SIMBAD references

Reliable identification of ongoing nuclear activity in nearby galaxies is challenging when central regions are heavily obscured, especially for low-luminosity active galactic nuclei (AGN) whose emission can also be contaminated by the host galaxy. Here, we demonstrate that mid-infrared variability from Wide-field Infrared Survey Explorer observations can serve as direct evidence for obscured low-luminosity AGN. We present IC 2207, a nearby edge-on galaxy, as a case study. Despite showing no clear AGN signatures in its optical spectrum and light curves, IC 2207 exhibits stochastic variations in W1 and W2 bands. The “peak-to-valley luminosity difference” ΔνL_ν parameter, providing a host-independent AGN luminosity constraint, is greater than 10⁴² erg s^–1 in both bands. Mid-infrared image differencing localizes a point source at the galaxy center, showing that IC 2207 has a hidden AGN nucleus. We test common AGN identification criteria. Color variability can shift sources outside the AGN criteria in color-color diagrams. Due to low luminosity, multiband spectral energy distribution fitting and soft X-ray/radio luminosities are insufficient to confirm nuclear activity. However, the Swift-BAT hard X-ray detection provides independent AGN indication. The X-ray spectral analysis reveals a hydrogen column density ${N}_{{\rm{H}}}=1.9{4}_{-1.50}^{+1.86}\times 1{0}^{23}\,{{\rm{cm}}}^{-2}$, indicating that the AGN is highly obscured. The estimated Eddington ratio L_bol/L_Edd ≈ 0.007 lies near the theoretical transition threshold from standard thin disk to advection-dominated accretion flow, hinting at a special variability mechanism. This case study of IC 2207 implies that long-term mid-infrared variable point sources can serve as direct evidence for obscured low-luminosity nuclear activity in nearby galaxies, complementary to X-ray observations.