Continuing X-ray observations by Chandra of the kilonova from the merger of two neutron stars to form a black hole hint at new processes. Initially, a gamma-ray burst and subsequent X-ray emissions told of a jet of material produced by the merger, but X-rays from this jet should be dimming. They’re not, suggesting that ejecta from the merger, given an extra bounce from the merged neutron stars a second before collapse, is also generating X-rays.

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.

«If the merged neutron stars were to collapse directly to a black hole with no intermediate stage, it would be very hard to explain this X-ray excess that we see right now, because there would be no hard surface for stuff to bounce off and fly out at high velocities to create this afterglow,» said Margutti, UC Berkeley associate professor of astronomy and of physics. «It would just fall in. Done. The true reason why I’m excited scientifically is the possibility that we are seeing something more than the jet. We might finally get some information about the new compact object.»

Margutti and her colleagues, including first author Aprajita Hajela, who was Margutti’s graduate student when she was at Northwestern University before moving to UC Berkeley, report their analysis of the X-ray afterglow in a paper recently accepted for publication in The Astrophysical Journal Letters.

Story Source: Materials provided by University of California — Berkeley. Original written by Robert Sanders. Note: Content may be edited for style and length.