Mysterious energy burst seen in millimeter-wavelength radio astronomy for the first time

An artist's rendering of a collision between a star and a neutron star, triggering a gamma-ray burst, one of the most energetic phenomena in the universe.  (ALMA (ESO/NAOJ/NRAO), M. Weiss (NRAO/AUI/NSF))

An artist’s rendering of a collision between a star and a neutron star, triggering a gamma-ray burst, one of the most energetic phenomena in the universe. (ALMA (ESO/NAOJ/NRAO), M. Weiss (NRAO/AUI/NSF))

One of the most powerful flashes in the sky, the result of a star and a neutron star colliding, has been observed by millimeter-wavelength radio astronomy for the first time, providing an unprecedented view of one of the most violent events in the cosmos.

A research team led by Northwestern University in Illinois, and Radboud University in the Netherlands, used the Atacama Large Millimeter/submillimeter Array, or ALMA radio telescope in Chile to capture the afterglow of GRB 211106A, a brief gamma-ray burst (GRB) determined to have originated in a galaxy 20 billion light years away.

“This brief gamma-ray burst was the first time we tried to observe such an event with ALMA,” Wen-fai Fong, professor of physics and astronomy at Northwest, said in a statement. “Afterglow for short bursts is very difficult to obtain, so it was spectacular to see this event shine so brightly.

Dr Fong is one of many authors on a study about the observation which will be published in a forthcoming issue of the Astrophysical Journal Lettersand is available online now at the academic preprint archive arxiv.org.

GRBs are powerful bursts of gamma radiation that occur when massive stars collapse into black holes, or dense neutron stars in a binary system merge with their companion stars to form a black hole, an intensely catastrophic event believed to forge most of the heavier the elements in the universe such as gold and plutonium.

“These mergers occur due to gravitational wave radiation that removes energy from the orbit of the binary stars, causing the stars to spiral in towards each other,” Radboud University astronomer and lead author of the paper Tanmoy Laskar said in a statement. “The resulting explosion is accompanied by jets traveling at close to the speed of light. When one of these jets points toward Earth, we observe a short pulse of gamma radiation, or a short-lived GRB.”

Short GRBs can last only a fraction of a second, but their afterglow can persist in longer, less energetic wavelengths of light for minutes or even days.

Such was the case with GRB 211106A, whose afterglow was first detected in X-ray light by NASA’s Neil Gehrels Swift Observatory, then found in infrared light by the Hubble Space Telescope, and most recently in millimeter radio light by ALMA. It was only with the addition of the ALMA observation that the GRB was located in a distant galaxy.

“The Hubble observations revealed an unchanging field of galaxies,” Dr Laskar said in a statement. “ALMA’s unprecedented sensitivity allowed us to locate the GRB in that field with more precision, and it turned out to be in another faint galaxy, which is further away. This again means that this short-lived gamma-ray burst is even more powerful than we first thought, making it one of the most luminous and energetic on record.”

The millimeter wavelength also gave scientists a clearer picture of the structure and density of the environment around the GRB, according to Dr. Fong, and even allowed scientists to measure the apparent width of the beam that triggered the burst at just more than 15 degrees, one of the widest that has ever been measured.

The study highlights the value of observing complex phenomena at multiple wavelengths using the most sophisticated tools available, which now include the newly operational James Webb Space Telescope.

“In the future, we can also use JWST to capture infrared afterglows and study their chemical composition,” Dr Laskar said in a statement. “I am excited about these upcoming discoveries in our field.”

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