Why X-ray Astronomers Are Anxious for Good News From Troubled Hitomi Satellite

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An H-2A rocket carrying an X-ray astronomy satellite called "Hitomi" is launched from the Tanegashima Space Center in Kagoshima Prefecture, southern Japan, on Feb. 17, 2016. Kyodo News via AP

Astronomers use all sorts of electromagnetic radiation to learn about the universe—but X-ray spectra remain elusive. Philip Ronan, CC BY-SA

X-ray Astronomy Dreams

Astronomers use the electromagnetic spectrum—including visible or infrared light—to study stars, planets, galaxies and the universe as a whole. They have long used prisms and grisms to split the light into its components. Rather than just taking images, this spectroscopy allows astrophysicists to study the composition of objects in space and the conditions of the material that is emitting the light, including whether and how it moves around. Optical spectroscopy, for example, lets astronomers see how the stars in a galaxy move around and how old they are.

X-rays are near the far end of the eletromagnetic spectrum beyond the farthest ultraviolet, but not as far as Gamma rays.

Thanks to our atmosphere, X-rays from space don’t reach us at the Earth’s surface. That’s actually good news, since we'd all be in trouble: being constantly bombarded by X-rays leads to DNA damage, cancer and worse. But this also means we need to go to space to see X-rays from the cosmos. Astrophysicists have long wanted to put an X-ray high-resolution spectrograph into space—but the goal has so far remained elusive.

Perseus cluster of galaxies as seen by the Chandra X-ray Observatory. The X-rays come from million-degree gases around the galaxy cluster. Giant bubbles and cavities show where the supermassive black hole blasted energy into the gas. NASA/CXC/IoA/A.Fabian et al., CC BY

X-ray astronomy got its start in the 1950s and ‘60’s with the first X-ray telescopes being launched on sounding rockets and balloons. Space telescopes followed, and with these, astronomers could take X-ray images or low-resolution spectra and made amazing discovery after discovery: the first black hole in our Milky Way galaxy; clusters of galaxies bathed in the glow of million-degree gas; all the way to a mysterious X-ray “background.” Soon after its launch in 1999, the Chandra X-ray Observatory finally resolved that X-ray background into a multitude of growing supermassive black holes in the early universe.

But the history of X-ray spectroscopic measurements in space is somewhat star-crossed. Before Hitomi was ASTRO-EII, known as Suzaku. Suzaku carried an X-ray microcalorimeter, but just a few weeks after launch, the instrument’s cooling system suffered a series of failures and lost all its coolant. Before that came ASTRO-E, which was lost during launch in 2000 when its M-V-4 rocket failed. And before that, NASA planned to fly an X-ray microcalorimeter on a mission called AXAF-S, which got canceled.

Visions of the Hot and Energetic Universe

With a true high-resolution X-ray spectrograph in space we could finally see so much: we could see the motion, the ebb and flow, of million-degree gas sloshing around galaxy clusters as the supermassive black hole in the galaxy at the center of the cluster shoots unimaginable amounts of energy into it with its relativistic jets. We could watch the final gasps of matter as it falls into a feeding quasar, and see the distortion of spacetime itself due to Einstein’s general relativity. We could search for the “missing matter” which we believe must lurk in the vicinity of galaxies.

Artist's conception of the ATHENA X-ray observatory. ATHENA/ESA