68% (thick lines) and 90% (thin lines) confidence contour levels of the plasma emission measure per unit area, EM, versus the temperature, kT, for regions East (blue), Northwest (green) and Southwest (red). The dotted curve shows the expected trend for increasing ambient density (EM ∝ 1/(kT)²), assuming the same extension along the line of sight in all the regions (see Sect. 4).