A whisper of an extraordinary cosmic event reached our Earthly ear last year, leading the qualified on a quest to decipher it. Now, eleven months later, we learn about the cosmic catch.
Scientists have reported detecting a brief, powerful burst of high-energy radiation with NASA’s Fermi Gamma-ray Space Telescope on 26 August 2020. Shooting towards Earth for as long as nearly half the present age of the universe, the burst or signal is said to have beeped for less than a second — short, yet enough to make its powerful presence felt.
Though the signal, a gamma-ray burst (GRB), wouldn’t be puzzling simply by virtue of its 0.65-second life, what makes it interesting is that it mounts a challenge to the traditional classification of GRBs as used by scientists for years.
The short-lived burst was found to be caused by the death of a massive star, after scientists studied the afterglow of the event for weeks. Given its phenomenal source, the burst should have registered a presence of over two seconds. Instead, it made space for itself in the class of GRBs on the other side of this two-second marker, called short GRBs.
The understanding thus far has been that long GRBs originate from massive star core collapse events while short GRBs originate from the merger of either two neutron stars or a neutron star and a black hole. However, a short GRB arising from the death of a massive star, as observed in this reported case, forces a re-think of the classification.
“We already knew some GRBs from massive stars could register as short GRBs, but we thought this was due to instrumental limitations. Now we know dying stars can produce short bursts too,” Bin-bin Zhang at Nanjing University in China and the University of Nevada, Las Vegas has said.
Zhang is the lead author on one of the two papers published in the peer-reviewed scientific journal Nature Astronomy on 26 July. The papers present findings after extensive analyses on the observed gamma-ray data as well as the corresponding event in the universe that gave rise to it.
Scientists from around the world have contributed to this detective work, including Indian astronomers from institutes such as the Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital; the Inter-university Centre for Astronomy and Astrophysics (IUCAA), Pune; National Centre for Radio Astrophysics (NCRA) – Tata Institute of Fundamental Research, Pune; and Indian Institute of Technology Bombay, Mumbai.
Dr Shashi Bhushan Pandey, who is an astronomer with ARIES, told Swarajya that he was part of the collaboration that measured the red shift, or the distance to the burst source.
“Distance is very important to constrain any physical parameter. Unless you know the distance, you can't tell the energetics and other key parameters,” he said.
Red shift can be understood as the stretching of light towards the red part of the spectrum. In everyday life, we experience something similar through the Doppler shift when the source of a sound and the observer move relative to each other. This is noticeable when ambulance sirens change pitch as the vehicle moves relative to others on the road.
Light too exhibits such a quality. As galaxies are moving away from us, because of the expanding universe, the light coming in from these distant regions is ‘red shifted’. The red shift can then be used to tell the distance to these places. The further away an object is, the more it is found to be red-shifted.
Dr Pandey contributed to the analysis of the afterglow and identification of the host galaxy. He also had a look at this event using the 3.6 metre optical telescope — Asia’s largest — in Devasthal, a mountain peak located some 60 km from ARIES, near Nainital in Uttarakhand. His observation will come in handy during further analyses of the gamma-ray burst, the results of which are expected in the future.
From the analysis carried out in the last few months, the large global astronomer group has shown, for the very first time, that even a dying star can produce a short burst. “GRB 200826A”, named after its date of detection, has been credited as the shortest of its kind ever recorded.
Dr Pandey believes this is a unique and outstanding case because although various properties indicate a source associated with a long-duration burst, the actual burst recorded was a short-duration one.
“This poses some questions. Have we misunderstood or misqualified earlier gamma-ray burst cases? And the number density that we have for these bursts — are they correct or do they need a re-look?” Dr Pandey says, teasing new directions of research.
Gamma-ray bursts are expressions of some of the most violent events occurring in the universe. Whether it’s the death of a star or a scenario where two neutron stars or a neutron star and a black hole merge, GRBs accompany the formation of a black hole.
Rather than from the explosion, as one would guess, the bursts come from two jets of plasma that shoot out at near the speed of light in opposite directions from the core of the dying star. If at least one of these jets is pointed towards the Earth, a gamma-ray burst can be detected.
Doing justice to their colossal sources, these bursts light up on Earth up to about a as bright as the Sun for the period that they last, which typically ranges from milliseconds to minutes, depending on the source and mechanism of the burst.
During its brief life, they can outshine an entire galaxy. But since the Earth’s atmosphere blocks gamma rays, they have to be detected up high, either via balloons or space telescopes. Ground-based telescopes, though, have started to detect GRBs indirectly since 2019 with assistance from space-based counterparts.
Like visible, ultraviolet, or radio light, gamma rays are a form of electromagnetic radiation. They fall on the energetic side of the spectrum, adjacent to X-rays. Gamma photons are the most energetic photons in the spectrum.
Gamma rays were first observed in the year 1900, from the then newly discovered radium, by French chemist Paul Villard. However, the name “gamma rays” came from New Zealand physicist and 1908 Chemistry Nobel laureate Ernest Rutherford.
Gamma-ray bursts in astronomy came much later and had a more dramatic origin story. They were detected serendipitously when, during the Cold War, a suspicious United States (US) sent up military satellites with gamma-ray detectors to catch the Soviet Union conducting nuclear tests in violation of the atmospheric nuclear test ban treaty.
The gamma rays netted by the US in 1967 turned out to be not from nuclear explosions, but rather from a source somewhere in space. Research followed to identify the source of these bursts. A 1973 in the Astrophysical Journal catalogued 16 “short bursts of photons... observed between 1969 July and 1972 July using widely separated spacecraft”.
It wasn’t clear whether the bursts were coming from our solar system, within our galaxy, or well beyond our galactic neighbourhood.
In the 1990s, thousands of burst observations, thanks to the Burst and Transient Source Experiment on board the Compton Gamma-Ray Observatory, led to a better understanding of the source region. With the help of the Italian-Dutch satellite BeppoSAX, launched in 1996, it was learnt that the bursts came from extragalactic sources.
It was also that the merger of two collapsed stars in a binary (for example, two neutron stars or a neutron star and a black hole) best explained the burst source. Since then, we have learnt quite a bit more about gamma-ray bursts.
However, the new 2020 burst challenges current understanding of the mechanism driving these bursts. The second paper, with Tomás Ahumada of the University of Maryland and NASA Goddard Space Flight Center as the lead author, published in Nature Astronomy looks into this aspect.
“GRB 200826A is the shortest LGRB found with an associated collapsar; it appears to sit on the brink between a successful and a failed collapsar. Our discovery is consistent with the hypothesis that most collapsars fail to produce ultra-relativistic jets,” the authors write.
The observed GRB 200826A fits in with the idea that most massive stars die without producing long-lived jets and GRBs. This is also why long GRBs occur at a lower rate than the supernova type associated with them.
Additionally, the burst also tells scientists that the duration of the signal shouldn’t be taken on face value as an indicator of the burst origin.
“We think this event was effectively a fizzle, one that was close to not happening at all,” Ahumada has 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-duration GRBs ever seen.”
Further analysis is expected to shine more light on the matter, as well as on how to read gamma-ray bursts better going forward.
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