

Chandra observations show the flare-up and subsequent dimming of XT2, which matches predictions for the signal

from a pair of merging neutron stars and the birth of a magnetar.



X-ray: NASA/CXC/Uni. of Science and Technology of China/Y. Xue et al; Optical: NASA/STScI



Signals of a merger

In October 2017, astronomers announced the first detection of gravitational waves from the merger of two neutron stars earlier that year. The event also rung in the era of multi-messenger astronomy, as more than 70 telescopes observed the event’s afterglow in optical light, X-rays, gamma rays, and more. Now, an X-ray signal dubbed XT2 from a galaxy 6.6 billion light-years away has revealed another neutron star merger, which left behind a single, heavier neutron star with an incredibly powerful magnetic field: a magnetar.“We’ve found a completely new way to spot a neutron star merger,” said Yongquan Xue of the University of Science and Technology of China in a press release . Xue is lead author of a paper on the finding, published April 11 in Nature . Furthermore, he says, “The behavior of this X-ray source matches what one of our team members predicted for these events.”When neutron stars spiral together and merge, the event sends out signals across the electromagnetic spectrum, as well as gravitational waves. Depending on how the merger is oriented with respect to Earth, we might observe different types of light in the afterglow, such as gamma rays or X-rays . Thus, knowing the signs to look for, even in the absence of others, will help astronomers identify mergers even when gravitational waves cannot be detected.In the case of XT2, the Chandra X-ray Observatory spotted a surge of X-rays that flared and disappeared over the course of about seven hours on March 22, 2015. Though the signal was not identified at the time it was observed, it was discovered after the fact in the data, which were taken in a portion of the sky covered by the Chandra Deep Field-South — the deepest X-ray image ever obtained.The brightness and behavior of the XT2 signal matches predictions made in 2013 by study co-author Bing Zhang at the University of Nevada, Las Vegas, of the X-rays that should be unleashed when two neutron stars merge and birth a magnetar. Based on the X-rays observed, the team believes after the merger, the newly created fast-spinning magnetar briefly flared up and shone constantly in X-rays for about 30 minutes. But shortly after, the magnetar began losing spin energy by emitting a particle wind. Over the next 6.5 hours, the magnetar’s X-ray signal dimmed by more than 300x, ultimately fading from of Chandra’s view.