Scientists detect either first ‘kilonova’ afterglow, or new black hole’s first meal

Scientists have detected for the first time the afterglow of a kilonova, the violent merger of two neutron stars or black holes.

Astronomers first detected the original blast from the merger in 2017, when the advanced Laser Interferometer Gravitational-wave Observatory, or LIGO, picked up gravitational waves characteristic of a merger between neutron stars or black holes.

A burst of gamma radiation, visible and infrared light were also observed within hours of the detection of the gravitational waves.

In a new paper published in The Astrophysical Journal Letters, researchers from multiple US universities describe how continued monitoring of the merged stars — now known collectively as GW170817 — using Nasa’s Chandra X-ray Observatory may have led to either the first observation of kilonova afterglow or an equally unique observation of a new black hole’s first meals.

"We have entered uncharted territory here in studying the aftermath of a neutron star merger," Aprajita Hajela, a graduate student in astrophysics at Northwestern University and lead author of the study, said in a statement.

Not as bright as a supernova, astronomers have observed the flash of light from kilonovae before, and believe it results from the decay of radioactive elements generated in the merger. But the detection of a kilonova’s afterglow is new.

The Chandra observatory first picked up an X-ray emission from GW170817 in early 2018 and found that the signal became fainter over time until around the end of 2020 when the X-rays settled into a constant brightness.

That pattern of decreased X-ray emission before settling into constant brightness is what researchers believe signifies the afterglow of the kilonova, X-rays produced by a shock wave heating debris from the merger.

The researchers note there could be an alternative explanation: the X-ray emissions could be the results of material falling into a newly formed black hole.

Neutron star mergers do frequently result in the formation of a black hole, but the researchers believe the observed emissions indicate its unlikely in the case of GW170817 — a black hole formed by the merger would have limited the scale of the kilonova.

Further observations of GW170817 could be decisive. A kilonova afterglow should lead to increasingly bright radio emissions from the object, while a black hole should produce steady or declining X-ray emissions with no radio emissions.

Either outcome will be interesting to scientists.

"This would either be the first time we’ve seen a kilonova afterglow or the first time we’ve seen material falling onto a black hole after a neutron star merger," study co-author and University of California, Berkeley post-doctoral researcher in astronomy Dr Joe Bright said in a statement. “Either outcome would be extremely exciting.”