Think X-ray vision is a superpower found only in comics and movies? Unlike Superman and Supergirl, NASA has it for real, thanks to the X-ray observatories we’ve sent into orbit.
Now the Imaging X-ray Polarimetry Explorer – IXPE for short – has shot into space to enhance our superpower!
When dentists take X-ray pictures of a tooth, they use a machine that makes X-rays and captures them on a device placed on the opposite side. But X-rays also occur naturally. In astronomy, we observe X-rays made by distant objects to learn more about them.
IXPE will improve astronomers’ knowledge about some of these objects, like black holes, neutron stars, and the expanding clouds made by supernova explosions.
That’s because it will capture a piece of information about X-ray light that has only rarely been measured from space!
X-ray astronomers have learned a lot about the cosmos by measuring three properties of light – when it arrives, where it’s coming from, and what energies it has (think: colors). Picture these characteristics as making up three of the four sides of a pyramid. The missing piece is a property called polarization.
Polarization tells us how organized light is. This gives astronomers additional clues about how the X-rays were made and what matter they’ve passed through on their way to us. IXPE will explore this previously hidden side of cosmic X-ray sources.
What is polarization?
All light, from microwaves to gamma rays, is made from pairs of waves traveling together – one carrying electricity and the other magnetism. These two waves always vibrate at right angles (90°) to each other, with their peaks and valleys in sync, and they also vibrate at right angles to their direction of motion.
To keep things simple, we’ll illustrate only one of these waves – the one carrying electricity. If we could zoom into a typical beam of light, we’d see something like the animation above. It’s a mess, with all the wave peaks pointing in random directions.
When light interacts with matter, it can become better organized. Its electric field can vibrate in a way that keeps all the wave crests pointing in the same direction, as shown above. This is polarized light.
The amount and type of polarization we detect in light tell us more about its origin, as well as any matter it interacted with before reaching us.
Let’s look at the kinds of objects IXPE will study and what it may tell us about them.
Exploring star wrecks
Exploded stars create vast, rapidly expanding clouds called supernova remnants – like the Jellyfish Nebula above. It formed 4,000 years ago, but even today, the remnant’s heart can tell us about the extreme conditions following the star’s explosion.
X-rays give us a glimpse of the powerful processes at work during and after these explosions. IXPE will map remnants like this, revealing how X-rays are polarized across the entire object. This will help us better understand how these celestial cataclysms take place and evolve.
Some supernovae leave behind neutron stars. They form when the core of a massive star collapses, squeezing more than our Sun’s mass into a ball only as wide as a city.
The collapse greatly ramps up their spin. Some neutron stars rotate hundreds of times a second! Their magnetic fields also get a tremendous boost, becoming trillions of times stronger than Earth’s. One type, called a magnetar, boasts the strongest magnetic fields known – a thousand times stronger than typical neutron stars.
These superdense, superspinning supermagnets frequently erupt in powerful outbursts (illustrated above) that emit lots of X-rays. IXPE will tell astronomers more about these eruptions and the extreme magnetic fields that help drive them.
Closing in on black holes
Black holes can form when massive stars collapse or when neutron stars crash together. Matter falling toward a black hole quickly settles into a hot, flat structure called an accretion disk. The disk’s inner edge gradually drains into the black hole. Notice how odd the disk appears from certain angles? This happens because the black hole’s extreme gravity distorts the path of light coming from the disk’s far side.
X-rays near the black hole can bounce off the disk before heading to our telescopes, and this polarizes the light. What’s exciting is that the light is polarized differently across the disk. The differences depend both on the energies of the X-rays and on what parts of the disk they strike. IXPE observations will provide astronomers with a detailed picture of what’s happening around black holes in our galaxy that can’t be captured in any other way.
By tracking how X-ray light is organized, IXPE will add a previously unseen dimension to our X-ray vision. It’s a major upgrade that will give astronomers a whole new perspective on some of the most intriguing objects in the universe.
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