A recurring changing state AGN discovered in the SAMI Galaxy Survey and SDSS

The gist J0853 has gone through at least one full changing-look AGN cycle, from type 1.9 in 2001 to type 1 in 2015 and 2021, and then type 1.9 again in 2022

The Phenomenon of Changing-Look AGN

Active Galactic Nuclei (AGN) can undergo dramatic spectral shifts, losing or gaining broad emission lines and optical continuum emission over mere months to years, which are termed ‘Changing Look AGN’ (CLAGN). These rapid variations cannot be explained by static orientation-based unification models, making CLAGN crucial laboratories for probing short-term accretion disk instabilities, dynamic obscuration, and complex unknown physics in real-time. Ricci and Trakhtenbrot (2023) proposed that CLAGN actually contain two distinct populations: Changing-State AGN (CSAGN) and Changing-Obscuration AGN (COAGN). The former transition between observed spectral type due to intrinsic changes in the ionising radiation from the accretion disc, while the latter appear to change spectral type due to varying obscuration along the line of sight between the observer and the core of the AGN.

Spectroscopic Evidence of State Changes

The analysis of J0853 across four different spectral epochs reveals significant changes in its broad-line properties. Specifically, there is evidence that 'broad Hα is visible in the 2015 and 2021 spectra' and 'the 2015 and 2021 spectra also show evidence of increased Hβ emission, with the Hβ stellar absorption being infilled in 2015 and 2021'. The most recent spectrum from 2022 has a continuum shape that is bluer than the other cases, but this appears to be largely due to errors in relative flux calibration. The high-state spectra (in 2015 and 2021) show stronger broad components of FWHM ∼ 2000km s−1 dominating in the high type 1 state, compared to a weak very-broad component (FWHM ≃ 5000 − 6000 km s−1) visible in the low type 1.9 state.

Evidence for Physical State Change vs. Obscuration

Detailed spectral analysis plus WISE photometry demonstrate that the changes in this object are related to a physical state change, not changing obscuration. The WISE light curve suggests that this object may have briefly returned to a type 1.9 state in 2020. Furthermore, the WISE W1-W2 colours evolve from values typical of normal galaxies in the low state, to colours more typical of AGN in the high state. This evolution rules out the possibility that the changes we see are purely due to obscuration. The BLR structure of J0853 is not consistent with a single component responding to a change in central luminosity; instead, there are at least two separate components within the BLR.

Kinematic and Mass Estimates

The velocity centroid of the BLR with respect to the rest frame (defined by the stellar population) shows significant offsets between epochs, such as ∆vBLR = 166 ± 15 and −186 ± 22 km s−1 for the 2015 and 2021 spectra. The FWHM measurements of the two high-state spectra are consistent with each other, while the σBLR values are significantly different, suggestive of broader wings in the 2021 line profile. The black hole mass estimates derived from the Greene and Ho (2005) virial relation yield MBH values that are similar for high-state spectra, MBH ∼ 3 − 5 × 10 6 M⊙, although formally inconsistent with each other at the ∼ 1.7σ level.

Conclusion on AGN Type

Given the evidence, J0853 appears to be a CSAGN. The low-state spectra have significantly higher dust extinction than the high-state spectra, with values of E(B −V) ≃ 6 − 8 mag for the low state and E(B −V) ≃ 0.6 − 0.7 for the high state. The most compelling evidence for a CSAGN comes from the strong variation in WISE photometry. The data are not consistent with a simple model, suggesting that a more complex physical picture is needed, such as the low-luminosity BLR component coming from a distinct physical source compared to the narrower BLR in the high state.

The paper provides detailed spectroscopic and photometric analysis of galaxy J0853 to demonstrate its recurring changing-look AGN behavior. The findings suggest that this object has undergone at least one full cycle, transitioning between Type 1.9, Type 1, and Type 1.9 states over two decades. This study is significant because it probes the complex physics of short-term accretion disk instabilities and dynamic obscuration in real-time. The recurring nature of these events suggests that the BLR is not a single uniform structure but is made up of several components.

