Striving to see the high-energy sky with STROBE-X

Space agencies like the US National Aeronautics and Space Administration (NASA) have been launching space telescopes for decades, exploring the full spectrum of light, encompassing low-energy microwaves emitted from the far reaches of the known universe as well as the highest-energy gamma rays from violent stellar explosions. While telescopes like the James Webb Space Telescope and the Hubble Space Telescope get most of the press, a community of researchers is excited about a different mission: focusing on higher-energy light.

X-ray astronomers have developed plans for a new, mid-sized space telescope to add to NASA’s fleet: STROBE-X (Spectroscopic Time-Resolving Observatory for Broadband Energy X-rays). If funded, it will observe X-rays from neutron stars, the dense remnants of exploded stars, as well as black holes, whose gravity is so strong no light can escape.

“STROBE-X has the potential to touch on almost every area of astrophysics, from comets to blazars at the edge of the universe, with lots of things in between included,” says Tom Maccarone, a physics professor at Texas Tech University and a STROBE-X co-investigator.

The Hubble mostly uses the visible light spectrum. Higher energy X-ray and gamma-ray wavelengths are trickier to wrangle—they tend to be absorbed rather than reflected by Hubble’s mirrors. One way to deal with this for lower-
energy X-rays of 1/10th- to 1-nm wavelengths is to arrange the mirrors such that the X-rays graze past, deflecting them slightly to hit a detector. This is how NASA’s Chandra X-ray Telescope has produced many incredible images, such as those of leftover gas and dust from a supernova explosion.

Although gorgeous photos of space are always delightful, imaging alone can’t answer all the questions about the cosmos. The objects in space that generate X-rays are often quite small and compact, and the amount of light they emit fluctuates. Some change on timescales as fast as a fraction of a second, as in the case of rapidly spinning pulsars. Other events, known as transients, pop up in a sudden, bright flash when something big happens, such as two neutron stars slamming into each other and then disappearing.

In this image of the Tycho supernova remnant, low-energy X-rays (red) reveal expanding debris from the supernova explosion while high-energy X-rays (blue) show the blast wave, a shell of extremely energetic electrons. Credit: /CXC/Rutgers/K.Eriksen et al.

One way to study these rapidly changing objects is with X-ray timing, in which astronomers monitor and record changes in an object’s X-ray emission, says Paul Ray, an astrophysicist at the US Naval Research Laboratory and STROBE-X lead investigator. X-ray timing requires a specially equipped telescope sensitive to fast and sometimes faint variations in the amount of light. The Rossi X-ray Timing Explorer (RXTE), launched in 1995 and decommissioned in 2012, did so and found evidence of the smallest black hole yet—just three times the mass of the Sun—yielding a greater understanding of how black holes interact with surrounding stars and gas.

RXTE was big and clunky, the technology of a past era. It utilized large tanks of xenon gas to count interactions with X-ray photons. Even though it stayed operational for 16 years, its decommissioning left a void in the X-ray astronomer’s toolkit. Ray worked on RXTE as a postdoctoral fellow studying neutron stars. Similarly, Colleen Wilson-Hodge, an astrophysicist and STROBE-X project scientist at NASA Marshall Space Flight Center, used RXTE data as a doctoral candidate to observe how fast pulsars spin. “It was a big, heavy mission,” says Ray. “It was about as much as we could do with technology of the time.”

Ray and Wilson-Hodge were among the US X-ray astronomers who began planning for a new X-ray timing space telescope in the late 2000s, a project known at the time as AXTAR (Advanced X-ray Timing Array). It would use then-revolutionary solid-state silicon detectors. These detectors capture precise measurements of high-energy X-rays, but in a fraction of the volume that RXTE’s gas detectors required, perfect for a new space mission where minimizing size and weight is paramount to keep launch costs down.

However, the Astro2010 Decadal Survey didn’t recommend an X-ray timing mission like AXTAR, and the project was pushed aside in favor of other NASA priorities.

But after a meeting at an SPIE Astronomical Telescopes + Instrumentation symposium in 2016, a group of astronomers, including Ray and Wilson-Hodge, decided to pitch the concept to NASA again, this time as STROBE-X, in preparation for the Astro2020 Decadal Survey. NASA funded an in-depth study of how the STROBE-X mission would be designed. The goal was to show NASA that “here’s the science we can do, and why we can do it under a billion dollars,” says Ray. A further boost came when the recently released Astro2020 Decadal Survey recommended a medium-sized infrared or X-ray mission like STROBE-X as a priority.

Mirrors for STROBE-X (left) and NICER. Credit: NASA/Goddard Space Flight Center 

One of STROBE-X’s selling points to become NASA’s new probe is its well-developed technology, Ray and Wilson-Hodge say. They had already solved the problem of integrating silicon detector technology into an X-ray spacecraft in prior mission concepts like AXTAR and the European Space Agency’s (ESA) Large Observatory for X-ray Timing. As Abigail Stevens, a postdoctoral astronomer at Michigan State University and member of the STROBE-X steering committee, notes, “This gives us [the STROBE-X team] a fantastic advantage in our technical readiness level compared to other missions that will be proposed.”

The STROBE-X mission-concept study report details plans to use these detectors for two instruments: a wide field monitor to survey the whole sky regularly and search for changes with 15 times greater sensitivity than the all-sky monitor on RXTE, and a large-area detector for timing high-energy X-rays which are 1,000 times more energetic than visible light.

