New Insights Into the Spectral Variability and Physical Conditions of the X-Ray Absorbers in NGC 4151

Abstract

We investigate the relationship between the long term X-ray spectral variability in the Seyfert 1.5 galaxy NGC 4151 and its intrinsic absorption, by comparing the 2014 simultaneous ultraviolet/XRay observations taken with Hubble STIS Echelle and Chandra HETGS with archival observations from Chandra, XMM-Newton and Suzaku. The observations are divided into "high" and "low" states, with the low states showing strong and unabsorbed extended emission at energies below 2 keV. Our X-ray model consists of a broken powerlaw, neutral reflection and the two dominant absorption components identified by Kraemer et al. (2005), hereafter KRA2005, X-High and D+Ea, which are present in all epochs. The model fittings suggest that the absorbers are very stable, with the principal changes in the intrinsic absorption resulting from variations in the ionization state of the gas as the ionizing continuum varies. However, the low states show evidence of larger column densities in one or both of the absorbers. Among plausible explanations for the column increase, we discuss the possibility of an expanding/contracting X-ray corona. As suggested by KRA2005, there seem to be contributions from magnetohydrodynamic (MHD) winds to the mass outflow. Along with the ultra fast outflow absorber identified by Tombesi et al. (2010), X-High is consistent with being magnetically driven. On the other hand, it is unlikely that D+Ea is part of the MHD flow, and it is possible that it is radiatively accelerated. These results suggest that at a sufficiently large radial distance there is a break point between MHD-dominated and radiatively driven outflows.

Mass outflow of the X-ray emission line gas in NGC 4151

“…While the inclusion an unabsorbed power-law, at 3% of the flux of the continuum source, made a small statistical change (∆χ 2 = 158), visual inspection showed improvement of the continuum fitting, consistent with the suggestion by Kraemer et al (2005). The overall continuum fit was optimised to isolate the emission features, hence, while roughly consistent with Couto et al (2016), we did not require it to be an exact match.…”

“…However, their measurements included redshifted emission on the NE side of the nucleus. The emission line gas in that region lies behind the disk of the host galaxy (Das et al 2005) and could be covered by X-ray absorption similar to that described by Couto et al (2016). In fact, soft X-ray emission lines can be effectively suppressed by relatively small column density absorbers (e.g., N H ≤ 10 21 cm −2 , Kraemer et al 2011).…”

“…The ATABLE grid spanned a range from 22 ≤ logN H ≤ 23 and −0.5 ≤ logU ≤ 1.3, in intervals of 0.1 dex; the ranges are consistent with those used in Armentrout et al (2007). We also assumed a turbulence velocity of 100 km s −1 , in agreement with our previous studies (Armentrout et al 2007;Couto et al 2016). The MTABLES were generated via a two-step process.…”

“…To that end, we generated photoionisation model grids, computed using Cloudy (version 13.00; Ferland et al 2013 2 ). We assumed a continuum source with the same spectral energy distribution (SED) used in our analysis of the X-ray absorbers (Couto et al 2016), in the form of a power law L ν ∝ ν −α , with α = 1.0, for 1 ×10 −4 eV < hν < 13.6 eV, α = 1.3, for 13.6 eV < hν < 500 eV, and α = 0.5, for 500 eV < hν < 30 keV, with exponential cutoffs above and below the limits. In addition to the SED, the model results depend on the choice of input parameters, specifically: the radial distances of the emission-line gas with respect to the central source (r), number density (n H ), and column density (N H ) of the gas.…”

Mass outflow of the X-ray emission line gas in NGC 4151

“…The full, unfolded spectrum is shown in Figure 1 (left hand panel). The intrinsic continuum was heavily absorbed and in a low-flux state (Couto et al 2016), which enabled the isolation of numerous emission features, such as H-and He-like lines from O, Ne, Mg, and Si, Fe Kα, Si Kα and various radiative recombination continua.…”