Monday, January 17, 2022

From the Lab: 500 bn times more energy than Sun, here comes the blazar

What is important in a study like this is that we should be able to ‘observe’ an event not just through the visible light, but also through ultraviolet light, X-rays, and gamma rays.

Written by Amitabh Sinha |
July 12, 2015 1:28:17 am
Blazars, gamma-ray emission, elliptical galaxy, Compton Gamma-ray Observatory, X ray, knowledge A NASA image of another blazar, Cygnus, emitting extremely bright radiation.

C S Stalin and his team,
Indian Institute of Astrophysics, Bangalore

For the last three years, we have been trying to discover the location of  the gamma-ray emission site in “blazars”. They are a class of extremely bright objects in the sky now believed to be associated with super-massive black holes at the centre of an active, giant elliptical galaxy. Through our research, we have been trying to learn about the physical processes that lead to the emission of radiation at different wavelengths in blazars.

It was during this study that we noticed an object, deep in space, that had suddenly become very bright last year. It started releasing energy 500 billion times more than that emitted by the Sun. The existing literature described the source of these emissions as a “candidate gamma-ray emitter”. The explanation was based on the observations by the Compton Gamma-ray Observatory that was in space during the period 1991-2000. From here we began an in-depth study into the source of these new bright emissions.

What is important in a study like this is that we should be able to ‘observe’ an event not just through the visible light, but also through ultraviolet light, X-rays, and gamma rays. These observations also need to be simultaneous/near simultaneous (as the source shows brightness variations with time), so that we do not get a distorted picture. Such multi-wavelength observations are now possible owing to the availability of several ground-based and space-based telescopes. The observations in visible light can be made through telescopes on Earth.

However, to capture the information coming through ultra-violet, X-rays and gamma rays, we need to have telescopes in space as Earth’s atmosphere is opaque to such radiations. So, we sent a request for observations using the Swift space telescope (a NASA mission with international participation). Through our collaborators we were also able to get data from another space telescope, NuSTAR, that was launched in 2012. We then analysed data from the Fermi gamma-ray space telescope. Other than this, we also used data from two ground-based optical telescopes — Himalayan Chandra Telescope at Hanle, Ladakh, and Telescopio Nazionale Galileo at La Palma, Spain.

After thoroughly analysing all this data, we concluded that the object that flared about a year ago was a blazar powered by a black hole 250 million times bigger than the Sun. In fact blazars emit radiation over a wide range of wavelengths — from low energy radio waves to high energy gamma-rays.

Not many blazars are known and there is little understanding about such sources. We are yet to figure out the exact physical mechanisms that cause a sudden change in their brightness. Further observation and analysis will help unravel the same.

If you wish your research to be considered for this column, please write to Senior Editor Amitabh Sinha, who curates this column, at

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