NASA’s Fermi spots a strange pulse of high-energy radiation heading towards Earth

On August 26, 2020, NASA’s Fermi-Gamma-ray Space Telescope detected a pulse of high-energy radiation that had been heading toward Earth for nearly half the current age of the universe. Lasting only about a second, it turned out to be one of the records – the shortest gamma-ray burst (GRB) caused by the death of a massive star ever seen.

GRBs are the most powerful events in the universe, detectable across billions of light years. Astronomers classify them as long or short depending on whether the event lasts more or less than two seconds. They observe long bursts associated with the disappearance of massive stars, while short bursts have been linked to a different scenario.

Astronomers combined data from NASA’s Fermi Gamma Space Telescope, other space missions, and ground observatories to reveal the origin of GRB 200826A, a brief but powerful burst of radiation. It’s the shortest burst known to be fueled by a collapsing star – and it hardly ever happened. Credit: NASA Goddard Space Flight Center

“We already knew that some massive star GRBs could register as short GRBs, but we believed this was due to instrumental limitations,” Bin-bin Zhang told Nanjing University in China and the United Kingdom. ‘University of Nevada in Las Vegas. “This burst is special because it is certainly a short-lived GRB, but its other properties indicate its origin from a collapsing star. Now we know that dying stars can produce short bursts as well. “

Named GRB 200826A, after the date it occurred, the burst is the subject of two articles published in Nature Astronomy on Monday, July 26. The first, led by Zhang, explores gamma ray data. The second, led by Tomás Ahumada, a doctoral student at the University of Maryland, College Park and NASA’s Goddard Space Flight Center in Greenbelt, Maryland, describes the multi-wavelength afterglow of the GRB and the emerging light from the supernova explosion that followed.

“We believe this event was indeed a failure, one that was about to not happen at all,” Ahumada said. Even so, the Burst emitted 14 million times the energy released by the entire Milky Way galaxy over the same amount of time, making it one of the most energetic short-lived GRBs ever. seen. “

When a star much more massive than the Sun runs out of fuel, its core suddenly collapses and forms a black hole. As matter swirls toward the black hole, some of it escapes in the form of two powerful jets that rush outward at almost the speed of light in opposite directions. Astronomers only detect a GRB when one of these jets points almost directly at Earth.

Each jet pierces the star, producing a pulse of gamma rays – the most energetic form of light – that can last up to a few minutes. After the burst, the disturbed star then rapidly develops into a supernova.

Short GRBs, on the other hand, form when pairs of compact objects – such as neutron stars, which also form during stellar collapse – spiral inward over billions of years and enter in collision. Fermi observations have recently shown that in nearby galaxies, giant flares of isolated and supermagnetized neutron stars also disguise themselves as short GRBs.

GRB 200826A was a high energy emission explosion lasting only 0.65 seconds. After traveling for eons through the expanding universe, the signal had stretched for about a second when detected by Fermi’s gamma burst monitor. The event also appeared in instruments aboard NASA’s Wind mission, which orbits a point between the Earth and the Sun located approximately 930,000 miles (1.5 million kilometers), and Mars Odyssey, which orbits around the red planet since 2001. ESA (the agency’s European space INTEGRAL satellite) also observed the explosion.

All of these missions participate in a GRB tracking system called the InterPlanetary Network (IPN), for which the Fermi Project provides all US funding. Since the burst hits each detector at slightly different times, any pair of them can be used to help determine where in the sky it occurred. About 17 hours after the GRB, the IPN reduced its location to a relatively small portion of the sky in the constellation Andromeda.

Using the National Science Foundation-funded Zwicky Transient Facility (ZTF) at Palomar Observatory, the team scanned the sky for changes in visible light that could be related to afterglow. discoloration of GRB.

“Conducting this research is like trying to find a needle in a haystack, but IPN helps reduce the haystack,” said Shreya Anand, graduate student at Caltech and co-author of the article on afterglow. . “Of over 28,000 ZTF alerts on the first night, only one met all of our search criteria and also appeared in the region of sky defined by the IPN.”

Less than a day after the explosion, NASA’s Neil Gehrels Swift Observatory discovered waning x-ray emission from the same location. A few days later, a variable radio emission was detected by the Karl Jansky Very Large Array of the National Radio Astronomical Observatory in New Mexico. The team then began to observe afterglow with a variety of ground installations.

