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

Author： Bell M. E. Tzioumis T. Uttley P. Fender R. P. Arévalo P. Breedt E. McHardy I. Calvelo D. E. Jamil O. Körding E.

ISSN： 0035-8711

Source： Monthly Notices of the Royal Astronomical Society, Vol.411, Iss.1, 2011-02, pp. : 402-410

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Abstract

ABSTRACTWe 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 × 10⁻⁴L_Edd , 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 L_R∝M^0.6-0.8L^0.6_X , 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.