Unraveling the Variation Mechanism of CTA 102: A Combined Spectral and Polarization Investigation

Abstract The variation mechanism of blazars has been a longstanding question. As a target exhibiting extreme variability, CTA 102 offers a unique window into this puzzle. In this study, we collect multiwavelength flux data, optical spectra, and polarization data of CTA 102 spanning approximately 10 yr. We apply the localized cross-correlation function to analyze the correlation between the multiwavelength light curves. Based on the lag analysis, we find that the γ -ray, optical, and X-ray emitting regions are cospatial, all located at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>4</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>6</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1.1</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>1.1</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> pc upstream of the radio emitting region. On long timescales, the optical spectral behavior exhibits a transition from the redder-when-brighter trend to the bluer-when-brighter trend. Similarly, the γ -ray spectral behavior turns from the softer-when-brighter trend to the harder-when-brighter trend. The trend transitions could be explained by the two-component model, via the modulation of either the Doppler factor δ or the intrinsic peak frequency <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi>ν</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>p</mml:mi> </mml:mrow> <mml:mrow> <mml:mo accent="true">′</mml:mo> </mml:mrow> </mml:msubsup> </mml:math> . For the long-term behavior of the polarization degree (PD), we find that the two-component model can also quantitatively explain the correlation between the PD and the optical flux. On short timescales, the clockwise rotation on the q – u plane could be explained by the helical jet model, while the oscillation on the q – u plane may result from variations of the compression coefficient within the shock-in-jet model.