A recent study published in Nature Astronomy examines a new source of how gamma-ray bursts (GRBs) could be created from stars colliding or from the remains of stars in a tightly packed environment around a supermassive black hole which resides at the center of an old galaxy. This study holds the potential to help scientists better understand one of the most mysterious phenomena in the universe, as GRBs have long been hypothesis to be produced from the mergers of neutron stars or the explosion of massive stars.
“For every hundred events that fit into the traditional classification scheme of gamma-ray bursts, there is at least one oddball that throws us for a loop,” said Dr. Wen-fai Fong, who is an assistant professor in the Department of Physics and Astronomy at Northwestern University, and a co-author on the study. “However, it is these oddballs that tell us the most about the spectacular diversity of explosions that the universe is capable of."
Artist illustration of a gamma-ray burst (GRB). (Credit: International Gemini Observatory/NOIRLab/NSF/AURA/M. Garlick/M. Zamani)
For the study, the researchers examined GRB 191019A, which they estimate resides equal to or less than 100 parsecs (326 light-years) from the center of a distant galaxy more than a billion years old.
There are three longstanding hypotheses regarding what happens when stars die: small stars like our Sun eventually run out of hydrogen and expand into red giants before shedding their outer layers to become white dwarfs; much larger stars explode into supernovas, which create either neutron stars or black holes; and the third hypothesis involves the formation of binary stars from the remains of other stars that eventually collide. However, this new study presents a fourth hypothesis for how stars die.
“Our results show that stars can meet their demise in some of the densest regions of the universe, where they can be driven to collide,” said Dr. Andrew Levan, who is a professor in the Department of Physics at Radboud University, and lead author of the study. “This is exciting for understanding how stars die and for answering other questions, such as what unexpected sources might create gravitational waves that we could detect on Earth.”
The center of most galaxies has a supermassive black hole with a plethora of activity in its surrounding environment, including stellar remnants, neutron stars, white dwarfs, and smaller black holes. It is from this immense activity where scientists have long hypothesized that GRBs could be produced but have yet to make such a discovery.
GRBs are found by observing the bright flash of gamma rays, with any GRB lasting greater than two seconds being considered “long”. But in October 2019, scientists used NASA’s Neil Gehrels Swift Observatory to observe a GRB that lasted slightly over one minute, followed by long-term observations made by the Gemini South telescope in Chile.
These observations helped the researchers locate the source of the GRB at approximately equal to or less than 100 parsecs (326 light-years) from the center of an old galaxy more than one billion years old. The smoking gun of the findings was the researchers didn’t observe any evidence of a supernova that could have caused the GRB.
“The lack of a supernova accompanying the long GRB 191019A tells us that this burst is not a typical massive star collapse,” said Julian Rastinejad, who is a PhD student at Northwestern who carried out the calculations to make sure a supernova was not hidden in the data and is a co-author on the study. "The location of GRB 191019A, embedded in the nucleus of the host galaxy, teases a predicted but not yet evidenced theory for how gravitational-wave emitting sources might form."
For next steps, the researchers hope to use the Vera C. Rubin Observatory, scheduled to come online in 2025, to make more of these unique observations.
What new discoveries will scientists make about GRBs in the coming years and decades? Only time will tell, and this is why we science!
As always, keep doing science & keep looking up!