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

¹ St John's College Research Centre, University of Oxford, Oxford, OX1 3JP, UK
² SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
³ Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands

Received: 18 June 2008
Accepted: 15 January 2009

Abstract

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  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 and 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  pc. For one component we only have a lower limit of 0.3 pc. These distances are different from earlier suggestions.