Bengaluru: Once upon a time, billions of years ago, a star died. It let out a pulse of extremely high energy — a gamma ray burst (GRB), as the astronomers call it — that was detected on Earth just last year. Lasting less than a second, it was the shortest of its kind ever observed.
And it appears to have challenged some existing models that govern the way these bursts are seen.
Astronomers looking through NASA’s Fermi Gamma-ray Space Telescope observed a pulse of extremely high energy in the form of a GRB on 26 August 2020. The GRB, the astronomers say, is the shortest known to have been caused by the death of a star.
GRBs are the brightest and most energetic explosions or electromagnetic events that occur in the universe. They typically occur during a supernova, when a star of very high mass implodes under its own gravity to form black holes or neutron stars. A GRB can last anywhere from a few milliseconds to many hours. Some short GRBs have been known to occur when binary neutron stars merge.
The detection by Fermi, named GRB 200826A after the date on which it was observed, is believed to have been produced by a star collapse.
“We already knew some GRBs from massive stars could register as short GRBs, but we thought this was due to instrumental limitations,” said Bin-bin Zhang at Nanjing University in China, and lead author of one of the papers from the findings, in an accompanying press statement.
“This burst is special because it is definitely a short-duration GRB, but its other properties point to its origin from a collapsing star. Now we know dying stars can produce short bursts, too.”
Mechanism of GRB production
As a star runs out of fuel and implodes in a supernova, its matter collapses towards its core, which is in the process of becoming a black hole or another compact object.
A bit of the matter and energy speeding towards the black hole escapes in the form of two powerful jets of energy moving in opposite directions at nearly the speed of light.
A jet of this nature is a GRB, and it is detectable only if it points directly towards Earth.
GRBs can be detected across billions of light years as they have extremely high energy and travel long distances for long time periods. GRB 200826A took 6.6 billion years to reach us, traveling for nearly half the universe’s age — 13.8 billion years — before being detected on Earth for 0.65 seconds.
The GRB was also detected on other instruments such as NASA’s Wind mission, which is currently in an orbit 1.5 million km away from Earth, the European Space Agency’s INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) satellite, and the orbiter Mars Odyssey. These missions are part of a network called the InterPlanetary Network (IPN) that is used to hunt for and detect GRBs.
Findings from the afterglow
After a GRB is detected, longer-wavelength energies follow. These are described as the “afterglow” of a supernova. The energy is visible as X-rays, radio waves, and more.
The location of GRB 200826A was triangulated and narrowed down about 17 hours after it was detected, to a point in the Andromeda constellation. Meanwhile, astronomers scanned the skies for the afterglow.
“Conducting this search is akin to trying to find a needle in a haystack, but the IPN helps shrink the haystack,” said Shreya Anand of Caltech and a co-author on the afterglow paper, in the same statement. “Out of more than 28,000 ZTF (Zwicky Transient Facility at Palomar Observatory) alerts the first night, only one met all of our search criteria and also appeared within the sky region defined by the IPN.”
X-ray emission was observed from this location within a day of the burst by NASA’s Neil Gehrels Swift Observatory, while radio emissions arrived a couple of days later. Other telescopes that participated in studying the afterglow include the National Radio Astronomy Observatory’s Karl Jansky Very Large Array in New Mexico, and the Gran Telescopio Canarias on La Palma in Spain’s Canary Islands. The supernova itself was detected in the infrared about 28 days after the event by the Gemini North telescope in Hawaii.
Since the energies of the GRB and the afterglow were so weak and short in duration, the researchers theorise that the GRBs barely emerged from the star before they were shut down. But they conclude that such short GRBs from collapsing stars are rare occurrences and produced in conditions that “teeter on the brink of success or failure”.
(Edited by Sunanda Ranjan)