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A new signal for a neutron star collision discovered

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16 April 2019

A bright burst of X-rays discovered by NASA’s Chandra X-ray Observatory likely signals the merger of two neutron stars— dense stellar objects packed mainly with neutrons. The source of the X-rays, dubbed XT2, is located in the Chandra Deep Field South (CDF-S), a small patch of sky in the Fornax constellation. The wider field of view shows an optical image from the Hubble Space Telescope of a portion of the CDF-S field, while the inset shows a Chandra image focusing only on XT2. Credit: X-ray: NASA/CXC/Univ. of Science and Technology of China/Y. Xue et al.; Optical: NASA/STScI
A bright burst of X-rays discovered by NASA’s Chandra X-ray Observatory likely signals the merger of two neutron stars— dense stellar objects packed mainly with neutrons. The source of the X-rays, dubbed XT2, is located in the Chandra Deep Field South (CDF-S), a small patch of sky in the Fornax constellation. The wider field of view shows an optical image from the Hubble Space Telescope of a portion of the CDF-S field, while the inset shows a Chandra image focusing only on XT2. Credit: X-ray: NASA/CXC/Univ. of Science and Technology of China/Y. Xue et al.; Optical: NASA/STScI
A bright burst of X-rays has been discovered by NASA’s Chandra X-ray Observatory in a galaxy 6.6 billion light years from Earth. This event likely signaled the merger of two neutron stars—dense stellar objects packed mainly with neutrons—and could give astronomers fresh insight into how neutron stars are built. A paper describing the research, conducted by an international team of astronomers including Penn State researchers and alumni, appears in the journal Nature.

When two neutron stars merge they produce jets of high energy particles and radiation fired in opposite directions. If the jet is pointed along the line of sight to Earth, a flash, or burst, of gamma rays can be detected. If the jet is not pointed in our direction, a different signal is needed to identify the merger, such as the detection of gravitational waves—ripples in space time.

Now, with the observation of a bright flash of X-rays, astronomers have found a different kind of signal that could indicate a merger, and discovered that two neutron stars likely merged to form a new, heavier and fast-spinning neutron star with an extraordinarily strong magnetic field.

“We’ve found a completely new way to spot a neutron-star merger,” said Yongquan Xue, professor at the University of Science and Technology of China (USTC) and lead author of the paper; Xue was formerly a postdoctoral researcher at Penn State. “The behavior of this X-ray source matches what one of our team members predicted for these events.”

Chandra observed the source, dubbed XT2, as it suddenly appeared and then faded away after about seven hours. The source is located in a region of the sky known as the Chandra Deep Field-South, which is the focus of the deepest X-ray image ever taken, containing almost 12 weeks of Chandra observing time taken at various intervals over nearly 16 and a half years. XT2 appeared on March 22, 2015 and was discovered later in analysis of archival data.

“The serendipitous discovery of XT2 makes another strong case that nature’s fecundity repeatedly transcends human imagination,” said co-author Niel Brandt, Verne M. Willaman Professor of Astronomy and Astrophysics and professor of physics at Penn State and principal investigator of the relevant Chandra Deep Field-South data.

The researchers identified the likely origin of XT2 by studying how its X-ray light varied with time, and comparing this behavior with predictions made in 2013 by Bing Zhang, professor and associate dean for research at the University of Nevada, Las Vegas, and an author of the paper; Zhang was also formerly a postdoctoral researcher at Penn State. The X-rays showed a characteristic signature that matched those predicted for a newly-formed magnetar—a neutron star spinning around hundreds of times per second and possessing a tremendously strong magnetic field about a quadrillion times that of Earth’s.

The team thinks that the magnetar lost energy in the form of an X-ray-emitting wind, slowing down its rate of spin as the source faded. The amount of X-ray emission stayed roughly constant in X-ray brightness for about 30 minutes, then decreased in brightness by more than a factor of 300 over 6.5 hours before becoming undetectable. This behavior indicates that the neutron star merger produced a new, larger neutron star that survived at least a few hours rather than collapsing immediately into a black hole.

This result is important because it gives astronomers a chance to learn about the interior of neutron stars, objects that are so dense that their properties could never be replicated on Earth.

“We can’t throw neutron stars together in a lab to see what happens, so we have to wait until the Universe does it for us,” said Zhang. “If two neutron stars can collide and a heavy neutron star survives, then this tells us that their structure is relatively stiff and resilient.” 
Neutron-star mergers have been prominent in the news since the advanced Laser Interferometer Gravitational-Wave Observatory (LIGO)—including researchers from Penn State—detected gravitational waves from one in 2017. That source, known as GW170817, produced a burst of gamma rays and an afterglow in light detected by many other telescopes, including Chandra. The research team thinks that XT2 would also have been a source of gravitational waves, however it occurred before Advanced LIGO started its first observing run, and it was too distant to have been detected in any case.

The team also considered whether a different phenomenon, the collapse of a massive star, could have caused XT2 rather than a neutron star merger. However, XT2 is in the outskirts of its host galaxy, which aligns with the idea that supernova explosions that left behind the neutron stars kicked them out of the center a few billion years earlier. The galaxy itself also has certain properties – including a low rate of star formation compared to other galaxies of a similar mass – that are much more consistent with the type of galaxy where the merger of two neutron stars is expected to occur. Massive stars, by contrast, are young and are associated with high rates of star formation.

“The host-galaxy properties of XT2 indeed boost our confidence in explaining its origin,” said co-author Ye Li from Peking University.

The team estimated the rate at which events like XT2 should occur, and found that it agrees with the rate deduced from the detection of GW170817. However, both estimates are highly uncertain because they depend on the detection of just one object each, so more examples are needed.

“There must be more exciting transients that are still undiscovered in Chandra’s archival X-ray data,” said Guang Yang, graduate student in astronomy and astrophysics at Penn State and an author of the paper whose current research focuses on rate constraints for events like XT2. “These old data are really a gold mine.”

In addition to Xue, Brandt, Zhang, and Yang, the research team includes former Penn State scientists Bin Luo, professor of astronomy and space science at Nanjing University; Franz Bauer, associate professor of astrophysics, and Fabio Vito, postdoctoral fellow, at Pontifica Universidad Católica de Chile; Bret Lehmer, assistant professor of physics at the University of Arkansas; and Mouyuan Sun, postdoctoral researcher at USTC. Sun has recently been appointed as a faculty member at Xiamen University.

The team also includes Xuechen Zheng, Xu Kong, JY Li, and Junxian Wang at the USTC, Binbin Zhang at Nanjing University, and Hui Sun and Xue-Feng Wu at the Chinese Academy of Sciences.

The relevant data collected for this research were gathered by Chandra’s Advanced CCD Imaging Spectrometer (ACIS), built by a team from Penn State and the Massachusetts Institute of Technology. NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations.

[ Peter Edmonds and Gail McCormick ]

Contacts:
W. Niel Brandt: wnbrandt@gmail.com, (814) 865-3509
Gail McCormick (Penn State PIO): GailMcCormick@psu.edu, (814) 763-0901

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