ÌÇÐÄÊÓƵ

By analyzing the available data from various space observatories and ground-based telescopes, Indian astronomers have conducted a long-term multiwavelength study of a changing-look active galactic nucleus (AGN) known as NGC 3822. Results of the new study, October 7 on the arXiv pre-print server, provide more insights into the behavior of this AGN.

AGNs are among the most luminous and energetic sources in the universe. They are compact regions at the center of a galaxy, more luminous than the surrounding galaxy light, and very energetic due either to the presence of a black hole or star formation activity at the core of the galaxy.

Some AGNs show dramatic optical and X-ray spectral variabilities on timescales ranging from months to decades. They are known as changing-look AGNs (CL-AGNs), and many questions regarding their origin and nature still remain unanswered.

Located some 276 million light years away, NGC 3822 is a Seyfert galaxy hosting an AGN. It was discovered in 1784 and its supermassive black hole mass is estimated to be about 27 million solar masses. The changing-look behavior of the AGN in NGC 3822 was detected in 2022.

In order to better understand the behavior of AGN NGC 3822, a team of astronomers led by Narendranath Layek of the ÌÇÐÄÊÓƵical Research Laboratory in Ahmedabad, India, carried out a comprehensive study of this nucleus, based on the X-ray, ultraviolet, and optical data collected over a period of 17 years—since 2008.

"The dataset includes observations from Swift, XMM-Newton, and NuSTAR, the Very Large Telescope, and the Himalayan Chandra Telescope," the researchers wrote in the paper.

The observations identified flux variations across X-ray to optical/ultraviolet bands, with an increased variability amplitude at shorter wavelengths. The variability amplitude in the X-ray band was found to be approximately 63%, which gradually decreases in the ultraviolet bands: about 40% in W2, 39% in M2 and 33% in W1, followed by a further drop to 29% in the optical U band.

The X-ray continuum luminosity (in the 2–10 keV band) of AGN NGC 3822 was found to vary between 1.3 and 14 tredecillion erg/s and the corresponding Eddington ratio was found to change from about 0.0008 to 0.009. Therefore, the source remained in a sub-Eddington regime during this observation period.

X-ray spectral analysis conducted by the researchers points to the presence of intrinsic absorption of NGC 3822, which is attributed to clouds moving in and out of the line of sight. The astronomers noted that the absorber was clearly detected during the 2016 and 2022 observations, while it disappeared before and after these epochs.

The observations confirm changing-look behavior in NGC 3822, characterized by the appearance and disappearance of broad emission lines (BELs). The authors of the paper found that the changing-look transitions in this AGN are found to be driven by the change in the accretion rate.

© 2025 Science X Network