Astronomers may have detected a ‘sonic boom’ from a powerful explosion known as Kilonova

NASA’s Chandra X-ray Observatory has collected data on a kilonova – a powerful event pictured here that occurs when two neutron stars merge – associated with GW170817. This is the first cosmic event that produced gravitational waves and electromagnetic radiation, or light, that was detected on Earth. Credit: X-ray data from NASA, CXC and Northwestern Univ./A. Hajela; visual by NASA/CXC/M. Weiss

Astronomers may have detected a “sonic boom” of a powerful explosion known as a kilonova. This event, called GW170817, is the result of the merger of two neutron stars and is the first object for which both[{” attribute=””>gravitational waves and electromagnetic radiation, or light, have been detected form Earth. Continued detections of this light by " data-gt-translate-attributes="[{" attribute="">NASA’s Chandra X-ray Observatory—analyzed by a collaboration that includes Penn State researchers—revealed this cosmic phenomenon.

“Chandra has continued to detect electromagnetic radiation from this neutron star merger nearly four years after the event was first detected,” said David Radice, assistant professor of physics and of astronomy and astrophysics at Penn State and a member of the collaboration. “These observations provide important information about what happens after the initial collision, such as when and how the two merged objects might form a black hole.”

A kilonova occurs when two neutron stars – some of the densest objects in the universe – merge. On August 17, 2017, astronomers discovered gravitational waves from such a merger using the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) in the United States and the Virgo detector in Italy, coinciding with a burst of gamma rays. Since then, astronomers have been using telescopes all over the world and in space, including NASA’s Chandra X-ray Observatory, to study GW170817 across the electromagnetic spectrum, which includes X-rays.

“We have entered uncharted territory here in studying the aftermath of a neutron star merger,” said Aprajita Hajela of Northwestern University, who led the new study of GW170817.

Astronomers believe that after neutron stars merge, the debris generates light in the visible and infrared spectrum 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. In the case of GW170817, visible light and infrared emission were detected several hours after the gravitational waves.

Neutron star merging looked very different in X-rays. Right after the initial LIGO detection was announced, scientists asked Chandra to quickly switch from its current target to GW170817. At first they didn’t see any x-rays from the source, but on August 26, 2017, Chandra looked again and found a point source of x-rays.

This non-detection of X-rays followed quickly by detection provides evidence of a narrow jet of high-energy particles produced by the merging of neutron stars. The jet is “off-axis”, that is, it is not pointing directly at Earth. The researchers believe that Chandra initially saw the narrow jet from its side and therefore saw no X-rays immediately after the gravitational wave detection.

However, over time the material in the jet slowed and expanded as it hit the surrounding material. This caused the jet cone to begin to extend further into Chandra’s direct line of sight, and an X-ray emission was detected.

Since early 2018, the X-ray emission caused by the jet has steadily weakened as the jet slowed and grew. The research team then noticed that from March 2020 to the end of 2020, the decline stopped and the X-ray emission was roughly constant in brightness. It was a significant sign.

“The fact that the X-rays stopped fading rapidly was our best evidence yet that something more than a jet is detected in the X-rays in this source,” said co-author Raffaella Margutti of the University of California at Berkeley. “A completely different source of X-rays seems to be needed to explain what we’re seeing.”

One of the main explanations for this new X-ray source is that the expanding debris from the meltdown generated a shock, like the sonic boom of a supersonic aircraft. The emission produced by the shock-heated material is called kilonova afterglow. Another explanation is that the X-rays come from material falling toward a black hole that formed after neutron stars merged. GW170817 would be the first observation of either explanation.

“Further study of GW170817 could have far-reaching implications,” said co-author Kate Alexander, also from Northwestern University. “The detection of a kilonova afterglow would imply that the merger did not immediately produce a black hole. Alternatively, this object may offer astronomers a chance to study how matter falls on a black hole a few years after it is born.

To distinguish between the two explanations, astronomers will continue to monitor GW170817 in X-rays and radio waves. If it is a kilonova afterglow, the radio emission should get brighter over time and be detected again in the next few months or years. If the explanation involves material falling on a newly formed black hole, then the x-ray output should remain stable or rapidly decrease, and no radio emission will be detected over time. New Chandra observations of GW170817 from December 2021, which the team is currently analyzing, could help answer this question.

“This observation also paves the way for further study,” said co-author Ashley Villar, assistant professor of astronomy and astrophysics at Penn State. “When LIGO begins its fourth campaign of observations, we hope to find more kilonovae and really explore the diversity of these events, including how mass and energy signatures differ in afterglow and how non-thermal components like the structure throw may vary. The richness of this data set is essential to shed light on the physics behind this diversity.

For more information on this research, see:

Reference: “The emergence of a new X-ray source from the merger of binary neutron stars GW170817” by A. Hajela, R. Margutti, JS Bright, KD Alexander, BD Metzger, V. Nedora, A Kathirgamaraju, B. Margalit, D. Radice, E. Berger, A. MacFadyen, D. Giannios, R. Chornock, I. Heywood, L. Sironi, O. Gottlieb, D. Coppejans, T. Laskar, Y. Cendes , R. Barniol Duran, T. Eftekhari, W. Fong, A. McDowell, M. Nicholl, X. Xie, J. Zrake, S. Bernuzzi, FS Broekgaarden, CD Kilpatrick, G. Terreran, VA Villar, PK Blanchard, S. Gomez, G. Hosseinzadeh, DJ Matthews and JC Rastinejad, April 5, 2021, Astrophysics > High energy astrophysical phenomena.

An article describing these results appears in the latest issue of Letters from the Astrophysical Journal.

NASA’s Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory’s Chandra X-ray Center controls science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.

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