By observing the weak galaxy associated with the burst using the Gran Telescopio Canarias, a 10.4-meter telescope at the Roque de los Muchachos observatory in La Palma, Canary Islands, Spain, the team showed that its light takes 6.6 billion years to reach us. This is 48% of the universe’s current age of 13.8 billion years.

But to prove that this short burst came from a collapsing star, the researchers also had to catch the emerging supernova.

“If the explosion was caused by the collapse of a star, once the afterglow wears off, it should clear up again because of the explosion of the underlying supernova,” Leo said. Singer, Goddard astrophysicist and research advisor to Ahumada. “But at these distances, you need a very large, very sensitive telescope to spot the supernova’s light from the background glare of its host galaxy.”

To conduct the research, Singer obtained time on the 8.1-meter Gemini North telescope in Hawaii and the use of a sensitive instrument called the Gemini Multi-Object Spectrograph. Astronomers imaged the host galaxy in red and infrared light from 28 days after the burst, repeating the search 45 and 80 days after the event. They detected a near infrared source – the supernova – in the first set of sightings that could not be seen in subsequent ones.

The researchers suspect that this explosion was propelled by jets that barely emerged from the star before stopping, instead of the more typical case where long-lasting jets emerge from the star and travel considerable distances. If the black hole had fired weaker jets, or if the star was much bigger when it began to collapse, there might not have been any GRB at all.

The discovery helps solve a long-standing conundrum. While long GRBs must be coupled to supernovae, astronomers detect a much larger number of supernovae than long GRBs. This discrepancy persists even after taking into account that the GRB jets must tip almost in our field of view for astronomers to detect them.

The researchers conclude that the collapsing stars producing short GRBs must be marginal cases whose light-speed jets are on the verge of success or failure, a conclusion consistent with the idea that most Massive stars die without producing jets and GRB at all. More broadly, this result clearly demonstrates that the duration of a burst alone does not only indicate its origin.

The references:

“A particularly short-lived gamma-ray burst resulting from the collapse of the nucleus of a massive star” by B.-B. Zhang, Z.-K. Liu, Z.-K. Peng, Y. Li, H.-J. Lü, J. Yang, Y.-S. Yang, Y.-H. Yang, Y.-Z. Meng, J.-H. Zou, H.-Y. Yes, X.-G. Wang, J.-R. Mao, X.-H. Zhao, J.-M. Bai, AJ Castro-Tirado, Y.-D. Hu, Z.-G. Dai, E.-W. Liang and B. Zhang, July 26, 2021, Nature astronomy.
DOI: 10.1038 / s41550-021-01395-z

“Discovery and Confirmation of the Shortest Gamma-ray Burst of a Collapsar” by Tomás Ahumada, Leo P. Singer, Shreya Anand, Michael W. Coughlin, Mansi M. Kasliwal, Geoffrey Ryan, Igor Andreoni, S. Bradley Cenko, Christoffer Fremling, Harsh Kumar, Peter TH Pang, Eric Burns, Virginia Cunningham, Simone Dichiara, Tim Dietrich, Dmitry S. Svinkin, Mouza Almualla, Alberto J. Castro-Tirado, Kishalay De, Rachel Dunwoody, Pradip Gatkine, Erica Hammerstein, Shabnam Iyyani, Joseph Mangan, Dan Perley, Sonalika Purkayastha, Eric Bellm, Varun Bhalerao, Bryce Bolin, Mattia Bulla, Christopher Cannella, Poonam Chandra, Dmitry A. Duev, Dmitry Frederiks, Avishay Gal-Yam, Matthew Graham, Anna YQ Ho, Kevin Hurley, Viraj Karambelkar, Erik C. Kool, SR Kulkarni, Ashish Mahabal, Frank Masci, Sheila McBreen, Shashi B. Pandey, Simeon Reusch, Anna Ridnaia, Philippe Rosnet, Benjamin Rusholme, Ana Sagués Carracedo, Roger Smith, Maayane Soumagnac, Robert Stein, Eleonora Troja, Anastasia Tsvetkova, Richard Walters a Sd Azamat F. Valeev, July 26, 2021, Nature astronomy.
DOI: 10.1038 / s41550-021-01428-7

The Fermi Gamma-Ray Space Telescope is an astrophysics and particle physics partnership operated by NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Fermi was developed in collaboration with the US Department of Energy, with significant contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.