We present the Chandra discovery of soft diffuse X-ray emission in NGC 4151 ( L _ { 0.5 - 2 keV } \sim 10 ^ { 39 } erg s ^ { -1 } ) , extending \sim 2 kpc from the active nucleus and filling in the cavity of the HI material .
The best fit to the X-ray spectrum requires either a kT \sim 0.25 keV thermal plasma or a photoionized component .
In the thermal scenario , hot gas heated by the nuclear outflow would be confined by the thermal pressure of the HI gas and the dynamic pressure of inflowing neutral material in the galactic disk .
In the case of photoionization , the nucleus must have experienced an Eddington limit outburst .
For both scenarios , the AGN-host interaction in NGC 4151 must have occured relatively recently ( some 10 ^ { 4 } yr ago ) .
This very short timescale to the last episode of high activity phase may imply such outbursts occupy \gtrsim 1 % of AGN lifetime .