Key Findings Summary

** J0853 passes through at least one full changing-look cycle, as a Type 1.9 in 2001, a type 1 in 2015 and 2021, and a return to Type 1.9 in 2022. **

** The BLR changes in FWHM and E(B −V) across the observations: 'The low state spectra (in 2001 and 2022) have larger FWHM ≃ 5000 − 6000 km s−1 and higher E(B −V) ≃ 6 − 8 mag' and 'The high state spectra (in 2015 and 2021) have lower FWHM≃ 2000 km s−1 and lower E(B −V) ≃ 0.6 − 0.7'. **

** The AGN continuum contribution is '∼ 10 per cent of the spectral flux at 4000A is contributed by the AGN in the high state, while this is ∼ consistent with zero in the low state'. **

** The WISE W1 − W2 colours evolve from values typical of normal galaxies in the low state, to colours more typical of AGN in the high state. **

** The BLR structure of J0853 is not consistent with a single component responding to a change in central luminosity; instead, there are at least two separate components within the BLR. **

The number of changing-look AGN known has exploded in the last decade, driven by new time-domain imaging and spectroscopy. The hope is that we can uncover the detailed structure of the region around the SMBH, which is still at a spatial resolution that is impossible to reach in the optical.

The paper concludes that J0853 exhibits characteristics consistent with a CSAGN, where different components within the BLR respond differently to state changes. This complexity suggests that future observations will be necessary to fully understand the detailed structure of the region around the SMBH. The study highlights that optical spectral evolution is not sufficient to determine the nature of a state change, necessitating multi-wavelength and multi-epoch data for a complete picture.

The paper acknowledges various institutions and funding sources supporting this research, including the Australian Research Council Centre of Excellence for All-sky Astrophysics (CAASTRO). It also notes that the SAMI Galaxy Survey is based on observations made at the Anglo-Australian Telescope. The work utilizes data products from the Widefield Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology. This scientific work uses data obtained from Inyarrimanha Ilgari Bundara, the CSIRO Murchison Radio-astronomy Observatory. The CSIRO’s ASKAP radio telescope is part of the Australia Telescope National Facility.

The references section lists numerous foundational and contemporary works in AGN research, including those by Antonucci (1993), Baldwin & Ferland (1984), and Ricci & Trakhtenbrot (2023). The paper includes archived data obtained through the CSIRO ASKAP Science Data Archive. The SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions.

The analysis of radio flux shows no obvious radio variability across the time period sampled, suggesting that J0853 has relatively long term AGN activity. The spectral index found, α = −1.36 ± 0.14, suggests a peak at lower frequency and ageing of the synchrotron source electrons.

1. Improve AGN classification by integrating both intrinsic spectral state changes and physical obscuration models into prediction algorithms. This allows AI to distinguish between Changing-State AGN (CSAGN) and Changing-Obscuration AGN (COAGN) based on WISE color evolution versus optical continuum shifts, as noted in the paper: The WISE photometry rules out the possibility that the changes we see are purely due to obscuration.

2. Enhance time-domain prediction of AGN activity by incorporating multi-wavelength light curve analysis, specifically focusing on infrared bands. The AI system can predict state transitions by tracking WISE W1-W2 color evolution, as this rules out the possibility that the changes we see are purely due to obscuration and shows a shift from a colour typical of regular galaxies in the low state, to one that is more common for AGN in the high state.

3. Develop models for Broad Line Region (BLR) structure complexity by moving beyond simple single-component fits. The system can identify when the BLR structure of J0853 is not consistent with a single component responding to a change in central luminosity and instead suggests there are at least two separate components within the BLR.

4. Improve black hole mass estimation reliability by incorporating kinematic data from different epochs into virial relation fitting. The AI can assess the inconsistency in black hole mass estimates, noting that different epochs do not provide consistent values, and suggest that the geometric factor in the Keplerian relation may be different between epochs as a potential explanation.

5. Refine BLR velocity structure understanding by modeling line profiles as complex structures rather than simple functions. The system can learn from examples like J125809.31+351943.0, where the broadest (constant) BLR component was within or near the disk, and heated by the hot X-ray emitting corona.

We present a detailed analysis of the AGN in galaxy J085337.26+014303.6 at z=0.0582. A combination of SDSS and SAMI Galaxy Survey data shows that this object has gone through at least one full changing-look AGN cycle, from type 1.9 in 2001 to type 1 in 2015 and 2021, and then type 1.9 again in 2022. While the broad lines in this object show evidence of dust obscuration, detailed spectral analysis plus WISE photometry demonstrate that the changes in this object are related to a physical state change, not changing obscuration. In fact, the WISE light curve suggests that this object may have briefly returned to a type 1.9 state in 2020. In the optical, the host galaxy dominates the light (e.g. only 10 per cent contribution from the AGN at 4000 Angstoms within a central aperture), even in the high state. In contrast, the brightest WISE W2-band measurements place the AGN at a similar luminosity to the entire host galaxy. The broad Balmer lines show evidence of multiple components, with a weak very-broad component (FWHM=5000-6000 km/s) visible in the low type 1.9 state, and a stronger broad component of FWHM 2000 km/s dominating in the high type 1 state. We propose that this is caused by two spatially distinct regions within the broad line region.