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

Abstract

We report on one of the brightest flux levels of the Seyfert 2 galaxy NGC 2992 ever observed in X-rays, on May 2019. The source has been monitored every few days from March 26, 2019 to December 14, 2019 by Swift-XRT, and simultaneous XMM-Newton (250 ks) and NuSTAR (120 ks) observations were triggered on May 6, 2019. The high count rate of the source (its 2-10 keV flux ranged between 0.7 and $1.0\times 10^{-10}$ 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 EPIC pn spectral slices of $\sim$5 ks duration, we interpret them as a constant narrow iron K$\alpha$ line and three variable components in the iron K complex. When a self-consistent model accounting for the accretion disk emission is considered (KYNrline), two of these features (in the 5.0-5.8 keV and 6.8-7.2 keV bands) can be ascribed to a flaring region of the accretion disk located at $r_{in}\simeq 15$-40 r${}_g$ from the black hole. The third one (6.5-6.8 keV) is likely produced at much larger radii ($r_{in}>50$ r${}_g$). The inner radius and the azimuthal extension retrieved from the coadded spectra of the flaring states are $r_{in}=15\pm 3$ r${}_g$ and $\varphi =165^{\circ }-330^{\circ }$, suggesting that the emitting region responsible for the broad iron K component is a relatively compact annular sector within the disk. Our findings support a physical scenario in which the accretion disk in NGC 2992 becomes more active at high accretion rates ($L_{\mathrm{bol}}/L_{\mathrm{Edd}}\ge 4\%$).

12 pages, 12 figures, 1 table. Accepted for publication in MNRAS

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