Long-term Mid-infrared Color Variations of Narrow-Line Seyfert 1 Galaxies

TL;DR

This work analyzes long-term mid-infrared color variability in 1,718 Narrow-Line Seyfert 1 galaxies by leveraging 13–14 years of WISE/NEOWISE data, classifying color variations via the color index $C = \mathrm{W1-W2}$ and the correlation $\rho_{\mathrm{Pearson}}$ with W1 brightness. It demonstrates that thermal dust reprocessing and dust-temperature evolution dominate MIR color changes, with bolometric luminosity ($L_{bol}$) acting as the primary physical driver of BWB trends, while dilution from host or disk emission and jet contamination shape observed patterns in RQ vs RL sources. The analysis reveals systematic links between MCV strength and AGN parameters, including stronger BWB at higher $L_{bol}$ and a significant role for dust temperature, supported by a simple blackbody dust model that yields a mean $T_d \approx 1107$ K. By comparing NLSy1 MCV to MIR outbursts from other extragalactic transients (e.g., TDEs, CLAGNs, ANTs) from Yao25, the study shows dust-reprocessing as a common mechanism across diverse nuclear phenomena and provides a framework for interpreting MIR variability in jet-dominated systems and dusty tori geographies.

Abstract

We present a systematic investigation of long-term mid-infrared (MIR) color variability in 1,718 Narrow-Line Seyfert 1 galaxies (NLSy1s) using 14-year \textit{WISE}/NEOWISE monitoring data. Through Pearson correlation analysis between photometric magnitude and color, we identify: (1) a radio-quiet NLSy1 (RQ-NLSy1) population comprising 230 bluer-when-brighter (BWB) sources, 131 redder-when-brighter (RWB) sources, and 1,323 objects showing weak or statistically insignificant color variations; and (2) a radio-loud NLSy1 (RL-NLSy1) population containing 5 BWBs, 2 RWBs, and 27 sources with weak/no color variations. Our analysis reveals that the BWB tendency strengthens significantly in galaxies with redder mean MIR colors $\rm \left<W1-W2\right>$ and lower starlight contamination. Furthermore, this color-change pattern demonstrates that the most bolometric luminous sources exhibit the most pronounced BWB behavior. While similar trends exist for black hole mass and Eddington ratio, bolometric luminosity appears to be the primary physical driver. Potential origins of these variations (e.g., host galaxy contribution, accretion disk variability, and dust reprocessing) are discussed. We conclude that temperature-dependent dust reprocessing dominates the observed BWB, RWB, and no/weak variation patterns. This interpretation may also apply to similar MIR color variations observed in other extragalactic MIR transients, such as tidal disruption events, ambiguous nuclear transients, and changing-look AGNs. In addition, we find no significant difference in long-term MIR color variations between RL-NLSy1s and RQ-NLSy1s, however, RL-NLSy1s show significantly greater dispersion in intrinsic variability amplitude compared to RQ-NLSy1s due to jet-induced complexity, where non-thermal synchrotron emission from relativistic jets obscures thermal dust signatures.

Figures (6)

• Figure 1: Example MIR light curves for each classification in Section \ref{['sec:classification']}. For each panel, light curves (Upper left, Blue in W1 band and Red in W2 band), color variations (Lower left), and color variations following the photometric magnitudes in W1 band (Right) are shown.
• Figure 2: Distributions of the MCV Pearson correlation coefficient $\rho_{\mathrm{Pearson}}$ versus intrinsic amplitude of variability $\sigma_m$ in W1 band (Top) and W2 band (Bottom). Red rhombuses represent RL-NLSy1s, while black dots denote RQ-NLSy1s. Dashed lines are marked at $\rho_{\mathrm{Pearson}} = \pm0.6$.
• Figure 3: Distributions of the MCV Pearson correlation coefficient $\rho_{\mathrm{Pearson}}$ versus mean MIR color $\rm \left<W1-W2\right>$ (Top) and ${\rm r-\left<W1\right>}$ (Bottom). Symbols are consistent with those in Figure \ref{['fig:mcv_iav']}.
• Figure 4: MCV Pearson correlation coefficients v.s. AGN parameters in NLSy1s. Panel (a)--(c): $\rho_{\mathrm{Pearson}}$ v.s. black hole mass $\mathrm{M_{BH}}$, bolometric luminosity $\mathrm{L_{bol}}$, and Eddington ratio $\xi_\mathrm{Edd}$. RQ-NLSy1s and RL-NLSy1s are shown as black dots and red rhombuses, respectively. Dashed lines indicate $\rho_{\mathrm{Pearson}}=\pm 0.6$ for reference. Panel (d): Distribution of individual RQ-NLSy1s in the $\mathrm{M_{BH}}$-$\mathrm{L_{bol}}$ space, color-coded by $\rho_{\mathrm{Pearson}}$ values. Dashed rectangles denote 0.3 dex $\times$ 0.3 dex bins ($\geq$20 sources each). Panel (e): Median $\rho_{\mathrm{Pearson}}$ within each bin, color-coded to show the dominant variability trend. Diagonal lines indicate constant Eddington ratio $\xi_\mathrm{Edd}$, with values increasing from bottom to top.
• Figure 5: Distribution of the minimal MIR color ($C_{\rm min}$) versus maximal MIR color variation ($\Delta C=C_{\rm max}-C_{\rm min}$) for NLSy1s (red: RWB RQ-NLSy1s; blue: BWB RQ-NLSy1s; gray: weak/no MCV RQ-NLSy1s). Overplotted MIR outbursts from Yao25 include CLAGNs (green triangles), TDEs (black pentagrams), and ANTs (yellow crosses).