Another critical piece of the mission comes from NICER (Neutron star Interior Composition Explorer), a small X-ray timing instrument deployed by NASA on the International Space Station. It demonstrated the power of X-ray concentrating optics, which are lightweight pieces of aluminum, almost like foil, that guide X-rays to a detector. STROBE-X has plans to include an X-ray concentrator array, which Ray refers to as “Super-NICER”—a bigger, better version of the original with more area to collect X-ray photons.

Altogether, the three instruments will allow STROBE-X to chart X-ray sources over time in unprecedented detail. Looking at changes across time in other wavelengths has revolutionized astronomy in the past decade, from studies of transiting exoplanets and nearby stars to identification of gamma-ray bursts.

As leader of STROBE-X’s Science Working Group, Maccarone has spent the last few years figuring out what science results astronomers can look forward to from the mission and using those plans to inform the mission design.

According to the STROBE-X concept study, the mission has three main science goals. First, STROBE-X will help astronomers understand the interiors of neutron stars, which are so dense that a tablespoon would weigh as much as Mount Everest. In this extreme environment, scientists are unsure of what particle physics may be happening. STROBE-X will also gather data such as the sizes and spins of pulsars, a class of rapidly spinning neutron stars.

Second, STROBE-X will explore the sources of gravitational wave events, like merging black holes and neutron stars. Since 2015, LIGO (Laser Interferometer Gravitational-Wave Observatory) has been detecting these mergers, but a great deal more can be learned from a multimessenger approach that captures the X-ray “sights” as well as the “sounds” of gravitational waves. 

Last, STROBE-X proponents are interested in black holes that aren’t producing gravitational waves. The telescope will be able to see X-ray  photons emitted from matter accreting onto the black hole, which can provide extremely precise measurements of one of the three characteristics of black holes: spin. A supermassive black hole’s spin can reveal how it grew into the behemoth it is today, and spin is also critical to understanding other parts of the black hole ecosystem such as relativistic jets, material being thrown out from near the black hole at extremely high speeds.

One of the driving values of this mission is community, Ray says, and astronomers have contributed many different ideas for new research with STROBE-X beyond the mission’s main goals. “Everyone interested in compact objects [neutron stars and black holes, for example] should be interested in X-ray timing,” Ray says. Indeed, some 150 scientists, from stellar astronomers to particle physicists, have been added to the group, as shown in the long author list on the mission concept study paper.

Stevens, also a member of the STROBE-X Science Working Group, says she is excited about two neutron stars in particular: Sco X-1 and Cyg X-2. Conveniently located in our own galaxy, these two objects are particularly bright and currently accreting matter, providing an excellent opportunity to compare accretion of matter between black holes and neutron stars.

STROBE-X could even help search for hints of dark matter, a component of the universe scientists know exists but can’t directly detect, says Chanda Prescod-Weinstein, a theoretical physicist at the University of New Hampshire and STROBE-X steering committee member. With its neutron-star-measuring prowess, STROBE-X may be able to reveal signs of axions, a hypothetical particle that could be dark matter, trapped inside the cores of neutron stars. A neutron star with axions inside would cool differently than one made of regular neutrons, a subtle difference this mission might be able to detect.

With its sky-monitoring capabilities, STROBE-X might also provide clues to types of X-ray flashes astronomers don’t yet understand and other phenomena they can’t even imagine yet. If the future gravitational wave detector LISA (Laser Interferometer Space Antenna) finds mergers of supermassive black holes, STROBE-X may even be able to spot X-ray signs of those extreme events.

“This is something always worth thinking about with new missions, which is what the potential is for discovering something you weren’t looking for,” says Maccarone. “I’d argue that almost every space mission NASA has launched has had something in its prime mission highlights that wasn’t even anticipated at the time the mission was approved.”

There are a few other X-ray projects in the works, tackling other ways of observing the high-energy sky, such as NASA’s newly launched Imaging X-ray Polarimetry Explorer and ESA’s upcoming major X-ray imaging and spectroscopy mission, Athena. But what comes next for STROBE-X?

NASA says that the agency is moving forward with plans to fund and implement a new probe mission, particularly one that would operate in X-ray or infrared wavelengths. They’re particularly interested in ideas that would complement Athena, providing other capabilities like timing. STROBE-X fits this bill precisely. Wilson-Hodge is optimistic they’ll be selected for funding in the next, and final, round of proposals, saying, “I’m excited that maybe this time we’re really going to get to go somewhere with it. Maybe this time will really be the win.”

Ray says NASA’s final decision will be made around 2025, ideally leading to a launch sometime near the end of the decade. This mission, although it hasn’t crossed the finish line quite yet, has decades of history behind it and many astronomers supporting its journey. It’s been a long time coming, but that doesn’t dissuade Ray. “For anything of substantial scale like this, the ideas have to be around for a long time,” he says. “But when there’s an important measurement to be made, eventually people do it.”

Briley Lewis is a PhD Candidate and NSF Fellow at the University of California, Los Angeles, studying astronomy and astrophysics. She is a member of the Astrobites collaboration, and a freelance science writer. Follow her on Twitter @briles_34 or visit her website
www.briley-lewis.com