We present the analysis results of the simultaneous multifrequency observations of the blazar 4C +28.07. The observations were conducted by the Interferometric Monitoring of Gamma-ray Bright Active Galactic Nuclei (iMOGABA) program, which is a key science program of the Korean Very Long Baseline Interferometry (VLBI) Network (KVN). Observations of the iMOGABA program for 4C +28.07 were conducted from 16 January 2013 (MJD 56308) to 13 March 2020 (MJD 58921). We also used γ-ray data from the Fermi Large Array Telescope (Fermi-LAT) Light Curve Repository, covering the energy range from 100 MeV to 100 GeV. We divided the iMOGABA data and the Fermi-LAT data into five periods from 0 to 4, according to the prosody of the 22 GHz data and the presence or absence of the data. In order to investigate the characteristics of each period, the light curves were plotted and compared. However, a peak that formed a hill was observed earlier than the period of a strong γ-ray flare at 43–86 GHz in period 3 (MJD 57400–58100). Therefore, we assumed that the minimum total CLEANed flux density for each frequency was quiescent flux (Sq) in which the core of 4C +28.07 emitted the minimum, with the variable flux (Svar) obtained by subtracting Sq from the values of the total CLEANed flux density. We then compared the variability of the spectral indices (α) between adjacent frequencies through a spectral analysis. Most notably, α22–43 showed optically thick spectra in the absence of a strong γ-ray flare, and when the flare appeared, α22–43 became optically thinner. In order to find out the characteristics of the magnetic field in the variable region, the magnetic field strength in the synchrotron self-absorption (BSSA) and the equipartition magnetic field strength (Beq) were obtained. We found that BSSA is largely consistent with Beq within the uncertainty, implying that the SSA region in the source is not significantly deviated from the equipartition condition in the γ-ray quiescent periods.

Introduction
Observations and Data
    KVN
        Observations at 22, 43, 86, and 129 GHz
        Data Reduction and Imaging
    Fermi-LAT Data
Results
    Multiwavelength Light Curves
        22–129 GHz Light Curves
        -ray Light Curve
    Fractional Variability Amplitude
    Variable Flux
    Spectral Index
    Magnetic Field Strength
Discussion
    Multifrequency Variability Characteristics
    Spectral Characteristics
    Magnetic Characteristics
    Quiescent and Variable Emission Regions
    Comparison with Other Sources
Summary
Acknowledgments
Appendix A. Imaging

• Myoung-Seok Nam(Korea Astronomy and Space Science Institute, Daejeon 34055, Republic of Korea)
• Sang-Sung Lee(Korea Astronomy and Space Science Institute, Daejeon 34055, Republic of Korea, University of Science and Technology, Daejeon 34113, Republic of Korea) Corresponding author
• Whee Yeon Cheong(Korea Astronomy and Space Science Institute, Daejeon 34055, Republic of Korea, University of Science and Technology, Daejeon 34113, Republic of Korea)