Mid-infrared Variability from the Central Point Source as Direct Evidence for Obscured Low-luminosity Nuclear Activity in the Nearby Edge-on Galaxy IC 2207

Abstract 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” Δ νL ν parameter, providing a host-independent AGN luminosity constraint, is greater than 10 42 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>N</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>1.9</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1.50</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>1.86</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>×</mml:mo> <mml:mn>1</mml:mn> <mml:msup> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>23</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.25em"/> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">cm</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> </mml:math> , 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.