A comprehensive long-term study of the radio and X-ray variability of NGC 4051 Paper II

The origin of the low-luminosity radio emission in radio-quiet active galactic nucleus, is unknown. The detection of a positive correlation between the radio and X-ray emission would imply a jet-like origin, similar to that seen in 'hard-state' X-ray binary systems. In our previous work, we found no believable radio variability in the well-known X-ray bright Seyfert 1 galaxy NGC 4051, despite large amplitude X-ray variability. In this study, we have carefully re-analysed radio and X-ray observations using the same methods as our previous work; we again find no evidence for core radio variability. In direct contrast to our findings, another study claim significant radio variability and a distinctive anticorrelation between radio and X-ray data for the same source. The other study report only integral flux values and do not consider the effect of the changing array on the synthesized beam. In both our studies of NGC 4051, we have taken great care to account for the effect that the changing beam size has on the measured radio flux and as a result we are confident that our method gives more accurate values for the intrinsic core radio flux. However, the lack of radio variability we find is hard to reconcile because radio images of NGC 4051 do show jet-like structure. We suggest that the radio structures observed are likely the result of a previous period of higher radio activity and that the current level of radio emission from a compact nuclear jet is low.