Huge, mysterious blast detected in deep space

When two neutron stars spiral into one another and merge to form a black hole—an event recorded in 2017 by gravitational wave detectors and telescopes worldwide—does it immediately become a black hole?

Or does it take a while to spin down before gravitationally collapsing past the event horizon into a black hole?

Ongoing observations of that 2017 merger by the Chandra X-ray Observatory, an orbiting telescope,

suggests the latter: that the merged object stuck around, likely for a mere second, before undergoing ultimate collapse.

The evidence is in the form of an X-ray afterglow from the merger, dubbed GW170817, that would not be expected if the merged neutron stars collapsed immediately to a black hole.

The afterglow can be explained as a rebound of material off the merged neutron stars, which plowed through and heated the material around the binary neutron stars.

This hot material has now kept the remnant glowing steadily more than four years after the merger threw material outward in what's referred to as a kilonova.

X-ray emissions from a jet of material that was detected by Chandra shortly after the merger would otherwise be dimming by now.

While the excess X-ray emissions observed by Chandra could come from debris in an accretion disk swirling around and eventually falling into the black hole,

astrophysicist Raffaella Margutti of the University of California, Berkeley, favors the delayed collapse hypothesis, which is predicted theoretically.

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