The bright Seyfert 1 galaxy Mrk 509 was monitored by XMM-Newton and other satellites in 2009 to constrain the location of the outflow. We have studied the response of the photoionised gas to changes in the ionising flux produced by the central regions. We used the 5 discrete ionisation components A-E detected in the time-averaged spectrum taken with the RGS. Using the ratio of fluxed EPIC and RGS spectra, we put tight constraints on the variability of the absorbers. Monitoring with the Swift satellite started 6 weeks before the XMM-Newton observations, allowing to use the ionising flux history and to develop a model for the time-dependent photoionisation. Components A and B are too weak for variability studies, but the distance for component A is known from optical imaging of the [O III] line to be ~3 kpc. During the 5 weeks of the XMM-Newton observations we found no evidence of changes in the 3 X-ray dominant ionisation components C-E, despite a huge soft X-ray intensity increase of 60% in the middle of our campaign. This excludes high-density gas close to the black hole. Instead, using our time-dependent modelling, we find low density and derive firm lower limits to the distance of these components. Component D shows evidence for variability on longer time scales, yielding an upper limit to the distance. For component E we derive an upper limit to the distance based on the argument that the thickness of the absorbing layer must be less than its distance to the black hole. Combining these results, at the 90% confidence level, component C has a distance of >70 pc, component D between 5-33 pc, and component E >5 pc but smaller than 21-400 pc, depending upon modelling details. These results are consistent with the upper limits from the HST/COS observations of our campaign and point to an origin of the dominant, slow (v<1000 km/s) outflow components in the NLR or torus-region of Mrk 509.
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