tokens: et al. 2013 ). Thus, the rather small source size of R = 4.8 × 10 15 cm obtained from the SSC fitting for the core of Cen A suggests that the emission region is located very close to the nucleus. To verify that a higher magnetic field can be observed closer to the core, we plotted the obtained magnetic fields from the SED fits for various FR I AGNs as functions of the distance from the core, as shown in Figure 3 (no error is included). It should be noted that for the sake of simplicity, the distance of the emission region from the nucleus is defined as , where R is the source radius obtained from previous studies (Chiaberge et al. 2003 ; de Jong et al. 2015 ; Fraija et al. 2017 , 2018 ; Baghmanyan et al. 2018 ; Tanada et al. 2018 ) and θ op is the opening angle of the jet. The VLBI observations revealed that the opening angle is collimated to the range of 2°–8° at a distance of 1 pc from the core; therefore, we set the angle as 5° (Junor et al. 1999 ; Horiuchi et al. 2006 ). For the distances of the AX2 and BX2 knots, we used the directly measured values, as shown in Table 1 . As can be seen in Figure 3 , the magnetic field decays as a power-law function , where the Schwarzschild radius of Cen A is r s = 2 G M BH / c 2 ≃ 1.6 × 10 13 cm. The obtained fitting parameters are the initial magnetic field of B 0 = (1.5 ± 0.2) × 10 4 G at r s and α = 0.88 ± 0.14. The conservation of particle number in a freely expanding conical jet requires the particle number density of N ( d ) ∝ d −2 , which implies B ( d ) ∝ d −1 from equipartition (Hirotani 2005 ) and it is consistent with our results. Furthermore, the core-shift analysis with the Very Long Baseline Array observations of blazars shows that the attenuation of the magnetic field at the vicinity of the core is described by B ( d ) ∝ d −1 appropriately (O’Sullivan Gabuzda 2009 ). According to the theory of the magnetically powered jet (Komissarov et al. 2007 ; O’Sullivan Gabuzda 2010 ), the initial magnetic field strength for a black hole launched jet can be calculated by G, where L rad ≃ 1.5 × 10 43 erg s −1 is the total jet luminosity of Cen A obtained from the SSC fitting, which is about one-fourth of the fitted value. Although there is a discrepancy between the theory and fitted value, which might be caused by the rough estimations such as the distance of the emission zones, these results support our suggestion that the SSC emission region of Cen A is located near the nucleus. Figure 3. Magnetic fields obtained from the SED fits for various FR I AGNs as functions of the distance from the core. The blue filled circle, square, and triangle represent the magnetic field strength of the Cen A core, AX2, and BX2, respectively. The blue open marks indicate the magnetic field strength of the core of the other FR I AGNs. The distances of AX2 and BX2 are directly measured using the Chandra analysis (Kraft et al. 2002 ). Except for these knots, the distance is calculated with the emission region size
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