The structure of the X-ray absorber in Mrk 915 revealed by Swift

TL;DR

This study uses Swift-XRT monitoring to analyze X-ray variability in Mrk 915, revealing changes in spectral states linked to intrinsic power and ionized absorber properties, suggesting a clumpy or homogeneous absorber structure near the torus or broad line region.

Contribution

First detailed X-ray monitoring of Mrk 915 revealing spectral variability and constraining the location and structure of the ionized absorber.

Findings

Significant count rate variation over days.

Spectral changes linked to intrinsic power and absorber properties.

Absorber located inside the torus, possibly near the broad line region.

Abstract

In this paper we present the results obtained with a monitoring programme (23 days long) performed with Swift-XRT on the local Seyfert galaxy Mrk 915. The light-curve analysis shows a significant count rate variation (about a factor of 2-3) on a time-scale of a few days, while the X-ray colours show a change in the spectral curvature below 2 keV and the presence of two main spectral states. From the spectral analysis we find that the observed variations can be explained by the change of the intrinsic nuclear power (about a factor of 1.5) coupled with a change of the properties of an ionized absorber. The quality of the data prevents us from firmly establishing if the spectral variation is due to a change in the ionization state and/or in the covering factor of the absorbing medium. The latter scenario would imply a clumpy structure of the ionized medium. By combining the information provided by the light curve and the spectral analyses, we can derive some constraints on the location of the absorber under the hypotheses of either homogeneous or clumpy medium. In both cases, we find that the absorber should be located inside the outer edge of an extended torus and, in particular, under the clumpy hypothesis, it should be located near, or just outside, to the broad emission line region.