Optical Variability as a Probe of Hidden Nuclear Activity in Type 2 AGNs

abstract · abstract

Optical variability in Type 2 active galactic nuclei (AGNs) is rarely explored because the direct accretion-disk continuum is obscured by circumnuclear dust. Nevertheless, detectable optical variations are present in multi-epoch surveys such as SDSS Stripe~82, indicating that some component of the nuclear emission is observed indirectly, for example through scattering or partial transmission. This study explores whether this variability is statistically connected to spectroscopic parameters of the narrow-line region (NLR), using the ALPAKA catalogue of spectral measurements. A subsample of 412 Type 2 AGNs was assembled by crossmatching SDSS Stripe 82 multi-epoch variability measurements in the $u,g,r,i,z$ bands with the ALPAKA spectroscopic catalogue. Correlations were then computed between the root-mean-square (RMS) variability amplitudes and the corresponding emission-line luminosities, kinematic widths and equivalent widths (EWs). Significant anti-correlations are found between the RMS amplitudes and [O III] 4949, [O III] 5007, [N II] 6548 and [NII] 6584 line luminosities. Velocity dispersions ($σ$) and EWs of forbidden-lines [O III] 5007 and [N II] 6584 also show moderate anti-correlations with RMS. The results demonstrate that even in obscured AGNs, optical variability carries information about the hidden nucleus. The anti-correlation between RMS and line luminosity suggests a connection between accretion stability and ionising output. Anti-correlations between RMS and the [O III] and [N II] velocity dispersions indicate a secondary correlation between optical RMS variability and the integrated kinematic state of the NLR. In addition, the anti-correlation between RMS and EW shows that the EW variations are primarily driven by changes in the continuum level, while the narrow-line flux itself remains effectively constant on the relevant timescales.

S3.SS2: 3.2 Control sample, non-AGN Galaxies / block 1 · html

After matching, the redshift and ⟨ r ⟩ \langle r\rangle distributions of the two samples are statistically consistent (KS p = 0.81 p=0.81 for both parameters). Figure 3 illustrates the resulting redshift and RMS cor , r \mathrm{RMS}_{\mathrm{cor},r} distributions for the matched samples. No statistically significant difference is found between their RMS cor , r \mathrm{RMS}_{\mathrm{cor},r} distributions (KS p = 0.32 p=0.32 ; Mann–Whitney p = 0.30 p=0.30 ), indicating comparable variability amplitudes under matched observational conditions.

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Results. Significant anti-correlations are found between the RMS amplitudes and [O III] 4949, [O III] 5007, [N II] 6548 and [N II] 6584 line luminosities. Velocity dispersions ( σ \sigma ) and EWs of forbidden-lines [O III] 5007 and [N II] 6584 also show moderate anti-correlations with RMS.

S3.SS3: 3.3 Robustness against noise-induced selection effects / block 1 · html

retaining all objects regardless of the sign of Δ \Delta . The sample was divided into bins of [O III] λ ​ 5007 \lambda 5007 luminosity, and the median value of Δ \Delta was computed in each bin. The resulting trends (Fig. 5 ) show the same qualitative behaviour as the RMS cor \mathrm{RMS}_{\mathrm{cor}} analysis, with the median Δ \Delta decreasing toward higher luminosities in the g g , r r , i i , and z z bands, while no clear trend is observed in the u u band.

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Conclusions. The results demonstrate that even in obscured AGNs, optical variability carries information about the hidden nucleus. The anti-correlation between RMS and line luminosity suggests a connection between accretion stability and ionising output. Anti-correlations between RMS and the [O III] and [N II] velocity dispersions indicate a secondary correlation between optical RMS variability and the integrated kinematic state of the NLR. In addition, the anti-correlation between RMS and EW shows that the EW variations are primarily driven by changes in the continuum level, while the narrow-line flux itself remains effectively constant on the relevant timescales.

S4.SS2: 4.2 Cross-validation of line-width measurements and the origin of the RMS– σ \sigma correlation / block 1 · html

As a representative example, results for the i i band are reported here. The observed anti-correlation between RMS cor,i and σ SDSS \sigma_{\mathrm{SDSS}} is statistically significant for both emission lines. For [N II] λ ​ 6584 \lambda 6584 : ρ R ​ σ ≃ − 0.38 \rho_{R\sigma}\simeq-0.38 , while for [O III] λ ​ 5007 \lambda 5007 the correlation is ρ R ​ σ ≃ − 0.30 \rho_{R\sigma}\simeq-0.30 .

