[Paper Review] The bluest changing-look QSO SDSS J224113-012108

Xueguang Zhang|arXiv (Cornell University)|Jul 20, 2021

Galaxies: Formation, Evolution, Phenomena1 references5 citations

TL;DR

This study identifies SDSS J224113-012108 as the bluest changing-look quasar (CLQSO) to date, with a spectral index αλ ≈ −5.21 in 2017 due to a flux ratio decline of broad Hα to Hβ from 7 to 2.7 between 2011 and 2017. The discrepancy between virial black hole mass (∼10⁸ M☉) and M-sigma relation mass (∼3×10⁶ M☉) suggests unique accretion dynamics, while photometric variability and lack of dust obscuration favor a tidal disruption event (TDE) as the likely cause of the transition.

ABSTRACT

In this manuscript, we report a new changing-look QSO (CLQSO) SDSS J2241 at $z=0.059$. Based on the multi-epoch SDSS spectra from 2011 to 2017, the flux ratio of broad H$α$ to broad H$β$ has been changed from 7\ in 2011 to 2.7\ in 2017, leading SDSS J2241 with spectral index $α_λ\sim-5.21\pm0.02$ ($λ< 4000$Å) in 2017 to be so-far the bluest CLQSO. Based on the SDSS spectrum in 2011, the host galaxy contributions with stellar velocity dispersion $\sim86{ m km/s}$ can be well determined, leading to the M-sigma relation expected central BH mass $\sim3 imes10^6{ m M_\odot}$. However, through properties of the broad H$α$, the virial BH mass is $\sim10^8{ m M_\odot}$, about two magnitudes larger than the mass through the M-sigma relation. The different BH masses through different methods indicate SDSS J2241 is one unique CLQSO. Meanwhile, the long-term photometric light curve shows interesting variability properties, not expected by DRW process commonly applied in AGN but probably connected to a central TDE. Furthermore, based on continuum emission properties in 2017 with no dust obscurations, only considering the moving dust clouds cannot be preferred to explain the CLQSO SDSS J2241, because the expected intrinsic reddening corrected continuum emissions were unreasonably higher than the unobscured continuum emissions in 2017.

Motivation & Objective

• To identify and characterize a new changing-look quasar (CLQSO) with extreme spectral variability.
• To determine the intrinsic continuum properties and spectral energy distribution (SED) of SDSS J2241 in different epochs, particularly in the ultraviolet and optical bands.
• To assess the physical mechanisms behind the type transition in CLQSOs, especially distinguishing between dust obscuration and accretion rate variations.
• To evaluate the role of tidal disruption events (TDEs) in driving the observed spectral changes, particularly given the lack of dust obscuration and unusual photometric variability.
• To reconcile discrepancies between virial black hole mass estimates and M-sigma relation predictions in this unique AGN system.

Proposed method

• Multi-epoch SDSS spectroscopy (2011, 2016, 2017) was used to track changes in broad emission line fluxes, particularly Hα and Hβ.
• The stellar population synthesis (SPS) method was applied to the 2011 spectrum to model and subtract the host galaxy contribution, enabling accurate AGN continuum and emission line measurements.
• The spectral index αλ was calculated for λ < 4000 Å using the power-law fit to the continuum, yielding αλ ≈ −5.21 ± 0.02 in 2017.
• The M-sigma relation was used to estimate the black hole mass from the stellar velocity dispersion (σ* ≈ 86 km/s), while the virial mass was derived from the broad Hα line width and continuum luminosity.
• Photometric light curves were analyzed to assess variability patterns, comparing them to the damped random walk (DRW) model and TDE decay profiles (e.g., −5/3 slope).
• Dust extinction corrections were applied to the 2011 spectrum, and the resulting intrinsic SED was compared to the 2017 unobscured SED to rule out dust obscuration as the primary mechanism.

Experimental results

Research questions

• RQ1 What is the nature of the spectral transition in SDSS J224113-012108, and what physical mechanism drives the change from a broad-line to a weak-line state?
• RQ2 Why is SDSS J224113-012108 the bluest known changing-look quasar, and what does its extreme spectral index (αλ ≈ −5.21) imply about its intrinsic continuum?
• RQ3 How do the virial black hole mass and M-sigma relation mass estimates compare in this system, and what does the discrepancy suggest about its accretion history?
• RQ4 Can the long-term photometric variability of SDSS J224113-012108 be explained by standard AGN variability models like DRW, or does it suggest a transient event such as a tidal disruption event (TDE)?
• RQ5 Why is dust obscuration unlikely to explain the observed type transition, given the intrinsic SED properties in 2011 and 2017?

Key findings

• SDSS J224113-012108 is the bluest changing-look quasar known, with a spectral index αλ ≈ −5.21 ± 0.02 for λ < 4000 Å in 2017, indicating an extremely steep ultraviolet-to-optical continuum.
• The flux ratio of broad Hα to Hβ decreased from 7 in 2011 to 2.7 in 2017, confirming a significant spectral type transition from a broad-line to a weak-line state.
• The host galaxy stellar velocity dispersion was measured as σ* ≈ 86 km/s, yielding an M-sigma relation-based black hole mass estimate of ∼3×10⁶ M☉.
• The virial black hole mass derived from broad Hα line width and continuum luminosity is ∼10⁸ M☉, approximately two magnitudes higher than the M-sigma estimate, indicating a significant discrepancy.
• The long-term photometric light curve shows smooth, gradual variability inconsistent with the damped random walk (DRW) model, resembling a TDE decay profile with a slope of −5/3.
• Dust obscuration models are ruled out because the intrinsic, reddening-corrected continuum in 2011 would have been unreasonably higher than the observed unobscured continuum in 2017, contradicting the data.