tokens: Long Baseline Array (VLBA) images at 43 GHz (Agudo et al. 2011 ). A quasi-stationary feature within 2 mas of the core associated with a bent jet, was analyzed using Global mm-VLBI Array (GMVA) observations at 86 GHz (Hodgson et al. 2017 ). The spectral index showed different behavior between the core and stationary component at millimeter wavelengths. The spectral index of the core at 15–43 GHz and 43–86 GHz was inverted. When the core was flaring, the spectral index changed to become steep (Hodgson et al. 2017 ). In this paper, we report results from simultaneous multiple frequency VLBI observations of OJ 287 using the KVN at 22, 43, 86, and 129 GHz, from 2013 January until 2016 December. The paper is organized as follows. In Section 2 , we present the description of the observations for both the KVN data and all archival data used in this paper. Sections 3 and 4 describe results and various analyses of the flux density variability at multiple frequencies, respectively, and Section 5 includes a discussion on the results and analysis. Finally, we summarize this paper in Section 6 . We use luminosity distance D L = 1630 Mpc at a redshift of z = 0.306, assuming H =68 , , and = 0.302 (Spergel et al. 2015 ; Hodgson et al. 2016 ). Spectral index α is defined as . 2. Observations and Data Reduction 2.1. KVN Observations OJ 287 was observed by the KVN, which is a 500 km scale VLBI network consisting of three 21 m radio telescopes located in Seoul (KVN Yonsei), Ulsan (KVN Ulsan), and Jeju (KVN Tamna), Korea. The observations were conducted at 22, 43, 86, and 129 GHz bands simultaneously with a mean cadence of roughly a month since 2012 December except for maintenance seasons from June to August. The angular resolution of the KVN is 6 mas at 22 GHz, 3 mas at 43 GHz, 1.5 mas at 86 GHz, and 1 mas at 129 GHz. We used data obtained in 31 epochs in the period from 2013 January 16 (MJD 56308) until 2016 December 28 (MJD 57750). In each epoch, the source was observed with up to eight 5 minute scans per 24 hr. The observations were recorded in left circular polarization mode at a recording rate of 1 Gbps. The observing frequencies are 21.700–21.764, 43.400–43.464, 86.800–86.864, and 129.300–129.364 GHz, with a total bandwidth of 256 MHz (thus 64 MHz in each band). Detailed descriptions of the iMOGABA observations are reported in Lee et al. ( 2016b , 2017a ). 2.2. Data Calibration The obtained data were processed with the DiFX software correlator in Daejeon, Korea (Deller et al. 2007 , 2011 ), and the data was calibrated following the standard procedures (phase and amplitude calibration, fringe fitting, and bandpass correction) in the AIPS package from the National Radio Astronomy Observatory (NRAO). For the calibration, the KVN pipeline was used (Hodgson et al. 2016 ). The visibility phases in the 22 GHz band were transferred to those in the higher frequency bands (i.e., 43, 86, and 129 GHz bands) using the pipeline in order to increase the coherence time
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