S4.SS3: 4.3 Implications of the variability–spectral correlations / block 1 · html

The results reveal a clear anticorrelation between RMS cor and several narrow-line optical properties of Type 2 AGNs. Using only reliable narrow-line parameters from the ALPAKA survey (i.e. [O III] λ ​ λ ​ 5007 , 4959 \lambda\lambda 5007,4959 and [N II] λ ​ 6584 \lambda 6584 luminosities, EWs, and kinematic widths), more variable sources tend to exhibit (i) lower forbidden-line luminosities, (ii) smaller EWs, and (iii) narrower line profiles (Table 1 , Fig. 1 ).

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This work presents a study of the correlations between optical RMS variability from Stripe 82 and spectroscopic parameters from the ALPAKA survey for a sizeable, uniformly selected sample of Type 2 AGNs. These correlations enable an investigation of how the variability amplitude relates to the physical conditions of the NLR and to the obscured accretion flow. The main findings can be summarised as follows:

S5: 5 Conclusions / block 1 · html

Optical RMS variability in Type 2 AGNs exhibits significant anti-correlations with luminosities of narrow [O III] λ ​ 5007 \lambda 5007 , [O III] λ ​ 4959 \lambda 4959 , [N II] λ ​ 6584 \lambda 6584 and [N II] λ ​ 6548 \lambda 6548 lines, as well as with EWs and velocity dispersion of [O III] λ ​ 5007 \lambda 5007 and [N II] λ ​ 6584 \lambda 6584 . These combined trends show that AGNs with larger long-term continuum RMS variability tend to have lower narrow-line luminosities and smaller integrated line widths, both of which primarily trace the overall AGN power. Because narrow-line fluxes and kinematics probe kiloparsec-scale gas on timescales much longer than those sampled by the RMS variability, the observed RMS amplitude does not reflect changes in the NLR itself, but instead traces the underlying accretion state that governs both the NLR emission and the continuum variability.

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Variability amplitude decreases with narrow-line luminosity for Type 2, consistent with the well-established result that more luminous Type 1 AGNs exhibit smaller optical RMS variability.

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To investigate this effect on Mrk 268, we calculated the SF data from the ZTF light curve after subtracting the host component assuming its contribution of 50% of the minimum flux of the original light curve data. As a result, while the derived SF values increase, it does not show a clear dependence on time separation as in the original SF data. This result indicates that even when considering host contamination, the ZTF light curve of Mrk 268 does not show a significant flux variation due to the AGN.

S6.SS1.SSS1: VI.1.1 MCG -3-34-64 / block 1 · html

As a result, we obtained the intrinsic broad H α \alpha luminosity of MCG -3-34-64 as log ⁡ ( L bH ​ α , int . / erg ​ s − 1 ) = 41.82 \log(L_{\mathrm{bH\alpha,int.}}\mathrm{/erg\ s^{-1}})=41.82 with a typical uncertainty of 0.4 dex [ 171 ] . As a result, L bH ​ α , obs . L_{\mathrm{bH\alpha,obs.}} is consistent with L bH ​ α , int . L_{\mathrm{bH\alpha,int.}} within the uncertainty. In other words, the ratio of L bH ​ α , obs . L_{\mathrm{bH\alpha,obs.}} to L bH ​ α , int . L_{\mathrm{bH\alpha,int.}} of MCG -3-34-64 is significantly larger than predicted from the scattering model. We note that, while we do not account for stellar absorption of the H α \alpha line in our spectral fitting, its effect is thought to be negligible because the typical equivalent width (EW) of stellar absorption [ 127 , 25 , E ​ W ∼ EW\sim a few Å, e.g.] is much smaller than that of the fitted broad H α \alpha component of MCG -3-34-64 ( E ​ W ∼ 300 EW\sim 300 Å).

S3: III Spectroscopic results of SDSS J2334 / block 1 · html

Based on the best descriptions to stellar absorption features, stellar velocity dispersion σ ∗ \sigma_{*} is well measured as 112 ± \pm 14km/s, leading the central BH mass to be estimated about 10.2 − 4.5 + 7.6 × 10 6 ​ M ⊙ 10.2_{-4.5}^{+7.6}\times 10^{6}{\rm M_{\odot}} as shown in Fig. 3 through the well-known M B ​ H − σ ∗ M_{BH}-\sigma_{*} relation (between BH mass and stellar velocity dispersion) [ 34 , 36 , 72 , 62 , 8 , 10 ] widely accepted in quiescent galaxies [ 83 ] , in active galaxies [ 101 ] and also in TDEs [ 115 ] . The M B ​ H − σ ∗ M_{BH}-\sigma_{*} relation determined BH mass is well consistent with theoretical TDE model determined BH mass, providing further clues to support the central TDE and also support the SSP determined best descriptions in SDSS J2334. Here, in Fig. 3 , the 89 quiescent galaxies from G. A. D. Savorgnan & A. W. Graham [83] and the 29 reverberation mapped AGN from J.-H. Woo et al. [101] and the 12 TDEs from Z. Q. Zhou et al. [115] are considered to draw the linear correlation between stellar velocity dispersion and BH mass through the Least Trimmed Squares robust technique [ 23 ]

