First neutron star collision in millimeter light – ALMA telescopes show afterglow of one of the strongest gamma-ray bursts yet

Explosive end: Astronomers have observed for the first time one of the most energetic millimeter-wave explosions in the cosmos – the collision of a neutron star with a companion star. The resulting brief gamma-ray burst was one of the strongest ever detected. Analyzing the afterglow in the long-wave millimeter range allowed the researchers to determine the origin of the burst and the width of its energetic jets – providing important information about such events.

gamma-ray bursts are the brightest and most energetic explosions in the cosmos: within a few seconds they can release as much radiation as our sun did in its entire lifetime. The cause of long gamma-ray bursts lasting up to a few minutes is probable Supernovaeshort bursts, on the other hand, go to the Collision of a neutron star with a second star back. They are the “factories” where the heavy elements of our universe are formed.

The explosion triggered by such a collision creates concentrated jets of energetic particles and radiation. If one of these moving bundles of rays hits the earth, it generates a short gamma-ray pulse, usually lasting only a few tenths of a second.

Millimetre-wave afterglow of gamma-ray burst GRB 211106A as seen by the ALMA observatory.© ALMA (ESO/NAOJ/NRAO), T. Laskar (Utah), S. Dagnello (NRAO/AUI/NSF)

Aiming afterglow from gamma ray bursts

Now, for the first time, astronomers have captured the millimeter-wave afterglow of such a brief gamma-ray burst. “The radiation from the afterglow is essential to find out from which galaxy the gamma-ray burst came and how the explosion happened,” explains first author Tanmoy Laskar from Radboud University in the Netherlands. Of particular importance are the long-wave radiation components, which are caused by the interaction of the jets with the surrounding gas.

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The problem, however, is that so far astronomers have only been able to observe half a dozen short gamma-ray bursts in the radio wave range, but the telescopes available were too weak for millimeter waves and the bursts were too far away. Only now, thanks to the high resolution and sensitivity of the Atacama Large Millimeter/submillimeter Array (ALMA), has it been possible to capture the millimeter radiation of such a neutron star collision for the first time.

A bolt of lightning from a galaxy far, far away

Gamma-ray burst GRB 211106A was first detected on November 6, 2021 by ESA’s Integral Gamma-ray Observatory, after which astronomers pointed numerous other telescopes at the source, including NASA’s Swift X-ray Telescope, the Hubble Space Telescope and ALMA’s antennas. The Hubble images narrowed the field of candidate galaxies, but only millimeter-wave ALMA data revealed the true location and nature of the explosion.

According to this, this neutron star collision occurred at a distance corresponding to a redshift of z= 0.7 to 1.4. That means the gamma-ray burst happened when the universe was only 40 percent of its current age. “As a result, this neutron star collision was too far away to detect the gravitational waves it produced – it was beyond the range of our current gravitational-wave detectors,” says co-author Wen-fai Fong from Northwestern University.

Record breaking energy release

The information derived from the millimeter waves was all the more important. “Millimeter radiation can tell us something about the density of the environment around a gamma-ray burst. Combined with the data in the gamma and X-ray range, we can use this to determine the true energy of the explosion,” explains Fong’s colleague Genevieve Schroeder. In the case of GRB 211106A, part of the explosion energy released was apparently absorbed by surrounding dust clouds.

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If you take this into account, the explosion must have released a normal energy: the astronomers determined a value between 2 x 1050 and 6×1051 Erg. “The energy is thus among the highest ever determined for a short gamma-ray burst,” state Laskar and his colleagues. And the afterglow of GRB 211106A also shows several superlatives. The X-rays released in the aftermath of the explosion are said to be among the strongest ever detected in such a gamma-ray burst, and the same applies to radio emissions.

Jet with the widest opening angle

The millimeter radiation is also important for another reason: “The millimeter and radio waves gave us the information we needed to measure the aperture angle of the jet,” explains co-author Alicia Rouco Escorial from Northwestern University. “This is essential to determine the properties of short gamma-ray bursts in the cosmos and to compare them with, for example, the rate of neutron star collisions and neutron star black hole mergers.”

Analysis of the ALMA data showed that the jet from GRB 211106A had an aperture angle of around 15.5 degrees. “This is one of the widest jet spread angles ever observed in a brief gamma-ray burst,” the astronomers report. From their data, they conclude that ALMA could in future detect around one comparable event per year. “This surpasses all previous detection rates of short gamma-ray bursts in the radio range,” say the team.

New possibilities of astronomical research

Astronomers believe that observing gamma-ray bursts and their afterglows in the full spectrum of radiation opens up new possibilities for studying these still mysterious super-explosions. “These observations are just fantastic on many levels. They give us more information about the mysterious gamma-ray bursts and neutron star astrophysics in general,” comments Joe Pesce of the US National Science Foundation.

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The new James Webb space telescope is also of great importance. “With the Webb telescope, we can now analyze the light spectrum of the home galaxies of such outbursts and thus determine the distance more easily,” explains Laskar. “In addition, we can also use the JWST to study the afterglow in the infrared range and thus learn more about the chemical composition.” (The Astrophysical Journal Letters, Preprint; arXiv:2205.03419)

Quelle: National Radio Astronomy Observatory (NRAO), Northwestern University

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