54 research outputs found
A new physical interpretation of optical and infrared variability in quasars
Changing-look quasars are a recently identified class of active galaxies in
which the strong UV continuum and/or broad optical hydrogen emission lines
associated with unobscured quasars either appear or disappear on timescales of
months to years. The physical processes responsible for this behaviour are
still debated, but changes in the black hole accretion rate or accretion disk
structure appear more likely than changes in obscuration. Here we report on
four epochs of spectroscopy of SDSS J110057.70-005304.5, a quasar at a redshift
of whose UV continuum and broad hydrogen emission lines have faded,
and then returned over the past 20 years. The change in this quasar
was initially identified in the infrared, and an archival spectrum from 2010
shows an intermediate phase of the transition during which the flux below
rest-frame 3400\AA\ has decreased by close to an order of magnitude.
This combination is unique compared to previously published examples of
changing-look quasars, and is best explained by dramatic changes in the
innermost regions of the accretion disk. The optical continuum has been rising
since mid-2016, leading to a prediction of a rise in hydrogen emission line
flux in the next year. Increases in the infrared flux are beginning to follow,
delayed by a 3 year observed timescale. If our model is confirmed, the
physics of changing-look quasars are governed by processes at the innermost
stable circular orbit (ISCO) around the black hole, and the structure of the
innermost disk. The easily identifiable and monitored changing-look quasars
would then provide a new probe and laboratory of the nuclear central engine.Comment: 13 pages, 4 figures, 3 tables. Published in MNRAS. All code and data
links on GitHub, https://github.com/d80b2t/WISE_L
The first high-redshift changing-look quasars
We report on three redshift quasars with dramatic changes in their C IV
emission lines, the first sample of changing-look quasars (CLQs) at high
redshift. This is also the first time the changing-look behaviour has been seen
in a high-ionisation emission line. SDSS J1205+3422, J1638+2827, and J2228+2201
show interesting behaviour in their observed optical light curves, and
subsequent spectroscopy shows significant changes in the C IV broad emission
line, with both line collapse and emergence being displayed on rest-frame
timescales of 240-1640 days. These are rapid changes, especially when
considering virial black hole mass estimates of
for all three quasars. Continuum and emission line measurements from the three
quasars show changes in the continuum-equivalent width plane with the CLQs seen
to be on the edge of the full population distribution, and showing indications
of an intrinsic Baldwin effect. We put these observations in context with
recent state-change models, and note that even in their observed low-state, the
C IV CLQs are generally above 5\% in Eddington luminosity.Comment: 12 pages, 7 figures, 4 tables. All data, analysis code and text are
fully available at: github.com/d80b2t/CIV_CLQs. Comments, questions and
suggestions welcome and encourage
The first high-redshift changing-look quasars
We report on three redshift z > 2 quasars with dramatic changes in their C IV emission lines, the first sample of changing-look quasars (CLQs) at high redshift. This is also the first time the changing-look behaviour has been seen in a high-ionization emission line. SDSS J1205+3422, J1638+2827, and J2228 + 2201 show interesting behaviour in their observed optical light curves, and subsequent spectroscopy shows significant changes in the C IV broad emission line, with both line collapse and emergence being displayed on rest-frame time-scales of ∼240–1640 d. These are rapid changes, especially when considering virial black hole mass estimates of M_(BH) > 10⁹M⊙ for all three quasars. Continuum and emission line measurements from the three quasars show changes in the continuum-equivalent width plane with the CLQs seen to be on the edge of the full population distribution, and showing indications of an intrinsic Baldwin effect. We put these observations in context with recent state-change models, and note that even in their observed low-state, the C IV CLQs are generally above ∼5 per cent in Eddington luminosity
Chandra Observations of Candidate Subparsec Binary Supermassive Black Holes
