16-Dec-2022: Mysterious Circles of Radio Emission Detected in Space May Come From Supernova Explosions or Massive Black Holes

A new research offers plausible explanations for mysterious hazy circles of radio emission deep in celestial space called Odd Radio Circles (ORCs) detected recently using some of the most sensitive international radio telescopes.

Astronomers recently identified these ORCs using the Square Kilometer Array (SKA) in Australia & South Africa, the Giant Metrewave Radio Telescope (GMRT) in India, and the Low-Frequency Array (LOFAR) in the Netherlands. Such objects are seen only in radio and not in any other form of radiation. Some of these objects could be 1 million light-years across, about 10 times larger than our Milky Way. The ORCs are considered mysterious, as these objects could not be explained with any previously known astrophysical phenomena.

Dr. Amitesh Omar, Scientist at Aryabhatta Research Institute of observational sciences (ARIES), Nainital (an autonomous institute of DST, Govt. of India) in his research has proved that some of these ORCs could be remnants of thermonuclear supernovas triggered by the explosion of a white dwarf star in a binary system heavier than 1.4 times the mass of the Sun.

According to the research published in the Letters section of the U.K.’s Royal Astronomical Society Journal, these ORCs reside outside the Milky Way, lurking in the vast intergalactic space between its neighboring galaxies. The intergalactic supernova events, taking place outside galaxies, were already known from earlier optical surveys. Their remnants become bright in radio several thousands of years after the explosion and could be detected anywhere in the intergalactic space with the right sensitivity of the radio observations. As the sensitivity of the modern radio telescope arrays has increased many-fold, astronomers are now able to detect these objects. This explanation fits well for those ORCs which do not have any known optical objects at their centers.

However, some ORCs are most likely associated with distant galaxies as their centers have a known optical galaxy (galaxy that can be studied with the help of optical telescope), and hence these ORCs cannot be considered intergalactic supernovae. In order to explain such ORCs, Dr. Omar invoked a widely known mechanism of disruption of a star by extreme tidal forces exerted by a massive black hole as the star comes in close proximity of the central massive black hole in a galaxy. In this process, the star is destroyed, and about half its mass is thrown away from the black hole at very high speeds. This disruption process releases a huge amount of energy, similar to that produced in a supernova explosion. An occasional merger of two galaxies can cause millions of stars to get tidally disrupted by a black hole in a few million years, a cosmologically short period of time. The sudden release of huge energy creates shocks, which can traverse to around a million light-years in intergalactic space. These shocks energize omnipresent cosmic free electrons to the extent that the synchrotron radio emission is produced in the weakly magnetized intergalactic space. This explanation fits within the framework of the known astrophysical phenomena.

It is expected that with the completion of the SKA, the world’s most powerful radio telescope with international co-operation, includes participation by scientists and engineers working in various institutes under India’s Ministry of Science & Technology, many more of these objects will be detected in the future.

7-Dec-2022: First data taken by the 3.6-meter telescope detects unexpected kilonova emission from ‘a long-duration gamma-ray burst’

While tracing a burst of high-energy light detected on December 11, 2021, from the outskirts of the Milky Way located approximately 1 billion light-years away, astronomers have spotted the first astronomical event in which a long GRB has been accompanied by the unexpected discovery of a kilonova emission. Generally, kilonova are visible and infrared light associated with short-period gamma-ray bursts (GRBs) thought to be heat produced by the radioactive decay of heavier elements.

Photometric observations taken with the 3.6 m Devasthal Optical Telescope have provided vital information on the earliest phase of a kilonova ever detected, radically changing the understanding of scientists about the origin of GRBs.

GRBs are powerful astronomical cosmic bursts of high-energy gamma-ray. GRB emits more energy in a few seconds than our Sun will emit in its lifetime and has two distinct emission phases: the short-lived prompt emission (the initial burst phase that emits gamma-rays), followed by a long-lived multi-wavelength afterglow phase. The prompt emission (initial gamma-ray emission) of GRBs are automatically discovered by space-based gamma-ray missions such as NASA’s Fermi Gamma-ray Space Telescope, Neil Gehrels Swift Observatory, and India’s AstroSat. In recent years, scientists have discovered a special phenomenon called a kilonova of visible and infrared light with short-period GRBs, also known as a potential source of gravitational waves. It has been hypnotized that the heat produced by the radioactive decay of heavier elements may emit kilonova. This process also produces heavier elements, such as gold and platinum. However, observing kilonovas at near-infrared wavelengths is technically challenging, and only a few telescopes on Earth, including the 3.6-meter Devasthal Optical Telescope of the Aryabhatta Research Institute of Observational Sciences (ARIES), can detect kilonova and gravitational wave objects at these wavelengths upto faint limits.

