The Seyfert 1 Galaxy Mrk 335 at a very low flux state: mapping the soft X-ray photoionised gas

Context. This paper reports on an XMM-Newton observation of the Seyfert 1 Galaxy Mrk 335 performed as a target of opportunity when the source was in an unusually low flux state. Aims. The low level of continuum emission unveiled an underlying line-rich soft X-ray spectrum, which can be studied with the Reflection Grating Spectrometer. Methods. The emission features were analysed at high resolution with unprecedented detail for this source. Line ratio diagnostics from H-like and He-like ions indicate that the line emission arises in X-ray photoionised plasma. Extensive simulations were performed with the CLOUDY photoionisation code. The physical properties of the line emitting material were derived from the comparison of the expected and observed line intensities. Results. Because of the degeneracy in the ionisation parameter, a number of different solutions for the electron density and column density of the gas are consistent with the spectral diagnostics. This prevents us from uniquely determining the properties of the plasma; however, the location(s) of the X-ray photoionised gas can be constrained to within 0.06 pc (outer boundary). This limit places the X-ray line emitting gas at the inner edge of the material where the broad optical lines are produced (broad line region).