We present analysis of Chandra X-ray observations of seven quasars that were identified as candidate subparsec binary supermassive black hole (SMBH) systems in the Catalina Real-Time Transient Survey based on the apparent periodicity in their optical light curves. Simulations predict that close-separation accreting SMBH binaries will have different X-ray spectra than single accreting SMBHs, including harder or softer X-ray spectra, ripple-like profiles in the Fe K-α line, and distinct peaks in the spectrum due to the separation of the accretion disk into a circumbinary disk and mini disks around each SMBH. We obtained Chandra observations to test these models and assess whether these quasars could contain binary SMBHs. We instead find that the quasar spectra are all well fit by simple absorbed power-law models, with the rest-frame 2–10 keV photon indices, Γ, and the X-ray-to-optical power slopes, α_(OX), indistinguishable from those of the larger quasar population. This may indicate that these seven quasars are not truly subparsec binary SMBH systems, or it may simply reflect that our sample size was too small to robustly detect any differences. Alternatively, the X-ray spectral changes might only be evident at energies higher than probed by Chandra. Given the available models and current data, no firm conclusions are drawn. These observations will help motivate and direct further work on theoretical models of binary SMBH systems, such as modeling systems with thinner accretion disks and larger binary separations
Chandra Observations of Candidate Subparsec Binary Supermassive Black Holes
We present analysis of Chandra X-ray observations of seven quasars that were identified as candidate subparsec binary supermassive black hole (SMBH) systems in the Catalina Real-Time Transient Survey based on the apparent periodicity in their optical light curves. Simulations predict that close-separation accreting SMBH binaries will have different X-ray spectra than single accreting SMBHs, including harder or softer X-ray spectra, ripple-like profiles in the Fe K-α line, and distinct peaks in the spectrum due to the separation of the accretion disk into a circumbinary disk and mini disks around each SMBH. We obtained Chandra observations to test these models and assess whether these quasars could contain binary SMBHs. We instead find that the quasar spectra are all well fit by simple absorbed power-law models, with the rest-frame 2–10 keV photon indices, Γ, and the X-ray-to-optical power slopes, α_(OX), indistinguishable from those of the larger quasar population. This may indicate that these seven quasars are not truly subparsec binary SMBH systems, or it may simply reflect that our sample size was too small to robustly detect any differences. Alternatively, the X-ray spectral changes might only be evident at energies higher than probed by Chandra. Given the available models and current data, no firm conclusions are drawn. These observations will help motivate and direct further work on theoretical models of binary SMBH systems, such as modeling systems with thinner accretion disks and larger binary separations
Understanding extreme quasar optical variability with CRTS:II. Changing-state quasars
We present the results of a systematic search for quasars in the Catalina
Real-time Transient Survey exhibiting both strong photometric and spectroscopic
variability over a decadal baseline. We identify 73 sources with specific
patterns of optical and mid-IR photometric behavior and a defined spectroscopic
change. These "Changing-State" quasars (CSQs) form a higher luminosity sample
to complement existing sets of "Changing-Look" AGN and quasars in the
literature. The CSQs (by selection) exhibit larger photometric variability than
the CLQs. The spectroscopic variability is marginally stronger in the CSQs than
CLQs as defined by the change in H/[OIII] ratio. We find 36 sources with
declining H flux, 37 sources with increasing H flux and discover
seven sources with , further extending the redshift arm. Our CSQ
sample compares to the literature CLQ objects in similar distributions of
H flux ratios and differential Eddington ratios between high (bright)
and low (dim) states. Taken as a whole, we find that this population of extreme
varying quasars is associated with changes in the Eddington ratio and the
timescales imply cooling/heating fronts propagating through the disk.Comment: 43 pages, 22 figures, submitte
NuSTAR Observations of Candidate Subparsec Binary Supermassive Black Holes
© 2024 The Author(s). Published by the American Astronomical Society. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), https://creativecommons.org/licenses/by/4.0/We present an analysis of NuSTAR X-ray observations of three active galactic nuclei (AGN) that were identified as candidate subparsec binary supermassive black hole (SMBH) systems in the Catalina Real-Time Transient Survey based on apparent periodicity in their optical light curves. Simulations predict that close-separation accreting SMBH binaries will have different X-ray spectra than single accreting SMBHs. We previously observed these AGN with Chandra and found no differences between their low-energy X-ray properties and the larger AGN population. However, some models predict differences to be more prominent at energies higher than probed by Chandra. We find that even at the higher energies probed by NuSTAR, the spectra of these AGN are indistinguishable from the larger AGN population. This could rule out models predicting large differences in the X-ray spectra in the NuSTAR bands. Alternatively, it might mean that these three AGN are not binary SMBHs.Peer reviewe
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