Quasars, point-like glimmers of host galaxy are originally dubbed as “Quasi Stellar Radio Sources” due to their immense brightness in radio light. These luminous star-like objects are found in young galaxies that hosts an active supermassive black hole at their centre.
So, our home galaxy, Milky Way once hosted a Quasar! Since, the supermassive black hole of our galaxy, Sagittarius A is now silent, we do not own any Quasar right now.
Active supermassives are like fuel holders of Quasars. They are like wild hungers of normal matter in the universe. The material accreted around a black hole is accelerated at the velocities approaching speed of light that suck up to create these energetic objects.
Some of the particles are accelerated away from the black hole in the form of jets. The jets of Quasars are well recognized as the most powerful particle accelerators in the universe till now. Quasars are a part of active galactic nuclei (AGN) class objects.
AGN is a highly luminous region at the centre of galaxy, not lit by star light. The AGN have three classes of objects: Blazars, Seyfert Galaxies & our well-known Quasars. Blazars are compact Quasars that give out tremendous amount of energy while Seyfert galaxies release much lesser energy than Quasars, may be too young to hold such a large amount of energy.
All the three classes of objects release energy in the form of jets but so far no jets have been detected from Seyfert galaxies, possibly emitting jets away from us. These Galaxies are the lowest energy AGN, giving out just 100 keV of energy.
While Quasars emit millions, billions or even trillions of energy in eVs that exceed the energy of all the stars in a galaxy, shining 10-10000 times more than Milky Way.
Although Quasars can eat 1000-2000 solar masses per year, it has lifetime of just 100-1000 million years. Once they exhaust their fuel supply, it will turn ‘turn-off’ like black hole & may be never shine again in that galaxy. Jets released from these flickers are pointed directly towards Earth.
These jets interact with gas, emitting radio waves that appear as huge radio lobes in radio light. A hunt for these radio sources began in 1950s due to a static interference in transatlantic phone lines. Physicist Karl Jansky working at Bell Telephone Laboratories discovered that the interference is coming from outer space.
Also, afterwards National Astronomical Observatory of Japan found that only 10% of Quasars shine brightly in radio light & today we know Quasars emit light of all wavelengths except microwaves.
Earlier this year, American Astronomical Society (AAS) discovered the most distant Quasar J0313-1806 which was formed just after 670 million years of Big Bang & we have discovered more than a million of these class objects till now.
All the Quasars are similar possessing a strange characteristic of varying luminosity in short periods of time, popularly known as ‘changing-look’ Quasars. But we didn’t expected this from Quasars!
Although the thick dusty donuts of matter obscures much of Quasars’ emission, the level of obscuration should not change on human timescales because of their huge structures.
However, the brightness of ‘changing-look’ Quasars vary within a human year. For some Quasars, luminosity varies in a few light days concluding that their size cannot be more than a few light days.
Scientists are creating a comprehensive list of these Quasars to understand their strange behaviour of varying brightness. Normally, we look for objects that are bluer than stars with varying luminosity & high X-ray level, usually emitted by black hole systems.
Accordingly, scientists had developed a technique called photometric variability that relays on the variability of Quasars in a wide range of wavelengths. The technique holds good for highly luminous Quasars but failed for too faint Quasars.
Also, it was hard to recognize whether we are looking at black holes or Quasars. Probably, due to unknown masses of supermassive black holes, Quasars became hard to study!
The issue led astronomers to come up with a new technique of spectroscopy to detect faint Quasars. The researchers from University of Bath found four ‘changing-look’ Quasars using the spectroscopic data & two of them were too faint to detect using previous techniques.
A Former MPhys student at Bath, Bart Potts, author of the research paper says “We took a previous dataset and applied our new method to see if we could identify any new changing quasars that others had missed. This gave us a bigger set of changing-look quasars for further study, and validated that our methodology was more sensitive than others, which was great. It shows that our methodology is more sensitive to weaker luminosity.”
New techniques help scientists to assess very small changes in wavelength of the faint Quasars & enable them to find Quasars undergoing extreme changes in luminosity. A deeper understanding of Quasars & their supermassive black holes will tell us a lot about the evolution & growth of galaxies that ultimately help us to decide the possible fate of the galaxy.