The scientists from the ARIES, an autonomous institute of DST, used data from the  3.6 m Devasthal Optical Telescope of the Aryabhatta Research Institute of Observational Sciences (ARIES) along with other telescopes, including HST in studying the aftermath of the long GRB (GRB 211211A), detected by the NASA's Neil Gehrels Swift Observatory and the Fermi Gamma-ray Space Telescope on December 11, 2021. The high-energy outburst lasted about a minute, and follow-up observations taken from the 3.6-meter Devasthal Optical Telescope identified a kilonova.

The spectral energy distribution of GRB afterglow is usually explained in terms of non-thermal emission (due to synchrotron radiation). However, in this event, both thermal and non-thermal emissions were included in the spectral energy distribution of the afterglow, modeled using the magnificent and dim observations of the 3.6 m Devasthal Optical Telescope. After subtracting the afterglow contribution from the collected data taken using the 3.6 m telescope and 4Kx4K CCD IMAGER, the scientists, which include PI of the backend instrument Dr. Shashi Bhushan Pandey along with research students Rahul Gupta, Amar Aryan, Amit Kumar, and Dr. Kuntal Mishra found that the multiwavelength data could be well explained by additional thermal spectra and that this thermal emission could be explained in terms of kilonova emission.

"Several years ago, Neil Gehrels, an astrophysicist and namesake of Swift suggested that some long-duration GRBs may be produced by merging neutron stars. By GRB standards, this event was relatively nearby, allowing space and ground-based telescopes to capture the dim light of the kilonova. Kilonovae may also arise from more distant long GRBs, but we have not yet been able to see them through observations,” said Eleonora Troja, an astrophysicist at the University of Rome who led the team on the study.

 Dr. Shashi Bhushan Pandey and the team of Indian scientists involved in this work said that this discovery challenges our current understanding about the origin of GRBs and gives rise to new possibilities in this area of research. Professor Dipankar Banerjee, Director, ARIES, pointed out that future time-domain astronomy has a unique potential to make a lot of such discoveries using the 3.6 m Devasthal optical telescope.

In addition to the first data taken by the 3.6-meter telescope, this scientific discovery, published in the journal Nature, also used Hubble Space Telescope, Multicolor Imaging Telescopes for Survey and Monstrous Explosions, Color Alto Observatory, Devasthal Fast Optical Telescope, and many other spaces and ground-based telescopes. It will help in understanding the process of formation of heavy elements in the universe.

30-Jun-2021: Rebel behaviour of highest energy afterglow of a Gamma-Ray Burst detected in space may help probe stellar evolution

The highest energy afterglow detected in space so far seems to be a rebel. The emission from the most notable Gamma Ray Burst (GRB) explosion so far traced -- the afterglow from a galaxy 4.5 billion light years away was found to be complex in nature and did not follow the evolution expected in standard afterglow models. The detection of high energy photons (TeV Photons) from this GRB provides new insights and important clues to unravel the underlying physical processes at work which result in such explosions.

The GRB with ultra-high energy photons called GRB 190114C was detected for the first time on 14-January-2019. This discovery was reported in Nature by the Major Atmospheric Gamma Imaging Cherenkov Telescopes (MAGIC) collaboration.

As usual, the GRB lasted for a brief period, followed by an initial bright flash in high energies known as the ‘prompt emission’. A less luminous but long-lasting counterpart known as the ‘afterglow’ was detected after the prompt emission and offered scientists the chance to probe the GRBs.

Dr. Kuntal Misra from Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital, an autonomous institute of the Department of Science & Technology, along with significant contributions from national and international collaborators, carried out observations of the afterglow from GRB 190114C spanning over nearly 140 days after the burst. The paper has been published in Monthly Notices of the Royal Astronomical Society (MNRAS).

The optical observations of the afterglow from GRB 190114C were carried out with the Growth India Telescope (GIT), Himalayan Chandra Telescope (HCT) (both located in Hanle, Leh, India), and Devasthal Fast Optical Telescope (DFOT, located in Devasthal, Nainital, India) as well as with upgraded Giant Meter wave Radio Telescope (u-GMRT, located in Khodad, Pune, India), Australia Telescope Compact Array (ATCA, located in New South Wales, Australia) and the Atacama Large Millimeter Array (ALMA, located in Atacama Desert, Chile).

Detailed modelling of the afterglow using multi-band data indicates that the parameters describing the fraction of energy in electron population and magnetic field are evolving with time and not constant as generally seen in GRBs. The scientists suggested that the evolution of these parameters, at early times, may play a role in producing the bright TeV emission.