S2: II A TDE candidate in the long-term light curves of SDSS J2334 / block 1 · html

Based on the theoretical TDE model determined results, the expected tidal disruption radius is about 7 R G R_{G} (Schwarzschild radius). Once the central TDE can be expected, spatial size of the geometric structure of TDE debris can be simply checked. The TDE debris in SDSS J2334 around MJD-53000=3000 5000 have outer distance [ 47 ]

S2.SS3: 2.3 Data Reduction / block 1 · html

The Gemini/GNIRS observation contains two ABBA sequences, eight exposures for every target. The Pypeit pipeline produces four 2-D spectra for each object. We check these 2-D spectra to find the object manually and then coadd the 2-D spectra using the object as a reference. This results in one coadded 2-D spectrum. The 1-D spectra are extracted manually or automatically from the coadded 2-D spectra. Then we apply the flux calibration and telluric correction on the 1-D spectra using the standard star.

S3: 3 Results / block 1 · html

In this section, we present the emission line fitting results. We discuss the spectral diversity based on the line width of Ly α \alpha and H α \alpha , and the relation between the bolometric luminosity and the [O III] line luminosity.

abstract · abstract

We analyze 23 spectroscopically confirmed Type-2 quasars (QSOs) selected from the WISE 22$\rm μ$m band in the SDSS Stripe 82 region, focusing on their multi-band photometry and spectral energy distributions (SEDs). These objects were selected to be IR-luminous ($\rm flux_{W4} > 5mJy$, i.e., $12.62 < W4 < 14.62 \rm\ AB \, magnitude$), optically faint ($r > 23$) or with red color ($r - W4 >8.38$). Gemini/GNIRS observations were conducted for all 24 candidates, and 18/24 were also observed with Keck/LRIS. The observations confirm 23 to be real Type-2 QSOs in the redshift range $0.88 - 2.99$ (12 are at $z>2$). We collect multi-band photometry and conduct SED fitting. The composite photometry probes the wavelength from 0.1$\rm μ$m to 10$\rm μ$m at the rest frame. The IR emission is dominated by dust torus implying an average torus luminosity for the sample of $L_{\rm torus} 10^{46.84} \rm erg/s$. The origin of the rest-UV/optical light is not definitive, but we present three possible scenarios: scattered light, stellar emission, and the reddened accretion disk. Assuming an obscured:unobscured ratio of approximately 1:1, our targets have $L_{\rm bol} = 10^{46.28} \rm erg \,s^{-1} - 10^{47.49} \rm erg \,s^{-1}$ and around SMBH masses $\rm 10^{8.18} M_{\odot} - 10^{9.39} M_{\odot}$, assuming they accreate at the Eddington limit. Compared to previous Type-2 AGN SEDs, our targets have a brighter dust torus and redder optical-IR color. By comparing the SED to the results from JWST `little red dots' (LRDs), we find that these IR-selected Type-2 QSOs have similar SED shapes to the LRDs. This pilot Type-2 QSO survey demonstrates that mid-IR selection is an efficient way to find luminous Type-2 QSOs at $z>2$. Finally, the composite photometry and Type-2 QSOs SED model generated by this sample provide a guide for finding more Type-2 QSOs at higher redshift.

abstract · abstract

We present optical/UV photometric and spectroscopic observations, as well as X-ray and radio follow-up, of the extraordinary event AT2019cmw. With a peak bolometric luminosity of ~$\mathrm{10^{45.6}\,erg\,s^{-1}}$, it is one of the most luminous thermal transients ever discovered. Extensive spectroscopic follow-up post-peak showed only a featureless continuum throughout its evolution. This, combined with its nuclear location, blue colour at peak and lack of prior evidence of an AGN in its host lead us to interpret this event as a `featureless' tidal disruption event (TDE). It displays photometric evolution atypical of most TDEs, cooling from ~30 kK to ~10 kK in the first ~300 days post-peak, with potential implications for future photometric selection of candidate TDEs. No X-ray or radio emission is detected, placing constraints on the presence of on-axis jetted emission or a visible inner-accretion disk. Modelling the optical light curve with existing theoretical prescriptions, we find that AT2019cmw may be the result of the disruption of a star in the tens of solar masses by a supermassive black hole (SMBH). Combined with a lack of detectable star formation in its host galaxy, it could imply the existence of a localised region of star formation around the SMBH. This could provide a new window to probe nuclear star formation and the shape of the initial mass function (IMF) in close proximity to SMBHs out to relatively high redshifts.

