skyandtelescope.com • AUGUST 2019 21caused the spacecraft to spin out of control and, within hours,
break apart (S&T: July 2016, p. 11). Now, NASA and the Japa-
nese Aerospace Exploration Agency are working together on yet
another replacement mission: The X-ray Imaging and Spectros-
copy Mission (XRISM) is designed specifi cally to recover the
science lost with the Hitomi incident. Due to launch in early
2022, the spacecraft will house an X-ray imager in addition to
a calorimeter. Time will tell if this fourth try is the charm.Future of the Field
X-rays’ inability to penetrate Earth’s atmosphere means the
future of X-ray astronomy depends very much on which
instruments fl y into space. Without a full slate of projects
being proposed, designed, built, and launched, the fi eld is in
constant danger of going blind at these energies.
Fortunately, there’s no shortage of smaller missions, often
targeted at answering specifi c questions. But the open ques-
tions outnumber the upcoming missions. For now, 20-year-
old Chandra and XMM-Newton — both still going strong
— are holding down the fort. “We should be grateful to the
engineers who built them,” Kretschmar says. “If they had just
been built to spec, they would have been dead already.”
Ultimately, though, the fi eld needs another fl agship mis-
sion. “I think we are now on the verge — ‘verge,’ if you have
a patient timeline that allows for another decade or so — of
going to the next step,” he adds.
The next big thing is the Advanced Telescope for High
Energy Astrophysics (Athena), a European mission with
NASA involvement that’s due to launch in 2031. Hornsche-
meier calls its Wide Field Imager a “point source discovery
machine” thanks in part to its unprecedented 40-by-40-arc-
minute fi eld of view. The mission will also carry a calorimeter
that will be a vast improvement on the one fl ying on XRISM.
But for other astronomers, Athena’s capabilities are too
limited for it to be the sole fl agship, mainly because it sac-
rifi ces detail and depth for its incredible breadth. Its imager
will have fi ve-arcsecond resolution compared to Chandra’s
sub-arcsecond focus, and it won’t be able to fi nd sources as
faint as Chandra can.
These astronomers are proposing a new X-ray mission:
Lynx. It’s vying for priority as the astronomy community
meets to decide NASA’s science priorities for the next 10SERENDIPITOUS SOURCES
XMM-Newton’s ˡ eld of view is equivalent to the full
Moon, so any given X-ray image holds an extra 50 to 100
sources that weren’t the target of the observations. After
two decades, all of these extra sources add up — XMM-
Newton’s Survey Science Center has now amassed 531,454
unique X-ray sources over 1,000 square degrees on the sky.
The ˡ gure above shows the density of observed objects
in the 8th data release of the XMM-Newton Serendipitous
Source Catalogue, mapped in galactic coordinates.years. Lynx promises a focus comparable to Chandra’s on-
axis, while maintaining sharp imaging (better than 1 arc-
second resolution) over a 20-fold expanded fi eld of view.
Coupling this high resolution with increased photon-collect-
ing power, Lynx would reveal sources as much as 100 times
fainter than what Chandra can detect. Lynx could see the
very fi rst black holes, investigate the hot gas that envelopes
galaxies, and provide details on the energetic feedback com-
ing from supermassive black holes and stars forming within
galaxies. But it faces tough competition, running against
three other projects focusing on visible, ultraviolet, and
infrared wavelengths. Next year the National Academies will
identify which of these large missions should receive priority
for continued development.
As it has for decades, the fi eld of X-ray astronomy contin-
ues to brim with uncertainty, discovery, and promise.¢S&T News EditorMONICA YOUNG was an X-ray astronomer
in her fi rst life. Now she’s an editor with X-ray vision.FURTHER READING: Relive two decades on the frontier of dis-
covery with Chandra and XMM-Newton: https://is.gd/20xrays10 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
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