AGN STORM discovers a mirror in an AGN

Date: 
09/12/2019 - 4:00pm to 5:00pm
Location: 
BL 339
Speaker(s) / Presenter(s): 
Maryam Dehghanian, University of Kentucky

The AGN STORM (space telescope optical reverberation mapping) was an intense multi-wavelength campaign that observed NGC 5548, a type I Seyfert galaxy (z = 0.01717), for 180 months during 2014. The main goal was to determine the geometry and mass of the object through the reverberation mapping method, using 6 space-based and 21 ground-based telescopes.

NGC 5548 exhibited unusual behavior during the campaign: First, a strong soft X-ray absorption was present due to the appearance of a line of sight (LOS) obscurer between the central source and the observer. Then, it was discovered that broad emission lines and the UV continuum (emission-line holiday) decorrelated for 2 months during the observations. Finally, it was discovered that the same decorrelation happened between the narrow absorption lines and the continuum (absorption-line holiday).

We joined the campaign in 2017 to understand the behavior of NGC 5548 during the time of obscuration and holiday. We showed that LOS obscurer is the upper part of a symmetric continues disk-wind launched from the accretion disk. The base of this wind is called the equatorial obscurer and it persistently shields the BLR.  Based on Cloudy’s prediction, the variations of the covering factor of the LOS obscurer explains the absorption-line holiday, while changes in the column density/density of the equatorial obscurer explain the emission-line holiday.

Recently, we found out that the base of the wind has significant optical depth to electron scattering and must be a contributor to the Compton reflector. This reflector acts as a mirror and would reflect BLR emission. The wind’s optical depth explains why the far sides of the BLR are unexpectedly faint. During this talk, I will explain how our modeling resulted in this discovery, and also how the disk-wind model scenario fits into observations. 
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