IUCAA - Khagol # 129 - Oct 2022 - English

| KHAG L | No. 129 - OCTOBER 2022 | 10 magnetic flip is not uncommon in astrophysical objects. The Sun's magnetic field reverses polarity (north pole becomes south and vice versa) about every eleven years. On the longer timescales, even the Earth's magnetic field flips. The following scenario is based on the observations from NASA's Swift, Europe's XMM satellite, and g r ound-based op t i ca l and r ad i o telescopes. The visible and UV brightening happens when the flow of matter into the black hole increases. Thismay have started when the magnetic field in the outer parts of the disk began to flip. The accreted matter of reverse polarity reaches the corona and weakens it. The weakened magnetic field can no longer support the corona, which vanishes. When the flipped magnetic field gains strength, restoring the X-ray corona, but the inward flow of matter is still high, the X-ray emission is stronger than it was initially. Finally, the corona and the disk returned to their states before the flaring event, now with a flipped magnetic field. So far, a few dozen Changing-look AGNs have been detected. They have shown rapid changes in UV and visible light like this one, but this is the first time X-rays have been seen to drop out as the other wavelength brightens. These surprising events offer a tantalizing glimpse at the extreme forces at work near an actively feeding supermassive black hole. To know more! Link to paper: https://ui.adsabs.harvard.edu/abs/2022ApJ...931....5L/abstract Link to NASA video: https://www.youtube.com/watch?v=cHmXuo39qz4 type-1 AGNs, tracking the evolutionary scenario of quasars and hence the SMBH host galaxy interaction and the effect of the AGN on the kiloparsec-scale molecular outflows. He is currently a Postdoctoral Research Associate at NASA Goddard Space Flight Center Research: Astronomers have a growing consensus that each large galaxy harbours a supermassive black hole at its centre. Some of these galaxies are different as their central compact regions are much brighter compared to normal galaxies such as our Milky way galaxy. The compact r eg i on o f t he s e mo r e l umi nou s counterparts is called the active galactic nucleus (AGN), and the galaxy is called the active galaxy. AGNs have been traditionally classified into subclasses depending on different spectral properties to study them elaborately. The optical spectra of an AGN contain both narrow and broad emission lines. AGNs that are detected with broad emission lines in the optical spectra are classified as Type 1 and those with narrow emission lines as Type 2. According to the orientation-based unification scheme (Antonucci, 1993), Type-1 and Type-2 AGNs are the same objects, only viewed froma different angle. Recently discovered Changing-look AGNs challenge this idea as they tend to show change from a Type 1 to Type 2 state and vice versa. A group of astronomers have been analysing 1ES 1927+654 which is an AGN situated 236 million light-years away and was previously classified as a Type II AGN since there had been no detection of broad emission lines, nor was there any line-of- sight obscuration by the dust. At the end of 2017, this galaxy began a rare and dramatic transformation. The optical and UV luminosity or flux was found to increase by almost a factor of 100 within 150 days. As the visible and UV brightened, the X-ray luminosity or flux started to decrease and, in about approximately 200 days, reached a minimum of 1/100th time the initial value. The higher energy X-rays completely vanished. The X-ray flux soon recovered to a flux level of approximately 10 times that of the preflare flux in another 100 days, i.e., by April 2019. Previously astronomers described it as a tidal disruption event which occurs when a star wanders extremely close to a supermassive black hole that it is torn apart. But the disappearance of the higher energy X-rays that originate from a cloud of super-hot particles near the black hole could not be explained. This is called the corona and is formed by a strong magnetic field. With the newstudy of observations done by this international collaboration of astronomers spanning the entire event, a different cause has been suggested. The study argues that the event was triggered due to a flip in the magnetic field in the disk of material around the black hole. This Sukanta Bose , on being elected as Fellow, Indian Academy of Sciences (2021) Congratulations !