The lively accretion disc in NGC 2992 - I. Transient iron K emission lines in the high-flux state

We report on one of the brightest flux levels of the Seyfert 2 galaxy NGC 2992 ever observed in X-rays, on 2019 May. The source has been monitored every few days from 2019 March 26 to 2019 December 14 by Swift-X-Ray Telescope (XRT), and simultaneous XMM-Newton (250 ks) and NuSTAR (120 ks) observations were triggered on 2019 May 6. The high count rate of the source (its 2-10 keV flux ranged between 0.7 and 1.0 × 10^-10 erg cm^-2 s^-1) allows us to perform a time-resolved spectroscopy, probing spatial scales of tens of gravitational radii from the central black hole. By constructing a map of the excess emission over the primary continuum, we find several emission structures in the 5.0-7.2 keV energy band. From fitting the 50 European Photon Imaging Camera (EPIC)-pn spectral slices of ∼5 ks duration, we interpret them as a constant narrow iron Kα line and three variable components in the iron K complex. When a self-consistent model accounting for the accretion disc emission is considered (KYNRLINE), two of these features (in the 5.0-5.8 and 6.8-7.2 keV bands) can be ascribed to a flaring region of the accretion disc located at r_in ≃ 15-40r_g from the black hole. The third one (6.5-6.8 keV) is likely produced at much larger radii (r_in > 50r_g). The inner radius and the azimuthal extension retrieved from the co-added spectra of the flaring states are r_in = 15 ± 3r_g and ϕ = 165°-330°, suggesting that the emitting region responsible for the broad iron K component is a relatively compact annular sector within the disc. Our findings support a physical scenario in which the accretion disc in NGC 2992 becomes more active at high accretion rates ( $L_{\rm bol}/L_{\rm Edd}\ge 4$ per cent).