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Long burst short: an unexpected supernova

Read time: 8 mins
Long burst short: an unexpected supernova

Image by WikiImages via Pixabay

In AD 185, humans saw a new star appear in the night sky. However, it disappeared within a year of its appearance. This vanishing star was the first human record of what astronomers today call a ‘supernova’. Supernovae are explosions of stars that have much more mass than our Sun. In their dying moments, these massive stars explode most of their material and send out so much energy that they are sometimes as bright as the entire galaxy in which the star resided. That is why, despite originating in faraway galaxies, we can still observe the supernovae. With years of improvement in technology, astronomers today use powerful telescopes that routinely detect supernovae not observable by our naked eyes. These supernovae originate in distant galaxies, sometimes billions of light-years away.

Over the last century, astronomers have also detected another stellar explosion called ‘gamma ray bursts’, abbreviated to GRBs. Gamma ray telescopes around the Earth discovered these astrophysical events when their gamma ray photons suddenly bombarded them before vanishing forever. The spikes in the gamma ray light last for up to a few minutes before disappearing forever. Later, scientists identified most of these astronomical events as the dying moments of stars more than twenty times more massive than our Sun. When astronomers follow up the gamma ray burst with optical telescopes, they can sometimes locate the optical wavelength light coming from the galaxy in which the dying star resided, and at other times optical light from the dying star itself. Occasionally, astronomers tune large dish-shaped telescopes, capable of observing the universe in the radio wavelengths of the electromagnetic spectrum, to that part of the sky. A few of these times, they successfully detect the radio wavelength light emitted by the dying star. The emission tells them whether the star also exploded as a supernova or not.

Not all GRBs are dying stars, however. About a tenth of the GRBs, termed ‘short GRBs’, last less than two seconds. Neutron stars, stars made up entirely of neutrons, weigh about twice the mass of our Sun but span only a few kilometres in diameter. When two such neutron stars come close to each other under the influence of their mutual gravity and finally merge into a single object, either a bigger neutron star or a black hole, they emit gamma ray photons. Gamma ray telescopes register the photons as blips for at most two seconds –– the ‘short’ GRBs.

A new study by a team of international astronomers has discovered a short GRB that was not created by merging neutron stars. Instead, the short GRB was created by a single star exploding into a supernova. The discovery was possible due to a large group of astronomers coming together. The group consists of astronomers specialising in analysing data from gamma-ray, X-ray, optical, and radio telescopes located worldwide. Astronomers from the Indian Institute of Technology Bombay (IIT-Bombay), Mumbai, the Inter-University Centre for Astronomy and Astrophysics, Pune, the National Centre for Radio Astrophysics––Tata Institute of Fundamental Research, Pune, and Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital were also involved in the study. The study was published in the journal Nature Astronomy.

Fermi Gamma-ray Space Telescope, an astronomy satellite operated by the National Aeronautics Space Administration (NASA), discovered the short gamma ray burst with the help of its dedicated instrument, the ‘Gamma-ray Burst Monitor’. The burst goes by the astronomical name GRB200826A because it was the first GRB that Fermi detected on the 26th of August 2020 with the help of its onboard computer algorithms that continuously process large volumes of astronomical data. AstroSat, India’s first multiwavelength astronomy satellite consisting of five scientific instruments for observing the universe from optical to gamma ray wavelengths, also participated in the study. One of its astronomy instruments on board, the ‘Cadmium Zinc Telluride Imager’, detected the GRB simultaneously and confirmed that it lasted for less than two seconds.

The astronomers used the Zwicky Transient Factory, another optical wavelength telescope, to identify the exact region of the sky from which the short GRB originated. The brightness of the source measured by this telescope also tells them the distance at which the GRB originated. Then, the astronomers set into motion an extensive multiwavelength observation campaign using optical telescopes around the world via an international collaboration called the ‘Global Relay of Observatories Watching Transients Happen’ (GROWTH). The partnership has astronomers at 15 scientific universities across the world.

