# Direct observation of the Broad Line Region in X-rays during the low state of NGC4051

## TL;DR

This study presents the first direct X-ray measurement of the Broad Line Region in NGC 4051 during its low state, revealing its density, distance, and kinematics through emission line analysis.

## Contribution

It provides the first direct X-ray observational evidence of the Broad Line Region in a Seyfert galaxy, measuring its properties during a low flux state.

## Key findings

- The emission lines indicate a high-density region within the BLR.
- The BLR is located a few 10^{15} cm from the continuum source.
- Kinematic measurements match optical Broad Line Region lines.

## Abstract

NGC 4051 is one of the X-ray brightest and most variable Seyfert galaxies. During 2009 NGC 4051 was caught at its lowest state ever observed by $XMM-Newton$. The low source continuum emission allows a clear measurement of the He-like emission lines of N$^{+5}$ and O$^{+6}$. The exceptionally high intercombination line flux is a direct measurement of either a high density or a UV irradiated photoionized plasma. Either way, this provides an unambiguous distance diagnostic. We find that the line emitting region has a density of $\sim3\times10^{10}$ cm$^{-3}$ and distance of a few $10^{15}$ cm from the continuum source, placing it within the UV-optical Broad Line Region of NGC 4051. Both N$^{+5}$ and O$^{+6}$ independently result in similar measurements for density and distance. In addition, we find the kinematics of the broad O$^{+7}$ Ly$\alpha$ match those of the He$^{+1}\,\lambda4686$\,\AA\ line, which is associated with the NGC 4051 Broad Line Region. This is the first direct X-ray measurement of a Seyfert Broad Line Region.