X-ray and radio variability in the low-luminosity active galactic nucleus NGC 7213

Abstract

We present the results of a ∼3 yr campaign to monitor the low-luminosity active galactic nucleus (LLAGN) NGC 7213 in the radio (4.8 and 8.4 GHz) and X-ray bands (2–10 keV). With a reported X-ray Eddington ratio of 7 × 10−4LEdd, NGC 7213 can be considered to be comparable to a hard-state black hole X-ray binary. We show that a weak correlation exists between the X-ray and radio light curves. We use the cross-correlation function to calculate a global time-lag between events in the X-ray and radio bands to be 24 ± 12 d lag (8.4 GHz radio lagging X-ray) and 40 ± 13 d lag (4.8 GHz radio lagging X-ray), respectively. The radio–radio light curves are extremely well correlated with a lag of 20.5 ± 12.9 d (4.8 GHz lagging 8.4 GHz). We explore the previously established scaling relationship between core radio and X-ray luminosities and black hole mass LR∝M0.6-0.8L0.6X, known as the ‘Fundamental Plane of black hole activity’, and show that NGC 7213 lies very close to the best-fitting ‘global’ correlation for the plane as one of the most-luminous LLAGNs. With a large number of quasi-simultaneous radio and X-ray observations, we explore for the first time the variations of a single AGN with respect to the Fundamental Plane. Although the average radio and X-ray luminosities for NGC 7213 are in good agreement with the Plane, we show that there is intrinsic scatter with respect to the Plane for the individual data points.