High-resolution X-ray spectroscopy of the low and high states of the Seyfert 1 galaxy NGC 4051 with Chandra LETGS

Context: With the new generation of high-resolution X-ray spectrometers the understanding of warm absorbers in active galactic nuclei has improved considerably. However, the important questions regarding the distance and structure of the photoionised wind remain unsolved.
Aims: To constrain the distance of the photoionised wind, we study the variability of the continuum, absorption, and, emission in one of the brightest and most variable low-luminosity AGN: the narrow-line Seyfert galaxy NGC 4051.
Methods: We analyse two observations taken with the Low Energy Transmission Grating Spectrometer of Chandra. We investigated the spectral response to a sudden flux decrease by a factor of 5, which occurred during the second observation.
Results: We detect a highly ionised absorption component with an outflow velocity of -4670 km s^-1, one of the highest outflow velocity components observed in a Seyfert 1 galaxy. Furthermore, this is one of the only observations whereby the X-ray observed absorption component is unaccompanied by a corresponding UV absorption component with the same outflow velocity. The spectra contain a relativistic O VIII Lyα line with properties similar to those determined for this source with XMM-Newton, and four absorption components spanning a range in ionisation parameter ξ between 0.07 and 3.19 (log values, and units of 10^-9 W m). An emission component producing radiative recombination continua of C VI and C V appears during the low state. The black body temperature decreases with the drop in flux observed in the second observation.
Conclusions: For all absorber components we exclude that the ionisation parameter linearly responded to the decrease in flux by a factor of 5. The variability of the absorber suggest that at least three out of four detected components are located in the range 0.02-1 pc. For one component we only have a lower limit of 0.3 pc. These distances are different from earlier suggestions.