28 Feb 2022

Astronomers think that after neutron stars merge, the debris generates visible and infrared light from the decay of radioactive elements like platinum and gold formed in the debris from the merger. This burst of light is called a kilonova. Indeed, visible light and infrared emission were detected from GW170817 several hours after the gravitational waves.

Initially the neutron star merger likely produced a jet of high-energy particles that was not pointed directly at Earth, explaining an initial lack of X-rays seen by Chandra. The jet then slowed down and widened upon impact with surrounding gas and dust. These changes caused an increase in X-rays observed by Chandra followed by a decline in early 2018. However, since the end of 2020, the X-rays detected by Chandra have remained at a nearly constant level. The Chandra image from data taken in December 2020 and January 2021 shows X-ray emission from GW170817 and from the center of its host galaxy, NGC 4993.

A research team studying the Chandra data think this steadying of the X-ray emission comes from a shock — like a sonic boom from an airplane — as the merger debris responsible for the kilonova strikes gas around GW170817. Material heated by such a shock would glow steadily in X-rays giving a "kilonova afterglow", like Chandra has observed. The artist’s illustration shows the merger debris responsible for the kilonova in blue surrounded by a shock depicted in orange and red.

There is also an alternative explanation suggesting that the X-rays come from material falling towards a black hole that formed after the neutron stars merged. This material is depicted by a small disk in the center of the illustration. To avoid a coincidence, it is likely that only one of the two options — the kilonova afterglow or matter falling onto a black hole — is a significant source of the detected X-rays.

The two blue glowing arcs of material above and below the kilonova show where material from the now-faded jet has struck surrounding material.

To distinguish between the two explanations astronomers will keep monitoring GW170817 in X-rays and radio waves. If it is a kilonova afterglow, the radio emission is expected to get brighter over time and be detected again in the next few months or years. If the explanation involves matter falling onto a newly-formed black hole, then the X-ray output should stay steady or decline rapidly and no radio emission will be detected over time.

Researchers recently announced a source was detected in new Chandra observations performed in December 2022. Analysis of that data is ongoing. No radio detection has yet been reported.

[Image]

(A) Illustration of GW170817

(B) X-ray.