Active galactic nuclei (AGN) are among the most energetic objects in the Universe. They are believed to play a critical role in the evolution of their hosts by providing the energy input required to explain the relation between black hole masses and the bulge velocity dispersion, known as the MBH-σ relation. Recently, a team of researchers led by Anna Trindade Falcao has reported the discovery of extended hard X-ray emission in the red and blue Fe Kα 6.4 keV bands associated with the CT AGN NGC 5728.
NGC 5728 is an active barred spiral galaxy at a distance of approximately 41 Mpc. This galaxy has a CT type 2 nucleus, presenting a radio jet along the prominent ionization bicone, and a star-forming ring in the circumnuclear region. The nucleus is associated with kpc-extended soft and hard X-ray emission.
The analysis involved 11 Chandra ACIS-S observations, with a cumulative exposure time of 260.8 ks. The data was processed and analyzed using CIAO 4.15, and all observations were astrometrically aligned to the longest observation. Spectral fitting was used to identify prominent emission lines. Sherpa was used to extract the spectrum of the nucleus for a circular region and two conical regions, namely NW and SE cones.
The results of the analysis showed that the extended hard emission is co-located with the extended soft X-ray emission, extended optical line emission, and radio jet. Spectral fitting results showed that models attempting to explain the blue wing as Fe XXV and Fe XXVI emission cannot reproduce the large observed EW. The observed [red, blue] wing could originate from [redshifted, blueshifted] Fe Kα 6.4 keV emission, with velocities of [∼19,000-42,000, ∼28,000] km s−1, if the emission is due to neutral Fe Kα.
The team’s physical models show that shocks of approximately 1000 km s−1 could be occurring in this region, resulting in the emission of hard X-rays. However, this still cannot fully explain the observed Fe Kα wings. The findings suggest that more detailed observations and simulations are required to understand the physical processes that give rise to the observed emission.
In conclusion, the discovery of extended hard X-ray emission in the red and blue Fe Kα 6.4 keV bands associated with the CT AGN NGC 5728 is an exciting development in our understanding of active galactic nuclei. Further research in this area is likely to shed light on the complex physical processes that drive these fascinating objects and could help to unlock new insights into the evolution of galaxies and the Universe as a whole.
Source: Falcão, A.T., Fabbiano, G., Elvis, M., Paggi, A., & Maksym, W.P. (2023). Discovery of Extended Fe Ka Complex X-ray Emission in NGC 5728. https://doi.org/10.48550/arXiv.2303.00782