A new kind of signal points to neutron star mergers

"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..."

Gail McCormick-Penn State • futurity
April 18, 2019 8 minSource

NASA’s Chandra X-ray Observatory has discovered a bright burst of X-rays 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.

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, astronomers can detect a flash, or burst, of gamma rays. If the jet is not pointed in our direction, a different signal is necessary 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.

xt2
A bright burst of X-rays that NASA’s Chandra X-ray Observatory discovered 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/U. of Science and Technology of China/Y. Xue et. al.; Optical: NASA/STScI)

“We’ve found a completely new way to spot a neutron-star merger,” says 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.”

XT2 discovery

Chandra observed the source, dubbed XT2, as it suddenly appeared and then faded away after about seven hours. The source is 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 scientists discovered it later in analysis of archival data.

“The serendipitous discovery of XT2 makes another strong case that nature’s fecundity repeatedly transcends human imagination,” says coauthor Niel Brandt, professor of astronomy and astrophysics and professor of physics at Penn State, as well as 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 coauthor Bing Zhang, professor and associate dean for research at the University of Nevada, Las Vegas.

The X-rays showed a characteristic signature that matched those astronomers 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,” says 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.”

Massive mergers

Neutron-star mergers have been prominent in the news since the advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves from one in 2017. That source, known as GW170817, produced a burst of gamma rays and an afterglow in light many other telescopes, including Chandra, detected. 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 associated with high rates of star formation.

“The host-galaxy properties of XT2 indeed boost our confidence in explaining its origin,” says coauthor 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,” says 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.”

Additional researchers from Penn State, Nanjing University, Pontifica Universidad Católica de Chile, University of Arkansas, USTC, and the Chinese Academy of Sciences contributed to the research. Chandra’s Advanced CCD Imaging Spectrometer gathered the relevant data for this research. 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.

Source: Penn State

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