abstract · abstract

Balmer absorption lines are increasingly observed in the little red dots (LRDs) discovered by the James Webb Space Telescope, potentially tracing dense circumnuclear gas around rapidly accreting black holes. Motivated by this connection, we search for Balmer absorption using homogeneously analyzed spectra of a representative parent sample of 14,584 low-redshift ($z<0.35$) type 1 active galactic nuclei selected from the Sloan Digital Sky Survey. We identify seven sources with robust Balmer absorption (occurrence $\sim 0.05\%$) and model them with a partially covering absorber model, accounting for the spectral resolution. By fitting H$α$, H$β$, and H$γ$ simultaneously and tying their optical-depth ratios to theoretical values, we constrain optical depth at the line center ($τ_0$) and the covering factor ($C_f$). All sources with robust modeling require optically thick H$α$ absorption and typically moderate covering factors ($C_f\approx 0.2-0.6$), while the LRD analog J1025 shows $C_f \gtrsim 0.8$ consistent with recent measurements of high-redshift LRDs. The absorbers have modest velocity offsets ($\sim 150-850\,\mathrm{km\,s^{-1}}$) and narrow intrinsic widths ($\sim 20-200\,\mathrm{km\,s^{-1}}$). Multi-epoch spectroscopy of three sources reveals Balmer-absorption variability on both year and month timescales. Three objects exhibit exceptionally weak Fe II emission, high Eddington ratio, and low gas-phase metallicity, an atypically rare combination of properties that might elevate the incidence of Balmer-absorption to $\sim$10%. We argue that low-metallicity conditions may suppress disk winds and help retain dense neutral gas along the line-of-sight in systems of high accretion rate.

abstract · abstract

Changing-look active galactic nuclei (CLAGNs) are known to change their spectral type between 1 and 2 (changing-state) or change their absorption between Compton-thick and Compton-thin (changing-obscuration) on timescales of years or less. The physical mechanism and possible connection between the two types of CLAGNs are still unclear. We explore the evolution of the broadband X-ray spectra from Nuclear Spectroscopic Telescope Array (\nustar\,) and column density in five CLAGNs with moderate inclination viewing angles, which have shown significant variations of both optical types and X-ray absorption. Based on a phenomenological and two clumpy torus models, we find that the X-ray photon index ($Γ$) and the Eddington-scaled X-ray $2-10$ keV luminosity ($L_{\rm X}/L_{\rm Edd}$) are positively correlated for the five sources, which are similar to other bright AGNs and optical CLAGNs at type 1 phase. We find a significant negative correlation between log$N_\mathrm{H,los}$ and log$L_{\rm X}/L_{\rm Edd}$ except for ESO 362-G18. Similar to changing-state AGNs, changing-obscuration AGNs may be also triggered by the evolution of the accretion disc. Our results support the disc wind scenario, where the disc wind proportional to the accretion rate and formed at moderate inclination angles would push the obscuration material further away and decrease the column density from the line of sight observed in the changing-look AGNs.

abstract · abstract

To investigate the physical origin of the Balmer decrement in the broad-line region of active galactic nuclei (AGNs), we measure the temporal variability of the fluxes of the broad H$β$ and H$α$ emission lines using multi-epoch spectroscopic data of low-redshift AGNs from the Sloan Digital Sky Survey. The analysis of the mean spectra reveals that the Balmer decrement shows no correlation with AGN luminosity, while it is inversely correlated with the Eddington ratio. However, the temporal variation of the Balmer decrement in individual objects exhibits an even stronger anti-correlation with AGN luminosity, suggesting that the change in AGN luminosity plays a dominant role in determining the Balmer decrement. By comparing the temporal evolution of the Balmer decrement with the continuum color, we find that reddening due to the AGN itself may not be the primary factor. Instead, radiative transfer effects and excitation mechanisms, which deviate from the Case B recombination, appear to be critical for the variation of the Balmer decrement. These results provide useful insights into the underlying physics of changing-look AGNs and high-$z$ AGNs, such as the ``little red dots'', which exhibit extreme values of the Balmer decrement that can be misinterpreted as evidence for dust.