The mystery of AGN NGC 5548

Date: 
04/04/2019 - 3:30pm
Location: 
Blazer 339
Speaker(s) / Presenter(s): 
Maryam Dehghanian

AGNs are the brightest persistent source of the electromagnetic radiation in the universe, enabling us to discover and study them across the cosmos. These are in the central regions of galaxies, and observing and tracing them informs us of the role of supermassive black holes (SBH) in the formation and evolution of galaxies. Fundamental properties such as the mass of the SBH and correlations with luminosity are based upon line-continuum reverberation mapping. Time delays between changes in the continuum luminosity and the response of the emission lines measure the physical size. The method is the basis for understanding phenomena near the SBH, which are far too small to be resolved even with the best telescopes.

During this talk, I will discuss how this correlation was broken in one of the well-studied AGNs, namely AGN NGC 5548.  In this object, the soft X-ray part of the SED was dramatically extinguished by an obscurer. During part of the time that this obscurer was present, the absorption and emission lines did not respond to variations of the continuum. We modeled the decorrelation of the absorption lines from the continuum in terms of a varying obscurer covering factor, and identify the physics which makes this possible. We identify a cycle in which the soft X-ray portion of the SED varies, causing changes in the ionization of helium.  The ionizing radiation produced in its recombination governs the ionization of the species observed with HST.  Photoionization models reproduce the sense of HST observations.  The obscurer is likely to be part of the broad-line region which happens to cover our sight line to the central object.  This shows the importance of cloud shadowing in understanding the physics of the emission-line clouds. 

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