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Accretion around compact objects

Chatterjee, Mondal, Palit, et al. 2025

Iron lines in X-ray spectroscopy

Fluorescent iron (Fe K-alpha), is among the most prominent emission lines observed in the X-ray energy band. Typically centered around 6.404 keV, it arises from the transition of an electron in iron atoms from the 2p to 1s energy level. The line is technically a doublet, with a weaker component at 6.398 keV, resulting from fine-structure splitting. Its prominence stems from the combination of a high fluorescence yield and the moderate cosmic abundance of iron (George & Fabian, 1991; Matt et al., 1991). Fe K-alpha lines are observed across a wide range of astrophysical systems, including:

1. X-ray Binaries (XRBs): Emitted from material near compact objects like black holes or neutron stars. 2. Active Galactic Nuclei (AGNs): Originates from reflection or ionized gas near the supermassive black hole. 3. Ultraluminous X-ray Sources (ULXs): Linked to accretion disks and wind structures. 4. Cataclysmic Variables (CVs): Seen in systems involving accreting white dwarfs. Supernova Remnants (SNRs): Produced by hot, shock-heated iron ejecta or ambient gas. 5. Galaxy Clusters Traces iron in the hot intracluster medium. The Fe K-alpha line serves as a vital probe into the physical properties and dynamics of these systems. It can reveal the ionization state, density, and composition of gas, while its relativistic broadening and skewness near black holes provide insights into black hole spin and accretion disk properties (Fabian et al., 2000). Additionally, the line helps map the distribution of matter and track the chemical evolution of the universe in environments like galaxy clusters and supernova remnants (Hitomi Collaboration, 2018). For detailed discussions, refer to works such as George & Fabian (1991), Fabian et al. (2000), and the Hitomi Collaboration (2018).

Origin of Soft-Excess

One of the enduring mysteries in accretion physics is the phenomenon of the soft X-ray excess. The primary emission from the X-ray corona is typically well-described by a power-law spectrum. However, at lower energies, observations reveal a notable surplus of emission—referred to as the soft excess.

Several interpretations have been proposed in the literature to explain this feature. These include extreme relativistic reflection, ionized wind absorption, and warm Comptonization. Among these, recent studies increasingly support the warm Comptonization scenario as a viable explanation.

For further insights, consider exploring works on the topic, such as Done et al. (2012), Petrucci et al. (2013, 2018, 2020), Mehdipour et al. 2011, and Palit et al. (2024). These studies provide a comprehensive understanding of the evolving perspectives on the soft X-ray excess.

Changing-Look AGNs

Changing-Look AGNs (CLAGNs) are a recently identified class of AGN that show dramatic luminosity changes over 5–10 years, far shorter than the million-year duty cycles predicted by standard accretion theory (Schawinski et al. 2015). Likely driven by rapid shifts in accretion rate onto the SMBH, these transitions can alter a galaxy’s Seyfert classification and reveal the need to consider accretion physics, not just orientation, in AGN unification (Antonucci 1993). Unlike Galactic black hole binaries (BHXBs), where accretion evolves on week-long timescales, most AGN vary slowly, making CLAGNs rare laboratories for observing accretion-flow evolution in real time. Their extreme variability—including luminosity swings and the appearance or disappearance of the soft X-ray excess (Done et al. 2016; Tripathi et al. 2021; Palit et al. 2025)—makes them strong analogues of BHXB state transitions (Arcodia et al. 2020).

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