2020MNRAS.493.3757J

We collect the long-term γ-ray, optical, and radio 15 GHz light curves of quasar object PMN J2345-1555, and make correlation analyses between them. We find that different samplings can lead to different time delays, and the periodicity of light curves can produce signals that are beyond 3σ significance level. The optical V and R band and the infrared J band light curves lead the radio 15 GHz light curve by 49⁺¹⁵_–12 d. There is no lag between the optical and γ-ray light curves. We use the time lags to derive the core size and its uncertainty. We find that the core region of 15 GHz is 1.5^+0.4_–0.5 parsec away from the jet base. The optical and γ-ray emitting regions coincide, which are located at 1.0^+0.3_–0.2 parsec upstream of the core region of 15 GHz. Thus, the optical and γ-ray emitting regions are most probably inside the broad-line region. For this target, the magnetic field and particle density at 1 parsec in the jet are derived to be 0.16 G and 247/γ_min cm^–3, respectively. The black hole mass is estimated to be 10^8.44 M_☉. The behaviour of δV - δR is complex, while the R - J shows a bluer-when-brighter trend. We proposed an r-dependent spectral index model to explain the colour index behaviours, which is complementary for the shock in jet model. The possibility that the spectral index behaviours are due to the contribution from the disc and the varying Doppler factor is not excluded.