Does it instantaneously become a black hole when two neutron stars spiral towards one another and combine to become a black hole, as seen by gravitational wave detectors and telescopes throughout the globe in 2017? Or does it take some time to spiral down before falling over the event horizon as a black hole?
Ongoing studies of that 2017 merger by the Chandra X-ray Observatory, an orbiting observatory, point to the latter: that the fused entity lingered for a fraction of a second before collapsing.
The proof comes in the form of GW170817, an X-ray afterglow from the merger that would not be predicted if the merging neutron stars fell instantaneously to a black hole. The afterglow is caused by the rebound of material from the merging neutron stars, which plowed through and heated the material around the binary neutron stars. More than four years after the merger flung material outward in what is known as a kilonova, this hot material has kept the remnant blazing persistently. X-ray emissions from a material jet observed by Chandra immediately after the merger would have dimmed by now.
While extra X-ray emissions recorded by Chandra might be caused by material in an accretion disk spinning about and ultimately falling into the black hole, astronomer Raffaella Margutti of the University of California, Berkeley, prefers the theoretically predicted delayed collapse explanation.
“If the merged neutron stars collapsed directly to a black hole with no intermediate stage, it would be very difficult to explain the current X-ray excess because there would be no hard surface for stuff to bounce off and fly out at high velocities to create this afterglow,” said Margutti, a UC Berkeley associate professor of astronomy and physics. “It’d simply tumble in. Done. The actual reason for my scientific excitement is the chance that we are witnessing something other than the jet. We could finally learn anything about the new compact item.”
Margutti and her colleagues, including lead author Aprajita Hajela, who was Margutti’s PhD student at Northwestern before coming to UC Berkeley, present their investigation of the X-ray afterglow in an article approved for publication in The Astrophysical Journal Letters.
A kilonova’s radioactive light
The Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) and the Virgo collaboration discovered gravitational waves from the merger on August 17, 2017. Satellite and ground-based observatories rapidly followed after, capturing a burst of gamma rays, visible and infrared emissions that validated the notion that numerous heavy elements are created in the aftermath of such mergers within heated ejecta, resulting in a dazzling kilonova. The kilonova glows because to light released during the disintegration of radioactive materials created in the merger debris, such as platinum and gold.
Chandra, too, shifted its focus to GW170817, but did not detect any X-rays until nine days later, indicating that the merger generated a narrow jet of material that collided with the material around the neutron stars, emitting a cone of X-rays that originally missed Earth. Only later did the jet’s head grow and start producing X-rays in a larger jet observable from Earth.
The jet’s X-ray emissions surged for 160 days following the merger, then gradually decreased as the jet slowed and expanded. However, Hajela and her colleagues discovered that from March 2020 (around 900 days after the merger) through the end of 2020, the drop halted and the brightness of the X-ray emissions stayed about constant.
“The fact that the X-rays stopped fading soon was our greatest evidence yet that anything other than a jet was identified in X-rays from this source,” Margutti added. “It looks that a whole other source of X-rays is required to explain what we’re seeing.”
The additional X-rays, according to the researchers, are caused by a shock wave that is separate from the jets created by the merging. This shock was caused by the delayed collapse of the combined neutron stars, most likely because its high spin counteracted the gravitational collapse for a short period of time. By remaining for an additional second, the material around the neutron stars received an extra bounce, resulting in an extremely rapid tail of kilonova ejecta that caused the shock.
“We believe that the kilonova afterglow emission is caused by shocked material in the circumbinary medium,” Margutti said. “The shock wave is driven by material that was in the surroundings of the two neutron stars and was shocked and heated up by the quickest edge of the kilonova ejecta.”
She said that the radiation is just now reaching us because it required time for the heavy kilonova ejecta to decelerate in the low-density environment and for the kinetic energy of the ejecta to be transformed into heat by shocks. This is the identical mechanism that generates radio and X-rays for the jet, but since the jet is much, much lighter, it is instantly decelerated by the environment and shines in the X-ray and radio from the beginning.
According to the researchers, an alternate possibility is that the X-rays are caused by material falling towards the black hole that created when the neutron stars combined.
“This would either be the first time we’ve witnessed a kilonova afterglow or material falling onto a black hole after a neutron star merger,” said co-author and UC Berkeley postdoctoral researcher Joe Bright. “Either conclusion would be really thrilling.”
The only observatory that can still detect light from this cosmic collision is Chandra. Follow-up observations by Chandra and radio telescopes, on the other hand, may be able to discriminate between the various hypotheses. Radio emission from a kilonova afterglow is likely to be discovered again in the coming months or years if it is a kilonova afterglow. If the X-rays are created by matter falling onto a freshly formed black hole, the X-ray production should remain constant or quickly diminish, and no radio emission should be observed over time.
Margutti believes that LIGO, Virgo, and other observatories will collect gravitational waves and electromagnetic waves from further neutron star mergers, allowing the sequence of events before and after the merger to be more accurately nailed down and revealing the mechanics of black hole creation. Until then, the sole sample accessible for research is GW170817.
“Further investigation of GW170817 might have far-reaching ramifications,” said co-author Kate Alexander, a postdoctoral researcher at Northwestern University. “The discovery of a kilonova afterglow would suggest that the merger did not instantly result in the formation of a black hole. Alternatively, astronomers may be able to investigate how matter falls upon a black hole a few years after its creation.”
Margutti and her colleagues recently stated that X-rays were found by the Chandra telescope during studies of GW170817 in December 2021. That data is currently being analyzed. There has been no report of radioactivity connected with the X-rays.