Kavli Affiliate: Erin Kara
| First 5 Authors: Ruancun Li, Claudio Ricci, Luis C. Ho, Benny Trakhtenbrot, Erin Kara
| Summary:
Time-domain studies of active galactic nuclei (AGNs) offer a powerful tool
for understanding black hole accretion physics. Prior to the optical outburst
on 23 December 2017, 1ES 1927+654 was classified as a "true" type~2 AGN, an
unobscured source intrinsically devoid of broad-line emission in polarized
spectra. Through our three-year monitoring campaign spanning X-ray to
ultraviolet/optical wavelengths, we analyze the post-outburst evolution of the
spectral energy distribution (SED) of 1ES 1927+654. Examination of the
intrinsic SED and subsequent modeling using different models reveal that the
post-outburst spectrum is best described by a combination of a disk, blackbody,
and corona components. We detect systematic SED variability and identify four
distinct stages in the evolution of these components. During the event the
accretion rate is typically above the Eddington limit. The correlation between
ultraviolet luminosity and optical to X-ray slope ($alpha_mathrm{OX}$)
resembles that seen in previous studies of type 1 AGNs, yet exhibits two
distinct branches with opposite slopes. The optical bolometric correction
factor ($kappa_{5100}$) is $sim 10$ times higher than typical AGNs, again
displaying two distinct branches. Correlations among the corona optical depth,
disk surface density, and $alpha_mathrm{OX}$ provide compelling evidence of a
disk-corona connection. The X-ray corona showcases systematic variation in the
compactness-temperature plot. Between 200 and 650 days, the corona is
"hotter-when-brighter", whereas after 650 days, it becomes
"cooler-when-brighter". This bimodal behavior, in conjunction with the
bifurcated branches of $alpha_mathrm{OX}$ and $kappa_{5100}$, offers strong
evidence of a transition from a slim disk to thin disk $sim 650$ days after
the outburst.
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