NASA sounding rocket mission searches for X-ray source emanating from inner galaxy

This image shows a “map” of the night sky in soft X-ray light in galactic coordinates, with the Sun positioned in the center. The horizontal line in the middle of the image runs along the plane of our disc-shaped galaxy. University of Wisconsin, Madison astronomer Dan McCammon and the XQC team will observe the bright spot in the center of the image, surrounded by a dotted line. It is the southern part of a roughly circular blob around the center of the galaxy, bisected by cold absorbing gas in the plane of the galaxy. Credit: Snowden et al., 1997

To human eyes, the night sky between the stars appears dark, the emptiness of space. But X-ray telescopes capture a profoundly different view. Like distant fireworks, our X-ray images of the sky reveal a universe blooming with activity. They hint at yet unknown cosmic eruptions originating from somewhere deeper in our galaxy.

To help find the source of these mysterious X-rays, University of Wisconsin Madison astronomer Dan McCammon and his team launch the Quantum X-ray Calorimeter or XQC instrument. XQC will make its seventh trip to space aboard a NASA suborbital rocket. This time, XQC will observe a spot of X-ray light with 50 times better energy resolution than ever before, key to revealing its source. The launch window opens at Equatorial Launch Australia’s Arnhem Space Center in the Northern Territory, Australia on June 26, 2022.

Because Earth’s atmosphere absorbs X-rays, our first views of cosmic X-rays have waited until the space age. In June 1962, physicists Bruno Rossi and Ricardo Giacconi launched the first X-ray detector into space. The flight revealed the first X-ray sources beyond our sun: Scorpius X-1, a binary star system some 9,000 light-years away, as well as a diffuse glow spread across the sky. The discovery founded the field of X-ray astronomy and later earned Giacconi a share of the 2002 Nobel Prize in Physics.

This animated image shows the two alternative sources of X-rays that the XQC mission is studying. Credit: NASA Goddard Space Flight Center

Scientists have now mapped the X-ray sky in increasingly fine detail with the help of other NASA X-ray missions. However, there are several bright spots whose sources are unknown. For the upcoming flight, McCammon and his team will target a patch of X-ray light partially visible from the northern hemisphere.

“It covers a lot of the galaxy, but we had to be in the southern hemisphere to see that part of the sky,” McCammon said. “We’ve been waiting for this expedition to Australia for a long time.”

Scientists believe the X-ray patch comes from diffuse, hot gases heated by supernovae, the bright flares of dying stars. The XQC mission is investigating two possible sources, shown in the graphic below.

One possibility is that the X-rays come from gases heated by “Type Ia” supernovae, the agony of massive stars that live tens to hundreds of millions of years. The inner part of our galaxy has a high enough concentration of this type of supernova to heat up the patch of X-rays McCammon is studying.

The other possible source is “Type II” supernovae. The stars that cause Type II supernova are even more massive, burn brighter and hotter, and only live a few million years before going supernova. They occur in active star-forming regions, such as those in one of our galaxy’s inner spiral arms.

To distinguish these possibilities, XQC will analyze the X-ray light, looking for traces of oxygen and iron. More oxygen indicates Type II supernovae, while less oxygen suggests Type Ia supernovae. The underlying physics is complex but ultimately stems from how long the stars burned before erupting. The smaller stars behind Type Ia supernovae burn longer, leaving behind less oxygen than Type II supernovae.

Of course, theft is likely to capture a lot more information as well. “This is exploration with a new capability – we want to see what we can see,” McCammon said. “Every time we look at the X-ray sky with a new ability, it turns out to be more complicated than we thought.”

After the flight, the team plans to recover the instrument. He will retire to Oak Ridge National Laboratories in Tennessee, where he will contribute to laboratory experiments.

This flight will be XQC’s final trip to space, but the very first in the new line of rockets from Arnhem Space Center in East Arnhem, Australia. XQC is part of a three-rocket campaign launched from the range in June and July 2022, NASA’s first launch from Australia since 1995.

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