaotic this emission region is and what accelerates the electrons. They might even show both processes are at work, Baring says. “That makes nature richer,” he says, “and keeps theorists busy.”
Source: Science Mag