No, not that one.
Much attention is being devoted this month to the James Webb Space Telescope, from NASA and the European Space Agency, which is set to launch Dec 22. But a more exclusive cadre of astronomers watched excitedly Thursday during the trip to space of a smaller, but also transformative, observatory.
NASA launched the Imaging X-ray Polarimetry Explorer, or IXPE mission, on a SpaceX Falcon 9 rocket from Kennedy Space Center in Florida at 1 am Eastern. The spacecraft cost a mere $188 million, compared with the James Webb’s mammoth budget of $9.7 billion, and is expected to demonstrate a new form of astronomy. It will, for the first time, perform imaging X-ray polarimetry in orbit, a technique that could offer astronomers insights that no other telescope can match.
“It’s giving us information about some of the most bizarre and exciting objects in space,” said Thomas Zurbuchen, the associate administrator of NASA’s science mission directorate.
IXPE (pronounced by the mission team as “ix-pee”) was placed into an orbit 340 miles above Earth after its launch. The telescope will spend several weeks there deploying its scientific instruments and testing its equipment, then begin its two-year mission.
X-rays are a useful way to observe the universe. Emitted from extremely energetic objects, they allow astronomers to probe events — superheated jets near black holes or explosions of stars, for example — in a way other wavelengths, such as visible light, cannot. But X-rays can be studied only from space because they are mostly absorbed by Earth’s atmosphere.
A variety of dedicated X-ray space telescopes and instruments have launched to orbit, most notably NASA’s Chandra X-ray and ESA’s XMM-Newton observatories, which both launched in 1999. With spacecraft like these, scientists have unveiled the birthplaces of stars inside gaseous nebulas and mapped the spread of dark matter in clusters of galaxies, among other pioneering work.
The use of imaging X-ray polarimetry sets IXPE apart from its predecessors. If you’ve ever worn a pair of polarised sunglasses, you may know that they use thin slits to block horizontal light, but that turning them sideways blocks vertical light instead. The same principle is employed in X-ray polarimetry. The technique will allow astronomers to observe the direction of the wavelike movement of the X-ray particles as they arrive, revealing the orientation of incoming electric and magnetic fields. Armed with this data, astronomers can glean more information from the X-rays emitted by astrophysical phenomena.
Instead of simply observing the X-rays with a single instrument, the spacecraft is actually three separate telescopes, each comprising 24 concentric mirrors, at the end of a 13-foot-long boom, which will extend during the telescope’s first week in space.
As the X-rays arrive, they will be focused by each telescope onto three detectors at the end of the boom. The detectors each contain a 10-millimeter layer of helium and a gas called dimethyl ether, or DME. This will reveal the polarisation of the X-rays, which will make tracks in the gas as they hit.
“These detectors will provide an image of the polarisation,” said Elisabetta Cavazzuti, a program manager from the Italian Space Agency, which designed the detectors.
There have been several attempts at conducting X-ray polarimetry in space before, Martin Weisskopf, the mission’s principal investigator at NASA’s Marshall Space Flight Center, said. In 1971, Weisskopf was involved in a successful experimental mission that performed brief X-ray polarisation observations of the Crab nebula in our galaxy using a sounding rocket, which goes straight up and down but doesn’t go into orbit. A later attempt to launch a more advanced polarimeter on the Soviet Spectrum-X spacecraft in the 1990s was interrupted by the collapse of the Soviet Union, according to Weisskopf.
“We’ve been waiting a long time to have a polarimetry mission,” he said.
His and other researchers’ patience paid off in 2017, when NASA selected IXPE as part of its Small Explorers program.
In the two years after it launches, the IXPE spacecraft will observe more than 100 cosmic targets, including black holes, supernovas and exotic stars.
One goal of the telescope is to observe the spin of relatively small black holes, those around 10 times the mass of our sun. X-ray polarimetry will be able to probe relativistic effects that occur very close to these black holes, where the polarisation angle of X-ray photons that have escaped is expected to be altered as they travel through the heavily warped space-time caused by the black hole’s spin.
“For the first time we can try and measure those distortions,” said Adam Ingram, an astrophysics lecturer at Newcastle University in England.
IXPE will also probe neutron stars, the remnant cores left behind after giant stars collapse. Scientists are particularly interested in pulsars, which are rapidly rotating neutron stars, and magnetars, which are highly magnetised ones.
By zeroing in on magnetars, researchers hope to see how ironclad the laws of physics are. IXPE will be able to investigate an effect near these stars called quantum electrodynamics, or QED, where the extremely strong magnetic fields should cause a high level of polarisation in the emitted X-ray particles.
“QED is the base of our understanding of physics,” Ilaria Caiazzo, a researcher from the California Institute of Technology, said. “If we found that it’s not right, that would really revolutionise everything. I expect we will confirm this effect.”
Elsewhere, IXPE could tell us more about the moments after a star’s explosion, a supernova. The spacecraft’s data will reveal how ejected material from a supernova interacts with the surrounding interstellar medium as it plows into it at extreme speeds, creating a shock front. Electrons can then pass back and forth across the shock front, a process known as diffusive shock acceleration.
“It’s a very important process in astronomy, but we don’t fully understand the details,” Ingram said. “It’s thought to be behind why supernova remnants are bright.”
IXPE’s primary mission is expected to last two years. But if NASA extends the mission, the spacecraft could last nearly two decades, Weisskopf said. With more time, astronomers could study other targets, such as Sagittarius A*, the supermassive black hole at the center of our galaxy. By looking for the reflections of X-rays on clouds of gas near the black hole, they could look for evidence of increased activity from Sagittarius A* in the past few centuries.
“The clouds wouldn’t be as bright as they seem unless the black hole was more bright several hundred years ago,” Weisskopf said. “You can calculate how long it takes for the X-rays to come to the cloud and bounce toward us. It’s a very tough experiment.”
When compared with super-telescopes like the James Webb, IXPE may be relatively modest. But it highlights the breadth of astronomy that scientists are now undertaking and the novel ways in which advanced machines are being used to explore our universe.
X-ray polarimetry, once a closed window on the cosmos, is being opened — and with it, a host of hidden secrets will be unlocked.
“It’s truly a new way of looking at the sky,” Zurbuchen said.
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