Astronomers from the National Centre for Radio Astrophysics––Tata Institute of Fundamental Research (NCRA–TIFR) used the Giant Metrewave Radio Telescope (GMRT) near Pune to go hunting for the radio emission from the GRB. They trained the GMRT on the GRB twice, 15 days and 20 days after Fermi and AstroSat had detected the GRB. The GMRT was upgraded in 2017, making it about three times more likely to detect the radio emission. “It is not impossible to measure these emissions with the GMRT,” says Poonam Chandra, associate professor at NCRA–TIFR and an author of the study. However, their search did not lead to any detection, leading them to conclude that the emission from the source was too weak at these wavelengths. The radio wavelength emission measured with the Karl G. Jansky Very Large Array (VLA) about two days after the Fermi detection corroborates this conclusion.

In astronomy, even such failures are of use, as they help astronomers understand the properties of the source. The researchers observed a significant increase and subsequent decrease in the optical wavelength light emitted by the source. This optical “bump” is a smoking gun signature of a supernova. Using the radio, optical, X-ray, and gamma-ray wavelength data from the multiple telescopes, the astrophysicists studied how the source’s brightness changed across the wavelengths as time progressed. They concluded that a supernova accompanied the short GRB. Although astronomers have detected such short GRBs before, this one had the shortest duration in gamma ray wavelengths amongst them. They concluded that special conditions around the exploding star gave rise to the gamma ray photons detected by Fermi and AstroSat. The physical conditions of the medium surrounding the star ensured that the gamma ray emission was weaker compared to “long” GRBs, thus explaining why it was a short GRB instead.

Navin Sridhar, an astrophysics graduate student at Columbia University, New York, who was not involved in the study, agrees with the explanation. “The evidence of a supernova “bump” disfavours most of the models that involve only black holes, neutron stars, and their mergers,” he says. The supernova “bump” could not have been created by a late explosion of energy by the neutron star or black hole resulting from two neutron stars merging, explains Pawan Kumar, professor at the University of Texas at Austin, also not involved in the study. “That explanation is problematic because of the tremendous amount of energy that such an outburst would then need to emit to produce the bump,” he says. Is it then possible that the gamma ray emission lasted much longer, but since it was weaker than other long GRBs, the telescopes could observe them for less than a second?

“Some of the observed parameters of the burst clearly disfavour that interpretation,” shares Navin.

Theoretical predictions

The astrophysicists involved in the present study carried out theoretical calculations based on the observed data to suggest that most supernovae fail to produce GRBs. A. R. Rao, visiting professor at the Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune, disagrees. “The conclusion is highly dependent on the physical model of electromagnetic emission of photons by the source,” he says. Particles like electrons moving towards the observer at speeds close to that of light emit the gamma ray photons eventually detected by the gamma ray telescopes. Because of this motion, the emission is ‘beamed’ towards the observer, like a lighthouse beacon. The authors of the present study did take this effect into account in their theoretical calculations that led them to the inference.

“However, the correction required could be more. Until more robust estimates are made, I would hesitate to draw such a conclusion,” he adds. Rao was not involved in the study.

Dipankar Bhattacharya, professor at IUCAA, who was also not involved in the study, agrees with Rao. “That most supernovae may not produce GRBs is not a new idea. The [calculations based on the] present observation provides peripheral support to it rather than being a strong independent inference,” says Dipankar.

More clarity can emerge from future observations of short GRBs accompanied by supernovae. However, executing such observations is difficult. “It requires the optical and infrared wavelength emissions from the GRB to be measured extensively, even when these emissions are weak, thus requiring very sensitive telescopes,” explains Rao. “The present discovery might encourage observers to look at short GRBs a bit more carefully,” he shares. Dipankar, however, is not entirely optimistic. “Observing such cases will require long and deep observations, after the GRB is detected, with large optical and infrared telescopes. Even in the future, I suspect only a small fraction will receive this kind of attention,” he cautions.

Contrarily, there are cases of opposite behaviour in GRBs that have intrigued astronomers. “Astronomers followed up GRB060614, classified as a traditional long GRB, in great detail with optical telescopes. But they found no supernova!” shares Dipankar. Gamma Ray Bursts keep astronomers on their toes and sometimes scratching their heads.