City astronomers discover rare radio sources

Manasi Saraf Joshi
Tuesday, 12 March 2019

Surjit Paul said, “We have found that this slope can be larger. In fact as per our findings it is 3. This research will throw light on the understanding of the evolution of galaxies, how they were formed and the time taken,”

PUNE: Breaking the existing understanding that a universal mechanism can explain the emission of radio waves in all radio sources, a team of researchers largely from National 

Centre for Radio Astrophysics (NCRA-TIFR), radio astronomers from the city, announced the discovery of a new, rare class of radio sources using the Giant Metrewave Radio Telescopes (GMRT).

This has been named ‘EISERS’, which stands for ‘Extremely Inverted Spectrum Extragalactic Radio Sources’. 

“This discovery has the potential to challenge the current understanding that a universal mechanism can explain the emission of radio waves in all radio sources. This research has been published in the international journal, Monthly Notices of the Royal Astronomical Society (MNRAS), London,” said the press statement from GMRT.

As per the existing understanding, the Standard Synchrotron logarithmic slope of the declining radio spectrum cannot be larger than +2.5, but the team has found that it can be more than that. One of the team members, Surjit Paul said, “We have found that this slope can be larger. In fact as per our findings it is 3. This research will throw light on the understanding of the evolution of galaxies, how they were formed and the time taken,” he said. 

A few years ago, a team led by Prof Gopal Krishna initiated the first systematic search for radio galaxies having a radio spectrum with a slope greater than +2.5. The team included SK Sirothia, M Mhaskey, P Ranadive, PJ Wiita, A Goyal, NG Kantharia and CH Ishwara-Chandra. They christened such rare radio galaxies as ‘EISERS’ and isolated 7 radio sources as potential EISERS. 

He said, “It was a fabulous feeling when we discovered it. Breaking the existing thing and discovering something new has its own happiness.”

He added, “The powerful radio emission is produced due to free electrons moving at speeds close to the speed of light in the presence of magnetic fields. This mechanism is called ‘Synchrotron Mechanism’ and the electrons producing the synchrotron radio waves, occur over a wide range of energies, such that their distribution across the energy range is believed to follow a specific shape which is a ‘power-law’ in the logarithmic space. The power-law distribution of electron energies has in fact been confirmed for numerous radio galaxies observed by radio astronomers since the 1960s. This has led to the belief that the power-law distribution of electrons is universal.”

“The question remains whether the power-law energy distribution is indeed universal, or non-power-law energy distributions arise under certain circumstances? The test requires accurate measurement of the radio spectra of the compact radio sources mainly at low radio frequencies. A key prediction from the standard synchrotron theory is that the logarithmic slope of the declining radio spectrum cannot be larger than +2.5, so long as the energy distribution of the radiating charged particles has a power-law shape. An even steeper spectral slope, if ever found, would signal a deviation from a power-law energy distribution. And the discovery of such galaxies would put a serious question mark on the popular hypothesis of the existence of a universal power-law mechanism,” he added.

THE FINDINGS
- These findings have been published in the same journal (MNRAS) in February 2019, by a team consisting of Mukul Mhaskey (SP Pune University), Gopal Krishna (ARIES, Nainital), Surajit Paul (SP Pune University), Pratik Dabhade (IUCAA), Sameer Salunkhe (SP Pune University) and Sandeep Sirothia (SKA-South Africa).
- This discovery re-opens the key question of whether the highly energetic electrons responsible for radio-wave emission in radio galaxies are accelerated by a single universal mechanism.

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